WITH SO MANY types of high end headphones with microphone on the market, choosing just the right ones for your needs can seem a daunting task. But by arming yourself with a little knowledge about the advantages and drawbacks of each headphone type, you can quickly narrow down the field and find the perfect choice.
The broad term headphones is often used to describe any personal listening device that sits on the head. But the category encompasses four distinct subcategories: earbuds, earphones, in-ear monitors, and headphones. Let’s look at each of these in turn.
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As earbuds are nonstarters for high-quality audio, I’ll move on to earphones, also called in-ear monitors. There’s a single difference between earbuds and earphones: earbuds fit in the outer ear, but an earphone is inserted into the ear canal, where, ideally, it creates an airtight seal. The difference in sound quality conferred by this apparently small distinction is profound. Creating an airtight seal greatly improves sound quality, particularly in the bass. A tight seal also prevents outside noise from intruding on your music. Earphones are often supplied with a variety of interchangeable eartips, made of soft silicone or compressible foam, which allows you to choose the ones that provide the best fit with your ears. Although earphones may be superficially similar to earbuds, very-high-quality sound is possible from good earphones, never from earbuds.
Although good earphones can provide superb sound, how well those earphones fit your ear canals and thus how good they sound is somewhat hit or miss, even with a range of flexible eartips. Because earphones’ sound quality depends so much on how well they fit, some manufacturers offer custom-fit in ear monitors earphones that have been custom molded to your ear canals.
A little history of how in-ear monitors came about helps in understanding their raison d’être. Remember seeing, at amplified concerts, those small black speakers sitting at the front of the stage, angled up toward the musicians? Those were stage monitors, which allowed the musicians to hear themselves playing and singing. Stage monitors are problematic they’re bulky, tend to generate feedback, must be turned up very loud to be heard above the house mix, and require that each musician stand in front of his or her individual monitor. The professional live-sound industry solved these problems with the custom-fit in-ear monitor.
Giving each musician personal in-ear monitors made stage monitors a thing of the past. But for in-ear monitors to be effective, they must form a perfect seal between the transducer (the tiny speaker) and the ear canal. Any leakage is unacceptable, because the sound from the huge m in speakers would swamp the sound from the in-ear monitors and confuse the musician.
The solution was to create impressions of the performer’s ear canals, then mold each earphone of a pair to provide a perfect fit with the corresponding canal. High-end audio has adapted this approach to creating very-high-quality in-ear monitors for music listening at home or on the go. The in-ear monitor not only provides high isolation from noise; the airtight fit allows the tiny transducer to perform optimally. A high-quality in-ear monitor can deliver spectacular performance, including bass of a depth and power not normally associated with tiny personal listening devices. Another benefit is comfort. Because in-ear monitors precisely match the shapes of your ear canals, you can almost forget they’re there. Moreover, the slightly larger size of in-ear monitors relative to earbuds gives designers room to include more elaborate transducers. Finally, a custom-fit in-ear monitor can provide as much attenuation of background noise as a headphone that uses active noise-canceling technology (described later).
To create the molds from which in-ear monitors will be cast, an audiologist trained in creating in-ear impressions injects a semi-liquid silicone or acrylic material into your ear. During this process, you’ll be asked to hold a plug in your opened mouth, to prevent your ear canals from moving as the material solidifies. About 15 minutes later, the audiologist removes the now-solid material, which retains a perfect impression of your ear canal. Each in-ear monitor is hand-cast from such a mold to create a shape unique to your ear. Note that the entire in-ear monitor—not just the tip—conforms to your ear canal’s shape. To ensure an absolutely perfect fit, some in-ear monitors are made from a pliable material that softens slightly when warmed by your ear canal. Many makers of inear monitors bring an audiologist to consumer hi-fi shows to create your ear-canal impressions right on the spot. The finished monitors are shipped to you several weeks later.
Because the market for musicians’ in-ear monitors is relatively large and the customers are so demanding, much research has gone into designing good-sounding products. Some in-ears are extremely elaborate, and may include five drivers and two crossover points one bass driver, two midrange drivers, and two tweeters, for example. Others rely on a single full-range driver. No matter their number, the drivers are usually miniature versions of the moving-coil cones found in freestanding loudspeakers. Another type of transducer used in in-ear monitors is called the balanced-armature driver, in which the armature is a tiny rod balanced on a pivot point, like a teeter-totter, within a magnetic field. The audio signal is applied to a coil surrounding the armature, which creates a fluctuating magnetic field that magnetizes the armature. The armature’s own magnetic field reacts with the fixed magnetic field created by the magnet, causing the armature to oscillate and, with it, transmit those vibrations to a tiny diaphragm connected to one end of the armature. An upper-end in-ear monitor may contain several balanced-armature transducers, each optimized for reproducing a different range of frequencies. Balanced-armature drivers are extremely efficient, requiring very little power to produce sound.
Custom-fit in-ear monitors require a greater commitment of time and money than off-the-shelf earphones, but their combination of comfort, noise isolation, and, most important, sound quality is unequaled among in-ear listening devices.
Which are the best headphones? There’s are as many right answers to this question as there are listeners and specific applications. Choosing the right headphones for you begins with defining how you will use the headphones, what combinations of sound and comfort you most value, and your budget. Let’s explore some of these criteria, keeping in mind that many of them overlap.
Location and intended use: Where will you use the headphones? If you plan to listen in public places such as airplanes, trains, or subways, you’ll want the headphones to provide isolation from outside noise as well as prevent pedestrians nearby from hearing your music. Earphones that provide a good fit and all custom in-ear monitors provide excellent isolation from ambient noise. (But remember that keeping outside noise from intruding on your listening experience can also prevent you from hearing sounds that warn of danger.)
Conversely, if you’re buying headphones because your home system is headphone rather than loudspeaker-based, or to enjoy music late at night without disturbing neighbors or family members, noise isolation and portability won’t be as important. For home listening, your top priorities should be sound quality and comfort.
Supra-aural and circumaural: Headphones are classified by how they fit on your head: supra-aural and circumaural. Supra-aural (literally, “over the ear”) headphones have pads that rest on the ears, which is why they’re also called “on-ear” headphones. By contrast, a circumaural (literally, “around the ear”) headphone completely covers and encloses the entire ear. Supra-aural headphones are generally lighter and smaller, and fit more easily in a travel bag. Their disadvantage is that they allow outside sounds to intrude on the listening experience, which can be distracting. (This also works in the other direction: Supra-aural headphones leak sound from the headphones to the outside world, potentially disturbing people nearby.
Supra-aural headphones are thus not a good choice for subway commuters or airline passengers.) By completely enclosing the ear, circumaural headphones provide greater isolation from outside sounds, and those nearby won’t hear your music. If you plan to listen in the presence of people who may be bothered by your music, choose circumaural over supra-aural headphones.
Open-back vs. closed-back: A second major division in headphone design is open-back and closed-back headphones. In open-back ’phones the side of the diaphragm facing away from the ear is open to the outside world. Conversely, a closed-back design completely seals the driver in an enclosure. Open-back headphones provide less isolation from outside noise, and allow others nearby to hear your music.
Generally speaking, open-back headphones have a more spacious soundstage and better imaging, and closed-back headphones have deeper, more powerful bass. Completely enclosing the driver, however, causes the sound radiated from the rear of the diaphragm to be reflected from the enclosure back into the diaphragm, introducing unwanted diaphragm motions that are heard as colorations and smearing. Generally, open-back ’phones offer better sound quality than closed-back designs. Closed-back ’phones can also make your ears feel hotter with extended use; openback ’phones “breathe.”
Portability: The ultimate in portability is provided by earphones and in-ear monitors, which are often supplied with a small travel case that easily fits inside a briefcase, backpack, or purse. Some listeners object to inserting objects in their ears, and will opt for headphones even for traveling. If portability is important, look for headphones that fold up for storage in a travel case.
Comfort: The best-sounding headphones in the world won’t be much good if they’re uncomfortable to wear. Headphones vary immensely in how they feel on the head and against or around the ears, and any discomfort will only increase the longer they are worn. There’s some agreement about which model headphones are generally comfortable and which aren’t, but everyone’s head is different. If possible, you should wear the headphones under consideration for an extended period before committing to a purchase.
