A Quick Guide to Headphone Accessories.

• Adapters and Converters
• Amplifiers For Headphones
  • Audiophile (Home Stereo) Amplifiers
  • Distribution and Mixer Amplifiers
• Audio Cables
• Carrying Cases
• Cord Managers
• Display Stands
• Equalizers
• Limiters
• RF/EMI Noise Suppressors
• Spatial Simulators (3D Sound)
  • Crossfeed Filters and Spatial Expanders
  • Virtualizers (3D Sound Processors)
• Vibration Transducers For Bass Enhancement
• Volume Controls and Remote Mixer Stations

Whether the “cans” are inexpensive lightweights or extravagant electrostatics, anyone who listens to headphones regularly will sooner or later consider buying accessories. In addition to the standard assortment of adapters and extension cables, today’s well-equipped headphone connoisseur may select from such technological wonders as crossfeed processors and surround-sound decoders that elevate listening to an art. Headphone accessories can cost several times the price of the headphones themselves. Along with descriptions, this guide includes tips on how to buy and use headphone accessories.

Adapters and Converters

There are two basic types of adapter functions: converting between 1/4″ and 1/8″ (3.5mm) plugs and converting between stereo and monaural. The headphone jack that comes on most professional and home audio equipment is meant to mate with a 1/4″ stereo plug. Portable stereos use the 1/8″ mini-stereo plug. The 1/4″ plug has larger contact surface area and forms a superior connection with the jack, but the mini-stereo plug is more compact. Convenience usually wins out in consumer markets, so the 1/8″ plug has become the standard (with an accessory 1/4″ adapter for use with the larger jacks).

Size adapters are available at audio supply stores that convert from 1/4″ to 1/8″ or vice-versa (both stereo-to-stereo and monaural-to-monaural). The “silver-colored” metal on most adapters is nickel. Gold-plated adapters will not oxidize and will maintain a better connection. Gold-plating is not mandatory, but it does make for good-looking parts. In equipment that generates a great deal of heat which accelerates oxidation, gold-plated connectors may be a better choice. Here are the Radio Shack part numbers for their adapters:

Stereo-To-Stereo (jack-plug):

1/4″ S to 1/8″ S: 274-336
1/4″ S to 1/8″ S: 274-875 (GOLD)
1/8″ S to 1/4″ S: 274-367
1/8″ S to 1/4″ S: 274-876 (GOLD)

Monaural-To-Monaural (jack-plug):

1/4″ M to 1/8″ M: 274-047
1/8″ M to 1/4″ M: 274-325

Monaural headphones are used mostly with communications equipment (such as speech recognition systems, shortwave radios and scanners) and often come with attached microphones or voice tubes. They are commonly referred to as “headsets,” although that term could also apply to stereophones. Mono headphones may actually cost more than their stereo counterparts due to their limited demand. If an integrated microphone is not needed, stereo headphones will work with a stereo-to-mono adapter Some mfrs install this adapter as a “universal” jack. Audio supply stores also sell them (and mono-to-stereo adapters). They come in all combinations of 1/8″ and 1/4″ inputs/outputs. Here are the Radio Shack part numbers for their adapters:

Stereo-To-Monaural (jack-plug):

1/4″ S to 1/8″ M: 274-361
1/8″ S to 1/4″ M: 274-348
1/4″ S to 1/4″ M: 274-360
1/8″ S to 1/8″ M: 274-368
1/8″ S to 1/8″ M: 274-882 (GOLD)

Monaural-To-Stereo (jack-plug):

1/4″ M to 1/8″ S: 274-361
1/8″ M to 1/4″ S: 274-363
1/4″ M to 1/4″ S: 274-1520
1/8″ M to 1/8″ S: 274-374

Amplifiers For Headphones

Headphone amplifiers were once an accessory exclusively for the recording studios, and then they were fairly utilitarian as is generally the case with professional equipment. The growth of the headphone market for audiophiles has spawned a new product category of headphone amplifiers for the consumer. Most of these are solid-state, but a few tube units have made it to market that even run off batteries (though they are bulky). Of course, these products are marketed with the same audiophile buzzwords as their larger cousins, the power amplifiers.