Generally, lighter headphones are more comfortable than heavier models. Supra-aural headphones that rest against the ear usually weigh less than circumaural ’phones that enclose the ear, but the pressure applied to your ears by supra-aural headphones can be irritating. Also consider the headband padding, which ranges from a vinyl cover over bare metal to a deep cushion. Because every listener’s head has a different shape and size, some brands or models of headphones will naturally fit you better than others.
Ruggedness: If your headphones will never leave your listening room, rugged build-quality won’t be an important consideration. For those who carry their ’phones everywhere they go, choosing headphones designed to withstand the rigors of daily travel is essential to realizing a long service life. Headphones designed for professional use are usually more durable.
Sound quality: The criteria for judging the sound quality of headphones and loudspeakers are the same: You want a smooth tonal balance with no colorations, particularly through the midrange. The bass should be extended and full, but not bloated and thick. Headphones vary considerably in their treble extension and sense of openness, and a model that lacks good treble extension will sound closedin. Transparency, resolution, and dynamic range are all important factors in how much musical satisfaction headphones deliver in the long term. Listen to familiar music through the headphones you’re considering, preferably for an extended audition.
Noise canceling: Active noise canceling is a technology that suppresses steadystate background noise, such as the din generated by airplanes, cars, and trains. Here’s how it works: A built-in microphone picks up ambient sound, amplifies that sound, inverts its polarity, and drives the headphones with the inverted signal. This inverted signal cancels, to some degree, the ambient sound leaking into your ears. Noise-canceling headphones greatly reduce fatigue on long flights. By lowering the level of background noise, this technology can also make music more intelligible and higher in resolution.
Noise-canceling headphones require a power source: either an integral rechargeable battery pack, or two or more replaceable AA or AAA batteries. They’re a little larger and more expensive than conventional headphones, but if you travel frequently, they can go a long way toward making travel more pleasant. There’s no reason why noise-canceling headphones need to compromise fidelity; I own a pair of terrific-sounding headphones (PSB M4U2) that happen to have noise-canceling technology. They greatly reduce the constant roar inside an airplane cabin; I wear them even when I’m not listening to music.
Amplifier matching: As later explained in technical detail, headphones perform best when their electrical characteristics match those of the amplifier driving them. The primary specification to look at is the headphones’ impedance. Briefly, inexpensive portable audio devices tend to work best when driving headphones of high impedance (i.e., higher than about 100 ohms). Low-impedance headphones perform better when driven by a more robust amplifier, such as those found in portable DACs and dedicated headphone amplifiers.
Balanced and unbalanced drive: Some headphones offer the option of balanced connection between the amplifier and headphones. Balanced connection carries the audio signal on three conductors rather than two. Balanced connection requires headphones that are wired to accept balanced connectors, as well as an amplifier designed for balanced operation.
Wireless headphones: Wireless headphones free the user from being tethered to the amplifier, but that convenience comes at a price: sound quality. Wireless headphones often use Bluetooth for transmitting the signal between the base station and the headphones. Bluetooth employs a coding and decoding system to reduce the bit-rate and thus the wireless signal’s bandwidth. For that reason, wireless headphones aren’t recommended except for casual use, or with the multichannel headphones described later.
Dynamic, electrostatic, and planar-magnetic headphones: An electrical signal can be converted into sound by various technologies, including the dynamic moving-coil driver, the electrostatic panel, and the planar-magnetic transducer. You’ll find a similar array of technologies among high-end headphones. Each of these technologies is simply a scaled-down version of the dynamic, electrostatic, and planar-magnetic drivers found in freestanding loudspeakers.
The vast majority of headphones have dynamic drivers. Dynamic headphones are rugged, small in size, and low in weight relative to electrostatic and planar-magnetic headphones. Although some dynamic headphones are expensive, they are generally less costly than electrostatic and planar-magnetic designs.
Electrostatic headphones are rare, but they have a cult following because of their outstanding transparency, resolution, and speed. These qualities, which they share with electrostatic freestanding loudspeakers, are conferred by the electrostatic diaphragm’s large surface area and extremely low mass. Such a lightweight diaphragm can respond very quickly to input signals, infusing the sound with a lifelike realism and a resolution of low-level detail that are generally lacking in dynamic designs. The treble of electrostatic headphones is free from distorting resonances and extends well beyond 20kHz. The downside of electrostatic headphones is the need for a high-voltage outboard power supply to charge the diaphragm, as well as an amplifier of sufficient voltage output to drive the ’phones. Because this power supply and amplifier must be plugged into an AC wall outlet, electrostatic headphones can’t be used for portable listening.
The classic electrostatic headphones the model that popularized the technology are the Stax SR Lambdas. Released in 1979 at the then whopping price of $340, the SR Lambdas brought unprecedented sound quality to headphones, and were a favorite model among audiophiles and location recording engineers.
The third primary headphone technology is the planar-magnetic driver: a very thin, lightweight diaphragm to which a conductor is bonded. The diaphragm is suspended in a magnetic field created by permanent magnets on both sides of the diaphragm. The audio signal passes through the conductor bonded to the diaphragm, creating a varying magnetic field. The variations in this magnetic field constitute an analog of the audio signal and interact with the permanent magnetic field, pushing and pulling the diaphragm to vibrate the air and thus create sound. The drivers in planar-magnetic headphones have much lower moving mass than dynamic drivers, a larger surface area, faster transient response, lower distortion, and greater frequency extension. Planar-magnetics can deliver exceptional sound quality, with resolution, dynamics, and tonal fidelity that rival those of state-of-the-art loudspeakers.
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Without inter-aural crosstalk, the brain is unable to construct a convincing illusion of sounds existing in space in front of us, as with loudspeakers. Let’s look at several techniques and technologies that attempt to make the headphone listening experience more like that of hearing live music:
Crossfeed circuits: A simple solution to the problem of “inside-the-head” imaging is the crossfeed circuit, which has been around since the 1950s. A crossfeed circuit electronically re-creates the inter-aural crosstalk that occurs when listening to two stereo loudspeakers. The circuit mixes into the left channel some right-channel information that has been greatly attenuated and slightly delayed, and vice versa.
Crossfeed circuits are built into some headphone amplifiers. A switch allows you to turn off the effect if you find the sound unnatural. Many listeners find crossfeed circuits a mixed blessing; they reduce “inside the head” imaging, but sometimes at the expense of blurring imaging, softening the treble, reducing resolution, and muting dynamic contrasts. There’s no definitive answer as to whether crossfeed circuits are a benefit; some listeners like the effect, some don’t.
Smyth Virtual Surround (SVS): The most innovative approach to overcoming the inside-thehead imaging of headphones is undoubtedly Smyth Virtual Surround (SVS). Developed by Stephen Smyth, who invented the codecs that later became the basis for the DTS multichannel surround-sound formats, SVS is a technology that produces a listening experience through headphones that is indistinguishable from listening to a stereo or multichannel loudspeaker system in a room. SVS is implemented in a commercially available hardware platform called the Smyth Realiser A8.
Here’s how it works. You insert in your ear canal earplugs in which are embedded tiny microphones. After connecting the mikes to the Realiser A8 audio processor (a component about the half the width of the average preamplifier), you play a test signal through your home audio system’s speakers while you sit in the listening position. You then put on a pair of headphones and play a second test signal, which is again picked up by the mikes inside your ears. The whole process takes about five minutes. After that, whenever you listen to music through headphones connected to the Realiser, you hear a startlingly realistic rendering of your speakers in your listening room.
The imaging is moved outside the headphones into an immersive three-dimensional space that is indistinguishable from the sound of the speakers in the room.
If you measured different speakers in a different room, the headphone sound will mimic those speakers in that room. As its name suggests, Smyth Virtual Surround works not only for stereo systems, but also for full surround-sound loudspeaker arrays. The impression of hearing surround information located far behind you through stereo headphones is uncanny.
There’s a reason the room/speaker system and headphone measurements were made with tiny microphones inside your ear canal. To understand that reason, you need to know that the outer ear, called the pinna, plays a vital role in locating sounds. The pinna’s pattern of folds and bumps (the medical terms are folds and bumps) create a complex series of sound reflections that result in a series of minute delays in the sounds reaching your eardrum. When the direct sounds striking your eardrum are combined with the sounds delayed by the reflections introduced by the pinna, the result is comb filtering: a series of notches in the frequency response. The patterns of the frequency-response notches change with the sound’s direction, and the brain analyzes these comb-filter patterns, and the changes in them, to instantly pinpoint the sound’s direction. The head and torso are also sources of reflected sound and play roles in this mechanism. This modification of a sound by the head and ear is called the head-related transfer function (HRTF).
During its measurement process, the Smyth Realiser A8 captures your particular HRTF (everyone’s is different). On playback, the Realiser uses digital signal processing (DSP) to impose on the audio signal your HRTF, fooling your brain into perceiving the sound as existing in three-dimensional space, complete with the tonal balance and spatial cues present in the room where the measurement was taken. Note that the Realiser also measures and then re-creates the “transfer function” of your loudspeakers and room the way the speakers and acoustics modify the test signal, and thus the musical signal, on playback.
This technology opens up startling possibilities. You could measure a worldclass audio system in a state-of-the-art listening room, then replicate that listening experience through headphones in your own home. Professionals use the Realiser to replicate, on location or in another studio, a known optimal monitoring environment.
Binaural listening: Smyth Virtual Surround simulates, via digital signal processing, the head-related transfer function that can immerse us in a three-dimensional soundfield when listening to stereo headphones. But there’s another way of creating that effect: to record music with an HRTF impressed on the recorded signal. The technique, called binaural recording, involves recording with microphones inside the sculpted ears of a dummy head, or Kunstkopf. When played back through stereo headphones, a binaural recording can re-create the impression of sounds emanating from any location around you. (I heard a binaural demonstration of a dummy head getting a “haircut” in reality, a recording of someone opening and closing a pair of scissors while moving in 360° around the dummy head. The spatial presentation was so realistic that it was unnerving.) Note that, to hear the intended spatial effect of binaural recordings, you must listen to them through stereo headphones.
Binaural recordings can be made without a dummy head: with a pair of microphones placed about 7" apart and facing away from each other, replicating the distance between a human head’s ears. These quasi-binaural recordings can provide greater spatial realism through headphones, but don’t approach the full spatial realism that binaural recordings made with a dummy head are capable of.
The ultimate binaural recording would be made using your HRTF. The dummy head is just a generic average of all human heads in size and shape, and thus doesn’t perfectly deliver precise spatial cues. Everyone’s HRTF is different, particularly the comb filtering created by the pinna’s folds and bumps. I’ve read of experiments in which recording microphones were placed inside the ear canals of a human sitting in a concert hall while an orchestra played; the recording was later played back to the same person through in-ear monitors. Although playback occurred in a tiny room, the effect was a near-perfect simulation of the spatial experience of sitting in a vast concert hall. As part of this experiment, the recording was played back to listeners other than the one whose HRTF was used to create the recording and to those listeners, the sound was horribly distorted. This was because the unique HRTFs of the listeners not involved in the recording had been replaced by the HRTF of the single listener who was involved. Each person’s brain is adapted to his or her own HRTF; the combination turns what is, objectively, a gross distortion produced by comb filtering into a coherent and natural perception of sound. The uniqueness of one’s own adaptation is starkly exposed when one listens to another person’s head-related transfer function.
Software for headphone listening: Finally, outside-thehead imaging can be created in software running on a personal computer. The software simulates the head-related transfer function of speakers in a room by applying DSP filters to the music signal on playback. The software offers the simulation of a wide range of well-known high-end speakers in various rooms, allowing you to choose the one that sounds best to you. The HRTF data was created by actually measuring those speakers in different rooms, producing a remarkable facsimile of the loudspeaker experience through headphones. The software package Out of Your Head from Darin Fong Audio (reviewed this issue) allows you to audition the various speaker/room combinations before you choose which to buy and download.
Multichannel headphones: Multichannel playback through stereo headphones can be achieved with less sophisticated technology than Smyth Virtual Surround, though not nearly as convincingly. Often called “Dolby headphones,” this product category uses DSP to mimic the HRTF, enabling the headphones to position sounds anywhere around the listener. Dolby headphones typically include a Dolby Digital decoder that takes a Dolby Digital bitstream from a TV, DVD Blu-ray Disc player, or other source, and outputs a signal to the headphones that creates a facsimile of hearing surround sound through stereo ’phones. With Dolby headphones, you hear dialogue from the center, music and effects at the left and right, and surround information from all around you. Dolby headphones, which are often wireless, are good for watching movies late at night and/or if you don’t want to disturb others. They can also be useful for those who are hard of hearing; the greater clarity provided compared to listening to the tiny speakers built into a television can open up a new world. This is particularly true of dialogue, which often gets lost when reproduced by television speakers but is much more intelligible when reproduced through headphones, largely because it’s spatially separated from the soundtrack’s music and effects.
The most common specification is frequency response, with which we’re all familiar. A headphone model’s frequency-response spec may read “20Hz–18kHz,” but this is misleading. Headphones don’t have flat measured response; instead, they’re designed to sound flat to listeners wearing them. It’s not unusual for headphones to have multiple amplitude peaks of as much as 10dB in the upper-midrange to mid-treble region (roughly 3–8kHz). Such peaks in a loudspeaker would make it unlistenable, but in headphones the peaks produce a more natural tonal balance. What’s more, frequency response measurements of headphones are inconsistent compared with speaker measurements; moving the headphone measurement microphone only 1⁄16" results in a radically different measured response. The measurement protocols for testing freestanding speakers are much more standardized and reliable.
Finally, there’s no correlation between good speaker frequency response and “good” measured headphone frequency response, because there’s no consensus about which headphone frequency response produces the most natural tonal balance. This is why you can’t tell much about how a pair of headphones will sound based on looking at their frequency-response spec, or even their frequency-response curve.
A more useful spec is of headphones’ impedance. Impedance is resistance to current flow (this is a simplified definition). The impedances of headphones span a far wider range of values than do those of freestanding speakers. A loudspeaker’s nominal impedance is typically between 4 and 8 ohms, but headphone impedances range from about 15 to 600 ohms.
The lower a speaker’s impedance, the more robust the amplifier driving it needs to be. A low-impedance speaker forces the power amplifier to deliver more current to the drivers. That’s why a low-impedance speaker is best driven with a powerful amplifier.
Similarly, low-impedance headphones sound best when powered by a high-quality amplifier.
The inexpensive amplifiers built into smartphones, tablets, low-end portable music players, and even some A/V receivers can’t deliver much output current to headphones, and are therefore not well suited to driving low-impedance models. The sound will be weak and anemic, and the bass mushy. The best amplifiers for low-impedance headphones are dedicated outboard headphone amplifiers, or the headphone amplifiers built into high-quality DACs. High-impedance headphones are a better choice with inexpensive mass-market electronics that include an integral headphone amplifier.
Knowing the headphones’ impedance is crucial to matching them with the amplifier that will drive them, specifically the amplifier’s output impedance. The rule of thumb is that the headphones’ impedance should be at least ten times the amplifier’s output impedance. If the amplifier’s output impedance is greater than one-tenth the headphones’ impedance, the headphones’ frequency response (tonal balance) will be affected. This is one reason the same headphones sound different when driven by different amplifiers. In addition, an amplifier with a high output impedance will have a low damping factor; that is, its ability to control the motion of the drivers’ diaphragms will be limited. Low damping factor is associated with lack of tautness, weight, and precision in the bass. Headphones driven by a headphone amplifier follow the same electrical laws as speakers driven by a home stereo amplifier.
If the headphone amplifier’s output impedance is less than 2 ohms, that amplifier should have no problem driving any headphones. But many headphone amplifiers have higher output impedances, especially those found in inexpensive portable devices. The designers of those budget devices add a 30-ohm resistor to the amplifier’s output to protect the op-amp driving the headphones.