Meanwhile the professional market is recognizing the importance of headphones for monitoring during live events and in the studio. The latest generation of headphone distribution amplifiers provide more features for custom outputs and convenience. Musicians can determine not only the volume levels, but also adjust the mix to suit their tastes.

Audiophile (Home Audio) Amplifiers

The headphone output of home audio components is perfectly adequate to drive modern headphones to ear-splitting volume, but since headphones are often regarded as toys, little care may be gone into designing the circuitry. A headphone output might be nothing more than a couple of series resistors off the power amplifier’s speaker terminals. Since power amps are supposed to drive loudspeakers, this configuration has the potential disadvantage of higher noise and output distortion than small signal amps, in addition to high output impedance, which could affect the sound of headphones.

Other headphone outputs have their own driver circuits, but the question remains as to their quality. A highly regarded preamplifier may, nevertheless, be using a poorly implemented headphone driver that either audibly distorts the signal or has inadequate drive capability. Portable stereos are notorious for their weak headphone outputs, which are deliberately underpowered to conserve battery life. Many listeners are surprised at the improvement in sound when they plug their portable phones into a home stereo.

Headphone amplifiers can be purchased as standalone units or as part of another audio component such as a preamplifier or integrated amplifier. Some models have built-in spatial processors to improve the sound of headphones (see the section on Spatial Processors for more info). The best way to evaluate a headphone amplifier to try it out with the intended headphones. Because modern headphones are much easier to drive than loudspeakers, most products will be able to meet minimum acceptable performance parameters. The following are guidelines for interpreting headphone amplifier specifications:

Power output: Modern dynamic headphones can reach maximum volume with only a few milliwatts of power. Therefore, unless the headphones are very inefficient, headphone amplifiers that are rated at over 500 milliwatts per channel are probably, extravagant – not to mention dangerous. On average, headphones need only 10-20 mW. (One exception is high impedance headphones, occasionally found in pro-applications, which can guzzle whole Watts of power.) However, make sure that the amplifier can deliver adequate output for the impedance of the intended headphones. While power amplifiers are typically rated for an 8-ohm load, headphone amplifiers do not have a standard. The easiest way to confirm compatibility is to plug in the headphones and listen. Whatever the setting of the volume control for comfortable listening, there should be plenty of gain to spare.

Distortion: Total harmonic distortion figures (THD) should be much less than 1% at rated output. Any other distortion figures quoted, such as intermodulation distortion (IM), should also be less than 1% at rated output. Again, headphones are much easier to drive than loudspeakers, and today’s power amplifiers have little trouble making these numbers.

S/N ratio: The Signal-to-Noise Ratio indicates how quiet the amplifier is when it is idling. Music must be above the noise floor to be audible. This S/N ratio is very important for headphone amplifiers because the close coupling of headphones to ears emphasizes equipment noise. It ratio is measured in decibels (dB) and comes in two versions: A-weighted and unweighted. A-weighted is better than unweighted, and both figures should be greater than 80 dB. Note: sometimes the S/N ratio is listed as a negative number: eg, -80 dB. Ignore the negative sign: -86 dB is quieter than -80 dB.

Damping factor: This spec is calculated by dividing the amp’s rated load by its output impedance and is more suited to amps driving loudspeakers than headphones. For loudspeakers, which can present a complex load, damping factor is an indication of an amplifier’s ability to drive the load. Headphones are essentially resistive loads that vary little over frequency. Larger damping factor figures are better. One caveat is that damping factors are rated for a specific load, say 8 ohms. If the headphones have a larger impedance, then the damping factor will be proportionally reduced. A good starting point is maybe 50. See also Output Impedance below.