Although this technique makes the amplifier more reliable and stable, it virtually ensures that the device won’t sound good with most of the headphones on the market. Many high-end headphone amplifiers have output impedances of a fraction of an ohm, and will drive any headphones with ease. Some high-quality headphones have a very low impedance; their designers assume that they will be driven by low-output-impedance amplifiers.
The last specification you should know about is sensitivity: a measure of how much sound the headphone produces for a given amount of input power. Specifically, the sensitivity spec expresses the sound-pressure level (SPL) the headphone outputs when driven by 1 milliwatt (1mW) of power. The sensitivity of headphones ranges from a low of about 70dB/mW to as high as 125dB/mW. The lower the sensitivity, the more amplifier power needed to achieve a given SPL.
Every pair of headphones needs to be driven by an amplifier, whether that amplifier is an integral component in a portable music player or computer, an outboard digital-to-analog converter, the headphone output built into a home-audio component, or a dedicated headphone amplifier. The amplifier’s quality plays a large role in how the headphones will sound.
The headphone amplifiers built into most portable devices are inexpensive operational amplifiers (op-amps) that can’t generate the voltage and deliver the current required by most headphones. To make matters worse, the high output impedance of op-amps introduces many problems, including an erratic frequency response that changes depending on the headphone model, a limited ability to deliver current to the headphones, and a low damping factor, all of which contribute to poor bass performance and limited dynamic range.
Higher-quality headphone amplifiers have very low output impedance (as low as 0.01 ohm) and thus a high damping factor, lower distortion, an ability to generate sufficiently high voltage and current to the headphones. The low output impedance also ensures that the amplifier won’t interact with the headphones’ impedance curve to introduce frequency-response errors. Virtually all standalone dedicated headphone amplifiers have these advantages. A few esoteric tubed designs, however, forgo low output impedance in favor of other qualities the designer believes are more important. One example is negative feedback in the amplifier: More negative feedback lowers an amplifier’s output impedance, but at the expense of sound quality, in the view of some designers. Such high-output-impedance amplifiers are best used with very-high-impedance (600 ohms) headphones. As always, before purchase, it’s best to audition a headphone amplifier with the headphones you plan to use with it.
If you choose in-ear monitors, it’s important to buy an amplifier with a high signal-to-noise ratio. The in-ear monitor’s high sensitivity converts more of the input signal to sound than do other types of personal listening devices, and that includes any noise generated by the amplifier.
Good amplifier performance isn’t strictly the province of dedicated headphone amplifiers; many portable DACs include a high-quality headphone amp. (Sometimes, such products are called “DAC/headphone amplifiers.”) Headphone jacks are appearing on an increasing number of products, such as integrated amplifiers and DACs designed for home use. However, the quality of the amplifiers driving those headphone jacks varies greatly with the product. In some components, the headphone jack is merely an afterthought, provided for convenience or to make the product more commercially appealing. In others, the headphone amplifier has received serious design attention and is built to a very high standard.
If headphone listening is important to you, search out those products that include a well-designed headphone amplifier.
The right-channel signal, carried by the ring, is connected to one side of the right-channel headphone driver’s voice coil. The left-channel signal, carried by the tip, is connected to one side of the left-channel headphone driver’s voice coil. The sleeve is connected to the other side of both the left and right drivers’ voice coils.
Some high-end headphones and headphone amplifiers offer balanced connection, sometimes carried via dual three-pin XLR connectors. For portable applications, balanced output is sometimes available on a four-pin XLR connector, or a 3.5 or 2.5mm tip-ring-ring-sleeve (TRRS) jack. The amplifier powers the voice coil in each headphone driver with two audio signals of opposite polarity; the amp is connected to both sides of the voice coil.
Balanced connection requires that the headphones be wired for balanced drive, and that the amplifier offer a balanced output. The advantages include better control over the diaphragm’s movement by the amplifier. This is because a balanced amplifier can swing twice the voltage of an unbalanced amplifier (all else being equal), and swing that voltage in half the time (a faster slew rate). Distortion is reduced, and crosstalk (i.e., unwanted signal leakage between channels) is eliminated because the left and right channels don’t share a ground, as in unbalanced connection.
A digital audio player and a pair of headphones is nothing less than a music server and stereo playback system that can fit in your hand. Together, this combination lets you enjoy your favorite music with true high-end sound quality, anywhere and anytime. In my view, that’s a compelling reason to invest in a high-quality mobile audio system. This is particularly true with today’s portable devices that are compatible with high-resolution audio, that allow you to stream music from lossless subscription services such as Tidal, and that will seamlessly connect with your home music server. A portable audio system can be as simple as a smartphone and a pair of earphones, or as complex as a dedicated player, miniature DAC, outboard headphone amplifier, and high-quality circumaural headphones.
The simplest way to get desktop audio is by adding a pair of small speakers with integral DACs and amplifiers. Connect the speakers to your computer via a USB cable, optical connection (TosLink), or SPDIF coaxial (on an RCA jack), and you’re ready to listen. (Note that powered “computer speakers” don’t contain an integral DAC; they’re fed from the computer’s analog output. As you’ll never get good sound from the computer’s analog output, computer speakers can be greatly improved by adding an external DAC.)
The next step up in quality is realized with passive desktop loudspeakers, an outboard DAC, and amplifier. The portable DAC/headphone amplifiers described above will also work in a desktop system. But rather than driving headphones, the DAC’s analog output connects to a small amplifier that will drive desktop speakers. Because nearly all desktop amplifiers have RCA input jacks, you’ll need a 3.5mm-to-RCA breakout cable.
DACs designed specifically for desktop audio have RCA outputs, obviating the need for the breakout cable. They also differ from their portable brethren in that they’re powered from an AC wall outlet (often through a “wall-wart” transformer) rather than from the computer via the USB bus. Powering a DAC from an AC wall outlet is a big sonic advantage because the power to the DAC circuitry can be cleaner and more stable. Desktop DACs may also include a volume-control knob.
High-quality DACs can also be built into integrated amplifiers, replacing two components on your desktop with one. Although the integrated amplifiers with built-in DACs designed for driving floorstanding speakers in a living room will work on a desktop, they tend to be large and expensive, and are often overkill for desktop audio. A number of companies offer small, affordable, unobtrusive integrated amps with high-quality DACs and power outputs that range from about 20 to 100Wpc. A few provide an output jack for connecting a powered subwoofer. Many are built around Class D output stages, which offer small size, low cost, light weight, and cool operation.
Desktop speakers are typically small two-ways specifically designed for nearfield listening. Smaller bookshelf speakers will also work on a desktop. Some desktop-specific speakers are angled so that the tweeters are aimed up toward your ears. Like any other speaker, passive desktop speakers need to be matched to the amplifier driving them. Small speakers tend to have low sensitivity, which requires more amplifier power. But because you’ll usually sit just a few feet away from the speakers, they don’t need to play very loudly.
If you want more bass than what’s possible from a speaker that fits on a desktop, you may choose to add a subwoofer. The guidelines for selecting and setting up a subwoofer for your living room also apply to subwoofers that augment the bass output of desktop speakers. It’s crucial that the signal driving the desktop speakers has been high-pass-filtered by a crossover, either in the integrated amplifier driving them or in the subwoofer itself.
Between your home system, portable audio player and headphones, and desktop-audio package, you’ll have great-sounding music whenever and wherever you want.
The broad term headphones is often used to describe any personal listening device that sits on the head. But the category encompasses four distinct subcategories: earbuds, earphones, in-ear monitors, and headphones. Let’s look at each of these in turn.