Output impedance: This figure has two possible meanings: the impedance of headphones that the amp is rated to drive (the rated load) or the actual output impedance of the amplifier itself. Both are measured in ohms. The rated load indicates the range of impedances that the amplifier can drive to full power (eg, 100mW at 30 ohms). Even if the amplifier is used with headphones that have a higher impedance, the reduction in power may be insignificant, as headphones tend to very efficient.

Instead of (or in addition to) the rated load, a specification may give the actual output impedance of the amplifier. This number is for mathethematically-inclined audiophiles who like to calculate their own hardware compatibility figures such as damping factor. For solid state amplifiers, a lower number is better – typically less than 1 ohm. The IEC 61938 international standard specifies that headphone manufacturers should assume that the amplifier has an output impedance of 120 ohms (the value of a typical output current-limiting resistor which protects the amp against shorts). Tube amplifiers may recommend a range of headphone impedances for optimal power transfer – usually if they use output transformers.

Distribution and Mixer Amplifiers

A headphone distribution amplifier takes a single headphone input and splits it to drive multiple headphones. It is standard equipment in recording studios, and at its most basic, could be any power amplifier with a fan of resistors at the speaker terminals – one resistor for each headphone – to limit the current to the headphones and to help maintain a reasonable load across the amplifier. These are called “mass-feed systems.” The disadvantages of mass-feed systems are that the amp now has a relatively high output impedance which might affect the sound of headphones and that the amplifier might sound noisier over headphones because it was not designed to output at lower voltages. If the headphones do not have the same impedance and efficiency, controlling overall volume might present difficulties. Some mass-feed systems feature passive attenuators at the headphone outputs.

A distributed-feed amplifier will have separate gain blocks and level controls for each output. For flexiblity, each output should have at least two selectable inputs – one for the main mix and another for a custom mix. Headphone mixer amplifiers have multiple inputs for a main stereo mix, various subgroup mixes (such as drums, background vocals or keyboards) and special effects loops plus an array of level controls so that perfomers can set their own mixes. The general consensus among audio engineers is that headphone mixers cannot have too many inputs (or outputs for that matter). See Volume Controls and Remote Mixer Stations for more mix options.

Although distribution amplifiers tend to be viewed as utilitarian, the specifications for audiophile amplifiers also apply to the outputs of a distribution amp. Since studio headphones may be less efficient (or tend to run louder than in consumer systems), the power specifications are particularly important. Overload indicators on each headphone output are a very useful feature for determining whether there is adequate drive capability. If not, adding an external amp can boost the output (taking care to avoid frying the cans and the musician’s ears). For more information about distribution amplifiers, see The Art of Monitoring and Mixing With Headphones and Designing a Headphone Distribution Amplifier.

Audio Cables

There are more articles on the sound of cables than there are stars in the sky (ok, so maybe I exaggerated a bit). Almost as numerous are the products with astronomical prices. Wire configurations may indeed affect sound (due, at least partially, to capacitance, inductance and resistance factors), but cables woven of exotic metals that cost hundreds of dollars per inch are too rich for this author’s ears. The recommendation is to purchase a good quality, inexpensive cable and later, if one feels compelled, spend one’s life savings on the Holy Grail of cables only after very, very careful auditioning. Among the manufacturers of high-end headphone cables are Clou Cable, DiMarzio, Inc. and Ted’s Excellent Cable. Clou Cable also makes a replacement headphone cord for the Sennheiser HD600 headphones.

A good quality cable is not necessarily the thickest cable, nor is necessarily it one terminated with the pretty, gold-plated connectors (although gold-plating is less prone to oxidation). In general, the less expensive cables will have connectors that are “molded” into the cable jacket, but that feature alone is not an indication of low quality. BEWARE any cable that looks or feels fragile. For example, if a jack on the cable is loose enough for a plug to slide in easily, it may also uncouple easily. Do the connectors have a flexible strain relief where they join with the cable to prevent damage to the wire? Is the cable so thin that it might break with after a few tugs?