Earbuds vs Earphones vs Headphones, and Custom-Fit In-Ear Monitors
Earbuds are the ubiquitous little plugs supplied with most portable music players. Earbuds have poor sound quality, which is one reason headphone sales have soared alongside the boom in music-playing mobile devices. Millions of listeners have discarded their earbuds and bought better-sounding devices.As earbuds are nonstarters for high-quality audio, I’ll move on to earphones, also called in-ear monitors. There’s a single difference between earbuds and earphones: earbuds fit in the outer ear, but an earphone is inserted into the ear canal, where, ideally, it creates an airtight seal. The difference in sound quality conferred by this apparently small distinction is profound. Creating an airtight seal greatly improves sound quality, particularly in the bass. A tight seal also prevents outside noise from intruding on your music. Earphones are often supplied with a variety of interchangeable eartips, made of soft silicone or compressible foam, which allows you to choose the ones that provide the best fit with your ears. Although earphones may be superficially similar to earbuds, very-high-quality sound is possible from good earphones, never from earbuds.
Although good earphones can provide superb sound, how well those earphones fit your ear canals and thus how good they sound is somewhat hit or miss, even with a range of flexible eartips. Because earphones’ sound quality depends so much on how well they fit, some manufacturers offer custom-fit in ear monitors earphones that have been custom molded to your ear canals.
A little history of how in-ear monitors came about helps in understanding their raison d’être. Remember seeing, at amplified concerts, those small black speakers sitting at the front of the stage, angled up toward the musicians? Those were stage monitors, which allowed the musicians to hear themselves playing and singing. Stage monitors are problematic they’re bulky, tend to generate feedback, must be turned up very loud to be heard above the house mix, and require that each musician stand in front of his or her individual monitor. The professional live-sound industry solved these problems with the custom-fit in-ear monitor.
Giving each musician personal in-ear monitors made stage monitors a thing of the past. But for in-ear monitors to be effective, they must form a perfect seal between the transducer (the tiny speaker) and the ear canal. Any leakage is unacceptable, because the sound from the huge m in speakers would swamp the sound from the in-ear monitors and confuse the musician.
The solution was to create impressions of the performer’s ear canals, then mold each earphone of a pair to provide a perfect fit with the corresponding canal. High-end audio has adapted this approach to creating very-high-quality in-ear monitors for music listening at home or on the go. The in-ear monitor not only provides high isolation from noise; the airtight fit allows the tiny transducer to perform optimally. A high-quality in-ear monitor can deliver spectacular performance, including bass of a depth and power not normally associated with tiny personal listening devices. Another benefit is comfort. Because in-ear monitors precisely match the shapes of your ear canals, you can almost forget they’re there. Moreover, the slightly larger size of in-ear monitors relative to earbuds gives designers room to include more elaborate transducers. Finally, a custom-fit in-ear monitor can provide as much attenuation of background noise as a headphone that uses active noise-canceling technology (described later).
To create the molds from which in-ear monitors will be cast, an audiologist trained in creating in-ear impressions injects a semi-liquid silicone or acrylic material into your ear. During this process, you’ll be asked to hold a plug in your opened mouth, to prevent your ear canals from moving as the material solidifies. About 15 minutes later, the audiologist removes the now-solid material, which retains a perfect impression of your ear canal. Each in-ear monitor is hand-cast from such a mold to create a shape unique to your ear. Note that the entire in-ear monitor—not just the tip—conforms to your ear canal’s shape. To ensure an absolutely perfect fit, some in-ear monitors are made from a pliable material that softens slightly when warmed by your ear canal. Many makers of inear monitors bring an audiologist to consumer hi-fi shows to create your ear-canal impressions right on the spot. The finished monitors are shipped to you several weeks later.
Because the market for musicians’ in-ear monitors is relatively large and the customers are so demanding, much research has gone into designing good-sounding products. Some in-ears are extremely elaborate, and may include five drivers and two crossover points one bass driver, two midrange drivers, and two tweeters, for example. Others rely on a single full-range driver. No matter their number, the drivers are usually miniature versions of the moving-coil cones found in freestanding loudspeakers. Another type of transducer used in in-ear monitors is called the balanced-armature driver, in which the armature is a tiny rod balanced on a pivot point, like a teeter-totter, within a magnetic field. The audio signal is applied to a coil surrounding the armature, which creates a fluctuating magnetic field that magnetizes the armature. The armature’s own magnetic field reacts with the fixed magnetic field created by the magnet, causing the armature to oscillate and, with it, transmit those vibrations to a tiny diaphragm connected to one end of the armature. An upper-end in-ear monitor may contain several balanced-armature transducers, each optimized for reproducing a different range of frequencies. Balanced-armature drivers are extremely efficient, requiring very little power to produce sound.
Custom-fit in-ear monitors require a greater commitment of time and money than off-the-shelf earphones, but their combination of comfort, noise isolation, and, most important, sound quality is unequaled among in-ear listening devices.
Headphones with microphone
Many listeners prefer headphones to in-ear products, particularly those for whom portability isn’t important. As good as high-quality custom in-ear monitors can sound, it’s headphones that provide a state-of-the-art personal listening experience. In addition, some listeners find in-ear monitors uncomfortable, and prefer the feel of headphones.Which are the best headphones? There’s are as many right answers to this question as there are listeners and specific applications. Choosing the right headphones for you begins with defining how you will use the headphones, what combinations of sound and comfort you most value, and your budget. Let’s explore some of these criteria, keeping in mind that many of them overlap.
Location and intended use: Where will you use the headphones? If you plan to listen in public places such as airplanes, trains, or subways, you’ll want the headphones to provide isolation from outside noise as well as prevent pedestrians nearby from hearing your music. Earphones that provide a good fit and all custom in-ear monitors provide excellent isolation from ambient noise. (But remember that keeping outside noise from intruding on your listening experience can also prevent you from hearing sounds that warn of danger.)
Conversely, if you’re buying headphones because your home system is headphone rather than loudspeaker-based, or to enjoy music late at night without disturbing neighbors or family members, noise isolation and portability won’t be as important. For home listening, your top priorities should be sound quality and comfort.
Supra-aural headphones are thus not a good choice for subway commuters or airline passengers.) By completely enclosing the ear, circumaural headphones provide greater isolation from outside sounds, and those nearby won’t hear your music. If you plan to listen in the presence of people who may be bothered by your music, choose circumaural over supra-aural headphones.
Open-back vs. closed-back: A second major division in headphone design is open-back and closed-back headphones. In open-back ’phones the side of the diaphragm facing away from the ear is open to the outside world. Conversely, a closed-back design completely seals the driver in an enclosure. Open-back headphones provide less isolation from outside noise, and allow others nearby to hear your music.
Generally speaking, open-back headphones have a more spacious soundstage and better imaging, and closed-back headphones have deeper, more powerful bass. Completely enclosing the driver, however, causes the sound radiated from the rear of the diaphragm to be reflected from the enclosure back into the diaphragm, introducing unwanted diaphragm motions that are heard as colorations and smearing. Generally, open-back ’phones offer better sound quality than closed-back designs. Closed-back ’phones can also make your ears feel hotter with extended use; openback ’phones “breathe.”
Portability: The ultimate in portability is provided by earphones and in-ear monitors, which are often supplied with a small travel case that easily fits inside a briefcase, backpack, or purse. Some listeners object to inserting objects in their ears, and will opt for headphones even for traveling. If portability is important, look for headphones that fold up for storage in a travel case.
Comfort: The best-sounding headphones in the world won’t be much good if they’re uncomfortable to wear. Headphones vary immensely in how they feel on the head and against or around the ears, and any discomfort will only increase the longer they are worn. There’s some agreement about which model headphones are generally comfortable and which aren’t, but everyone’s head is different. If possible, you should wear the headphones under consideration for an extended period before committing to a purchase.
Generally, lighter headphones are more comfortable than heavier models. Supra-aural headphones that rest against the ear usually weigh less than circumaural ’phones that enclose the ear, but the pressure applied to your ears by supra-aural headphones can be irritating. Also consider the headband padding, which ranges from a vinyl cover over bare metal to a deep cushion. Because every listener’s head has a different shape and size, some brands or models of headphones will naturally fit you better than others.
Ruggedness: If your headphones will never leave your listening room, rugged build-quality won’t be an important consideration. For those who carry their ’phones everywhere they go, choosing headphones designed to withstand the rigors of daily travel is essential to realizing a long service life. Headphones designed for professional use are usually more durable.
Sound quality: The criteria for judging the sound quality of headphones and loudspeakers are the same: You want a smooth tonal balance with no colorations, particularly through the midrange. The bass should be extended and full, but not bloated and thick. Headphones vary considerably in their treble extension and sense of openness, and a model that lacks good treble extension will sound closedin. Transparency, resolution, and dynamic range are all important factors in how much musical satisfaction headphones deliver in the long term. Listen to familiar music through the headphones you’re considering, preferably for an extended audition.
Noise canceling: Active noise canceling is a technology that suppresses steadystate background noise, such as the din generated by airplanes, cars, and trains. Here’s how it works: A built-in microphone picks up ambient sound, amplifies that sound, inverts its polarity, and drives the headphones with the inverted signal. This inverted signal cancels, to some degree, the ambient sound leaking into your ears. Noise-canceling headphones greatly reduce fatigue on long flights. By lowering the level of background noise, this technology can also make music more intelligible and higher in resolution.
Noise-canceling headphones require a power source: either an integral rechargeable battery pack, or two or more replaceable AA or AAA batteries. They’re a little larger and more expensive than conventional headphones, but if you travel frequently, they can go a long way toward making travel more pleasant. There’s no reason why noise-canceling headphones need to compromise fidelity; I own a pair of terrific-sounding headphones (PSB M4U2) that happen to have noise-canceling technology. They greatly reduce the constant roar inside an airplane cabin; I wear them even when I’m not listening to music.
Amplifier matching: As later explained in technical detail, headphones perform best when their electrical characteristics match those of the amplifier driving them. The primary specification to look at is the headphones’ impedance. Briefly, inexpensive portable audio devices tend to work best when driving headphones of high impedance (i.e., higher than about 100 ohms). Low-impedance headphones perform better when driven by a more robust amplifier, such as those found in portable DACs and dedicated headphone amplifiers.
Balanced and unbalanced drive: Some headphones offer the option of balanced connection between the amplifier and headphones. Balanced connection carries the audio signal on three conductors rather than two. Balanced connection requires headphones that are wired to accept balanced connectors, as well as an amplifier designed for balanced operation.
Wireless headphones: Wireless headphones free the user from being tethered to the amplifier, but that convenience comes at a price: sound quality. Wireless headphones often use Bluetooth for transmitting the signal between the base station and the headphones. Bluetooth employs a coding and decoding system to reduce the bit-rate and thus the wireless signal’s bandwidth. For that reason, wireless headphones aren’t recommended except for casual use, or with the multichannel headphones described later.
Dynamic, electrostatic, and planar-magnetic headphones: An electrical signal can be converted into sound by various technologies, including the dynamic moving-coil driver, the electrostatic panel, and the planar-magnetic transducer. You’ll find a similar array of technologies among high-end headphones. Each of these technologies is simply a scaled-down version of the dynamic, electrostatic, and planar-magnetic drivers found in freestanding loudspeakers.
The vast majority of headphones have dynamic drivers. Dynamic headphones are rugged, small in size, and low in weight relative to electrostatic and planar-magnetic headphones. Although some dynamic headphones are expensive, they are generally less costly than electrostatic and planar-magnetic designs.
Electrostatic headphones are rare, but they have a cult following because of their outstanding transparency, resolution, and speed. These qualities, which they share with electrostatic freestanding loudspeakers, are conferred by the electrostatic diaphragm’s large surface area and extremely low mass. Such a lightweight diaphragm can respond very quickly to input signals, infusing the sound with a lifelike realism and a resolution of low-level detail that are generally lacking in dynamic designs. The treble of electrostatic headphones is free from distorting resonances and extends well beyond 20kHz. The downside of electrostatic headphones is the need for a high-voltage outboard power supply to charge the diaphragm, as well as an amplifier of sufficient voltage output to drive the ’phones. Because this power supply and amplifier must be plugged into an AC wall outlet, electrostatic headphones can’t be used for portable listening.
The classic electrostatic headphones the model that popularized the technology are the Stax SR Lambdas. Released in 1979 at the then whopping price of $340, the SR Lambdas brought unprecedented sound quality to headphones, and were a favorite model among audiophiles and location recording engineers.
The third primary headphone technology is the planar-magnetic driver: a very thin, lightweight diaphragm to which a conductor is bonded. The diaphragm is suspended in a magnetic field created by permanent magnets on both sides of the diaphragm. The audio signal passes through the conductor bonded to the diaphragm, creating a varying magnetic field. The variations in this magnetic field constitute an analog of the audio signal and interact with the permanent magnetic field, pushing and pulling the diaphragm to vibrate the air and thus create sound. The drivers in planar-magnetic headphones have much lower moving mass than dynamic drivers, a larger surface area, faster transient response, lower distortion, and greater frequency extension. Planar-magnetics can deliver exceptional sound quality, with resolution, dynamics, and tonal fidelity that rival those of state-of-the-art loudspeakers.
Headphone Enhancement Technologies
As great as today’s state-of-the-art headphones have become and they have become spectacularly great they are nonetheless limited in their ability to create a three-dimensional soundstage. Rather than project instrumental and vocal images in front of the listener within an apparent acoustic space, headphones tend to produce “inside-the-head” imaging. To understand why, consider that when listening to speakers or live sounds, the left ear hears some right-channel information and vice versa, in a phenomenon called inter-aural crosstalk. The left-channel sound reaching the right ear also arrives slightly later at the right ear than it does at the left ear. But when listening to headphones, the right ear hears only right-channel information, and the left ear hears only left-channel information.Without inter-aural crosstalk, the brain is unable to construct a convincing illusion of sounds existing in space in front of us, as with loudspeakers. Let’s look at several techniques and technologies that attempt to make the headphone listening experience more like that of hearing live music:
Crossfeed circuits: A simple solution to the problem of “inside-the-head” imaging is the crossfeed circuit, which has been around since the 1950s. A crossfeed circuit electronically re-creates the inter-aural crosstalk that occurs when listening to two stereo loudspeakers. The circuit mixes into the left channel some right-channel information that has been greatly attenuated and slightly delayed, and vice versa.
Crossfeed circuits are built into some headphone amplifiers. A switch allows you to turn off the effect if you find the sound unnatural. Many listeners find crossfeed circuits a mixed blessing; they reduce “inside the head” imaging, but sometimes at the expense of blurring imaging, softening the treble, reducing resolution, and muting dynamic contrasts. There’s no definitive answer as to whether crossfeed circuits are a benefit; some listeners like the effect, some don’t.
Smyth Virtual Surround (SVS): The most innovative approach to overcoming the inside-thehead imaging of headphones is undoubtedly Smyth Virtual Surround (SVS). Developed by Stephen Smyth, who invented the codecs that later became the basis for the DTS multichannel surround-sound formats, SVS is a technology that produces a listening experience through headphones that is indistinguishable from listening to a stereo or multichannel loudspeaker system in a room. SVS is implemented in a commercially available hardware platform called the Smyth Realiser A8.
Here’s how it works. You insert in your ear canal earplugs in which are embedded tiny microphones. After connecting the mikes to the Realiser A8 audio processor (a component about the half the width of the average preamplifier), you play a test signal through your home audio system’s speakers while you sit in the listening position. You then put on a pair of headphones and play a second test signal, which is again picked up by the mikes inside your ears. The whole process takes about five minutes. After that, whenever you listen to music through headphones connected to the Realiser, you hear a startlingly realistic rendering of your speakers in your listening room.
The imaging is moved outside the headphones into an immersive three-dimensional space that is indistinguishable from the sound of the speakers in the room.
If you measured different speakers in a different room, the headphone sound will mimic those speakers in that room. As its name suggests, Smyth Virtual Surround works not only for stereo systems, but also for full surround-sound loudspeaker arrays. The impression of hearing surround information located far behind you through stereo headphones is uncanny.
There’s a reason the room/speaker system and headphone measurements were made with tiny microphones inside your ear canal. To understand that reason, you need to know that the outer ear, called the pinna, plays a vital role in locating sounds. The pinna’s pattern of folds and bumps (the medical terms are folds and bumps) create a complex series of sound reflections that result in a series of minute delays in the sounds reaching your eardrum. When the direct sounds striking your eardrum are combined with the sounds delayed by the reflections introduced by the pinna, the result is comb filtering: a series of notches in the frequency response. The patterns of the frequency-response notches change with the sound’s direction, and the brain analyzes these comb-filter patterns, and the changes in them, to instantly pinpoint the sound’s direction. The head and torso are also sources of reflected sound and play roles in this mechanism. This modification of a sound by the head and ear is called the head-related transfer function (HRTF).
During its measurement process, the Smyth Realiser A8 captures your particular HRTF (everyone’s is different). On playback, the Realiser uses digital signal processing (DSP) to impose on the audio signal your HRTF, fooling your brain into perceiving the sound as existing in three-dimensional space, complete with the tonal balance and spatial cues present in the room where the measurement was taken. Note that the Realiser also measures and then re-creates the “transfer function” of your loudspeakers and room the way the speakers and acoustics modify the test signal, and thus the musical signal, on playback.
This technology opens up startling possibilities. You could measure a worldclass audio system in a state-of-the-art listening room, then replicate that listening experience through headphones in your own home. Professionals use the Realiser to replicate, on location or in another studio, a known optimal monitoring environment.
Binaural listening: Smyth Virtual Surround simulates, via digital signal processing, the head-related transfer function that can immerse us in a three-dimensional soundfield when listening to stereo headphones. But there’s another way of creating that effect: to record music with an HRTF impressed on the recorded signal. The technique, called binaural recording, involves recording with microphones inside the sculpted ears of a dummy head, or Kunstkopf. When played back through stereo headphones, a binaural recording can re-create the impression of sounds emanating from any location around you. (I heard a binaural demonstration of a dummy head getting a “haircut” in reality, a recording of someone opening and closing a pair of scissors while moving in 360° around the dummy head. The spatial presentation was so realistic that it was unnerving.) Note that, to hear the intended spatial effect of binaural recordings, you must listen to them through stereo headphones.
Binaural recordings can be made without a dummy head: with a pair of microphones placed about 7" apart and facing away from each other, replicating the distance between a human head’s ears. These quasi-binaural recordings can provide greater spatial realism through headphones, but don’t approach the full spatial realism that binaural recordings made with a dummy head are capable of.
The ultimate binaural recording would be made using your HRTF. The dummy head is just a generic average of all human heads in size and shape, and thus doesn’t perfectly deliver precise spatial cues. Everyone’s HRTF is different, particularly the comb filtering created by the pinna’s folds and bumps. I’ve read of experiments in which recording microphones were placed inside the ear canals of a human sitting in a concert hall while an orchestra played; the recording was later played back to the same person through in-ear monitors. Although playback occurred in a tiny room, the effect was a near-perfect simulation of the spatial experience of sitting in a vast concert hall. As part of this experiment, the recording was played back to listeners other than the one whose HRTF was used to create the recording and to those listeners, the sound was horribly distorted. This was because the unique HRTFs of the listeners not involved in the recording had been replaced by the HRTF of the single listener who was involved. Each person’s brain is adapted to his or her own HRTF; the combination turns what is, objectively, a gross distortion produced by comb filtering into a coherent and natural perception of sound. The uniqueness of one’s own adaptation is starkly exposed when one listens to another person’s head-related transfer function.
Software for headphone listening: Finally, outside-thehead imaging can be created in software running on a personal computer. The software simulates the head-related transfer function of speakers in a room by applying DSP filters to the music signal on playback. The software offers the simulation of a wide range of well-known high-end speakers in various rooms, allowing you to choose the one that sounds best to you. The HRTF data was created by actually measuring those speakers in different rooms, producing a remarkable facsimile of the loudspeaker experience through headphones. The software package Out of Your Head from Darin Fong Audio (reviewed this issue) allows you to audition the various speaker/room combinations before you choose which to buy and download.
Multichannel headphones: Multichannel playback through stereo headphones can be achieved with less sophisticated technology than Smyth Virtual Surround, though not nearly as convincingly. Often called “Dolby headphones,” this product category uses DSP to mimic the HRTF, enabling the headphones to position sounds anywhere around the listener. Dolby headphones typically include a Dolby Digital decoder that takes a Dolby Digital bitstream from a TV, DVD Blu-ray Disc player, or other source, and outputs a signal to the headphones that creates a facsimile of hearing surround sound through stereo ’phones. With Dolby headphones, you hear dialogue from the center, music and effects at the left and right, and surround information from all around you. Dolby headphones, which are often wireless, are good for watching movies late at night and/or if you don’t want to disturb others. They can also be useful for those who are hard of hearing; the greater clarity provided compared to listening to the tiny speakers built into a television can open up a new world. This is particularly true of dialogue, which often gets lost when reproduced by television speakers but is much more intelligible when reproduced through headphones, largely because it’s spatially separated from the soundtrack’s music and effects.
Headphone Specifications
As with loudspeakers, reading a headphone’s specification sheet won’t tell you how the headphones sound or how comfortable they are. But, as with speakers, some headphone specs are useful in selecting the right model for your application.The most common specification is frequency response, with which we’re all familiar. A headphone model’s frequency-response spec may read “20Hz–18kHz,” but this is misleading. Headphones don’t have flat measured response; instead, they’re designed to sound flat to listeners wearing them. It’s not unusual for headphones to have multiple amplitude peaks of as much as 10dB in the upper-midrange to mid-treble region (roughly 3–8kHz). Such peaks in a loudspeaker would make it unlistenable, but in headphones the peaks produce a more natural tonal balance. What’s more, frequency response measurements of headphones are inconsistent compared with speaker measurements; moving the headphone measurement microphone only 1⁄16" results in a radically different measured response. The measurement protocols for testing freestanding speakers are much more standardized and reliable.
Finally, there’s no correlation between good speaker frequency response and “good” measured headphone frequency response, because there’s no consensus about which headphone frequency response produces the most natural tonal balance. This is why you can’t tell much about how a pair of headphones will sound based on looking at their frequency-response spec, or even their frequency-response curve.
A more useful spec is of headphones’ impedance. Impedance is resistance to current flow (this is a simplified definition). The impedances of headphones span a far wider range of values than do those of freestanding speakers. A loudspeaker’s nominal impedance is typically between 4 and 8 ohms, but headphone impedances range from about 15 to 600 ohms.
The lower a speaker’s impedance, the more robust the amplifier driving it needs to be. A low-impedance speaker forces the power amplifier to deliver more current to the drivers. That’s why a low-impedance speaker is best driven with a powerful amplifier.
Similarly, low-impedance headphones sound best when powered by a high-quality amplifier.
The inexpensive amplifiers built into smartphones, tablets, low-end portable music players, and even some A/V receivers can’t deliver much output current to headphones, and are therefore not well suited to driving low-impedance models. The sound will be weak and anemic, and the bass mushy. The best amplifiers for low-impedance headphones are dedicated outboard headphone amplifiers, or the headphone amplifiers built into high-quality DACs. High-impedance headphones are a better choice with inexpensive mass-market electronics that include an integral headphone amplifier.
Knowing the headphones’ impedance is crucial to matching them with the amplifier that will drive them, specifically the amplifier’s output impedance. The rule of thumb is that the headphones’ impedance should be at least ten times the amplifier’s output impedance. If the amplifier’s output impedance is greater than one-tenth the headphones’ impedance, the headphones’ frequency response (tonal balance) will be affected. This is one reason the same headphones sound different when driven by different amplifiers. In addition, an amplifier with a high output impedance will have a low damping factor; that is, its ability to control the motion of the drivers’ diaphragms will be limited. Low damping factor is associated with lack of tautness, weight, and precision in the bass. Headphones driven by a headphone amplifier follow the same electrical laws as speakers driven by a home stereo amplifier.
If the headphone amplifier’s output impedance is less than 2 ohms, that amplifier should have no problem driving any headphones. But many headphone amplifiers have higher output impedances, especially those found in inexpensive portable devices. The designers of those budget devices add a 30-ohm resistor to the amplifier’s output to protect the op-amp driving the headphones.
Although this technique makes the amplifier more reliable and stable, it virtually ensures that the device won’t sound good with most of the headphones on the market. Many high-end headphone amplifiers have output impedances of a fraction of an ohm, and will drive any headphones with ease. Some high-quality headphones have a very low impedance; their designers assume that they will be driven by low-output-impedance amplifiers.
The last specification you should know about is sensitivity: a measure of how much sound the headphone produces for a given amount of input power. Specifically, the sensitivity spec expresses the sound-pressure level (SPL) the headphone outputs when driven by 1 milliwatt (1mW) of power. The sensitivity of headphones ranges from a low of about 70dB/mW to as high as 125dB/mW. The lower the sensitivity, the more amplifier power needed to achieve a given SPL.
Headphone Amplifiers
Every pair of headphones needs to be driven by an amplifier, whether that amplifier is an integral component in a portable music player or computer, an outboard digital-to-analog converter, the headphone output built into a home-audio component, or a dedicated headphone amplifier. The amplifier’s quality plays a large role in how the headphones will sound.The headphone amplifiers built into most portable devices are inexpensive operational amplifiers (op-amps) that can’t generate the voltage and deliver the current required by most headphones. To make matters worse, the high output impedance of op-amps introduces many problems, including an erratic frequency response that changes depending on the headphone model, a limited ability to deliver current to the headphones, and a low damping factor, all of which contribute to poor bass performance and limited dynamic range.
Higher-quality headphone amplifiers have very low output impedance (as low as 0.01 ohm) and thus a high damping factor, lower distortion, an ability to generate sufficiently high voltage and current to the headphones. The low output impedance also ensures that the amplifier won’t interact with the headphones’ impedance curve to introduce frequency-response errors. Virtually all standalone dedicated headphone amplifiers have these advantages. A few esoteric tubed designs, however, forgo low output impedance in favor of other qualities the designer believes are more important. One example is negative feedback in the amplifier: More negative feedback lowers an amplifier’s output impedance, but at the expense of sound quality, in the view of some designers. Such high-output-impedance amplifiers are best used with very-high-impedance (600 ohms) headphones. As always, before purchase, it’s best to audition a headphone amplifier with the headphones you plan to use with it.
If you choose in-ear monitors, it’s important to buy an amplifier with a high signal-to-noise ratio. The in-ear monitor’s high sensitivity converts more of the input signal to sound than do other types of personal listening devices, and that includes any noise generated by the amplifier.
Good amplifier performance isn’t strictly the province of dedicated headphone amplifiers; many portable DACs include a high-quality headphone amp. (Sometimes, such products are called “DAC/headphone amplifiers.”) Headphone jacks are appearing on an increasing number of products, such as integrated amplifiers and DACs designed for home use. However, the quality of the amplifiers driving those headphone jacks varies greatly with the product. In some components, the headphone jack is merely an afterthought, provided for convenience or to make the product more commercially appealing. In others, the headphone amplifier has received serious design attention and is built to a very high standard.
If headphone listening is important to you, search out those products that include a well-designed headphone amplifier.
Balanced and Unbalanced Drive for headphones
Most headphone jacks accept the familiar 1⁄4" phone plug or the smaller 3.5 or 2.5mm stereo plugs. Each of these plugs has three sections tip, ring, and sleeve which is why they’re also called TRS plugs. The ring and tip respectively carry the right- and left-channel audio signals, and the sleeve is the common ground for both channels.The right-channel signal, carried by the ring, is connected to one side of the right-channel headphone driver’s voice coil. The left-channel signal, carried by the tip, is connected to one side of the left-channel headphone driver’s voice coil. The sleeve is connected to the other side of both the left and right drivers’ voice coils.
Some high-end headphones and headphone amplifiers offer balanced connection, sometimes carried via dual three-pin XLR connectors. For portable applications, balanced output is sometimes available on a four-pin XLR connector, or a 3.5 or 2.5mm tip-ring-ring-sleeve (TRRS) jack. The amplifier powers the voice coil in each headphone driver with two audio signals of opposite polarity; the amp is connected to both sides of the voice coil.
Balanced connection requires that the headphones be wired for balanced drive, and that the amplifier offer a balanced output. The advantages include better control over the diaphragm’s movement by the amplifier. This is because a balanced amplifier can swing twice the voltage of an unbalanced amplifier (all else being equal), and swing that voltage in half the time (a faster slew rate). Distortion is reduced, and crosstalk (i.e., unwanted signal leakage between channels) is eliminated because the left and right channels don’t share a ground, as in unbalanced connection.
Digital Audio Players
A digital audio player and a pair of headphones is nothing less than a music server and stereo playback system that can fit in your hand. Together, this combination lets you enjoy your favorite music with true high-end sound quality, anywhere and anytime. In my view, that’s a compelling reason to invest in a high-quality mobile audio system. This is particularly true with today’s portable devices that are compatible with high-resolution audio, that allow you to stream music from lossless subscription services such as Tidal, and that will seamlessly connect with your home music server. A portable audio system can be as simple as a smartphone and a pair of earphones, or as complex as a dedicated player, miniature DAC, outboard headphone amplifier, and high-quality circumaural headphones.Desktop Audio
The computer-audio revolution has provided a simple path for anyone to enjoy music on a desktop while working. You likely already use a computer running iTunes that is connected to the Internet’s streaming music services, so why not add a small amplifier and a pair of desktop speakers? In fact, I’m listening to my desktop-audio system as I write this.The simplest way to get desktop audio is by adding a pair of small speakers with integral DACs and amplifiers. Connect the speakers to your computer via a USB cable, optical connection (TosLink), or SPDIF coaxial (on an RCA jack), and you’re ready to listen. (Note that powered “computer speakers” don’t contain an integral DAC; they’re fed from the computer’s analog output. As you’ll never get good sound from the computer’s analog output, computer speakers can be greatly improved by adding an external DAC.)
The next step up in quality is realized with passive desktop loudspeakers, an outboard DAC, and amplifier. The portable DAC/headphone amplifiers described above will also work in a desktop system. But rather than driving headphones, the DAC’s analog output connects to a small amplifier that will drive desktop speakers. Because nearly all desktop amplifiers have RCA input jacks, you’ll need a 3.5mm-to-RCA breakout cable.
DACs designed specifically for desktop audio have RCA outputs, obviating the need for the breakout cable. They also differ from their portable brethren in that they’re powered from an AC wall outlet (often through a “wall-wart” transformer) rather than from the computer via the USB bus. Powering a DAC from an AC wall outlet is a big sonic advantage because the power to the DAC circuitry can be cleaner and more stable. Desktop DACs may also include a volume-control knob.
High-quality DACs can also be built into integrated amplifiers, replacing two components on your desktop with one. Although the integrated amplifiers with built-in DACs designed for driving floorstanding speakers in a living room will work on a desktop, they tend to be large and expensive, and are often overkill for desktop audio. A number of companies offer small, affordable, unobtrusive integrated amps with high-quality DACs and power outputs that range from about 20 to 100Wpc. A few provide an output jack for connecting a powered subwoofer. Many are built around Class D output stages, which offer small size, low cost, light weight, and cool operation.
Desktop speakers are typically small two-ways specifically designed for nearfield listening. Smaller bookshelf speakers will also work on a desktop. Some desktop-specific speakers are angled so that the tweeters are aimed up toward your ears. Like any other speaker, passive desktop speakers need to be matched to the amplifier driving them. Small speakers tend to have low sensitivity, which requires more amplifier power. But because you’ll usually sit just a few feet away from the speakers, they don’t need to play very loudly.
If you want more bass than what’s possible from a speaker that fits on a desktop, you may choose to add a subwoofer. The guidelines for selecting and setting up a subwoofer for your living room also apply to subwoofers that augment the bass output of desktop speakers. It’s crucial that the signal driving the desktop speakers has been high-pass-filtered by a crossover, either in the integrated amplifier driving them or in the subwoofer itself.
Between your home system, portable audio player and headphones, and desktop-audio package, you’ll have great-sounding music whenever and wherever you want.
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