Audio interconnect cables (as opposed to speaker cables) usually do not come packaged with specifications – that is because they measure pretty much the same. Wire is wire. Evaluating cable characteristics depends on the purpose of the cable:

Headphone extension cord: An extension cord simply extends the reach of the headphone plug to the amplifier. Configuration – 1/4″ stereo plug (or 1/8″ mini-stereo plug) to 1/4″ stereo jack (or 1/8″ mini-stereo jack). The cable can be a mix of 1/8″ and 1/4″ connectors – in which case it is both an extension and an adapter. A thick cable is not a good idea, if the listener plans to move around while wearing the headphones. Extension cables are also available coiled. Coiled extensions can be neater than straight cable, but they also tangle. Here are the Radio Shack part numbers for their extensions:

Standard Extensions:

1/8″ 6-ft extension (GOLD): 42-2482
1/8″ 20-ft extension: 42-2462
1/4″ 20-ft coiled extension: 42-2442
1/4″ 24-ft coiled extension: 42-2443

Converter Extensions (plug-jack):

1/8″ to 1/4″ 20-ft extension: 42-2464
1/4″ to 1/8″ 20-ft extension: 42-2460

Portable stereo to headphone amplifier: Configuration – mini-stereo (1/8″) plug to 2 RCA plugs. A high-end version of this cable was once custom made only, but now is in such demand that Monster Cable, the audio cable conglomerate, has created an entire gold-plated line around it (called “Interlink Portable”). Other high-end cable manufacturers maintain this configuration in inventory. Of course, Radio Shack has had a version of this cable on the racks for years (yes, Monster’s does look snazzier). Here are the Radio Shack part numbers for their cables:

1/8″ S plug to RCA: 42-2475
1/8″ S plug to RCA: 42-2613 (GOLD)

Preamplifier outputs to drive headphones: Configuration – 2 RCA plugs to stereo headphone jack (1/4″ or 1/8″). Rarely used and not recommended unless the preamplifier output specifications are known to be compatible with driving headphones. Preamp outputs may be both low current and high impedance, which would limit their ability to drive phones cleanly. The strongest candidates in this regard are preamps with buffered outputs. Here are the Radio Shack part numbers for their cables:

RCA to 1/4″ S Jack: 42-2471

Headphone amplifier to drive power amplifier: Headphone amplifiers make great line-drive preamps. If the headphone amplifier does not have RCA output jacks, then the power amplifier must be driven from the headphone output. Configuration – 1/4″ stereo plug (or 1/8″ mini-stereo) to RCA plugs. Headphone amplifiers that have volume controls can make great preamplifiers. See the HeadWize FAQs for more information. Here are the Radio Shack part numbers for their cables:

1/8″ S plug to RCA: 42-2475
1/8″ S plug to RCA: 42-2613 (GOLD)
1/4″ S plug to RCA: 42-2474

Sharing two headphones from one headphone jack: Configuration – 1/4″ stereo plug (or 1/8″ mini-stereo) to dual 1/4″ stereo jacks (or 1/8″ stereo jacks). This cable is generically called a Y-cable. A Y-connection will reduce the load impedance seen by the amplifier, but is unlikely to damage the amplifier. If the signal sounds distorted, then load impedance may be too low, and the Y-connector should not be used. Portable stereos may not have enough power to drive two sets of headphones to adequate volume levels. See the HeadWize FAQs for more information on driving multiple headphones from a single output. Here are the Radio Shack part numbers for their cables:

1/8″ S plug to (2) 1/8″ S jacks: 42-2463
1/4″ S plug to (2) 1/4″ S jacks: 42-2448

For the metric-minded, a 1/8″ plug or jack is 3.5mm. Interestingly, the 1/4″ stuff is not sold under a metric measurement – at least, not in the United States.

Carrying Cases

Carrying cases for headphone systems range from a tiny clasp attached to slip-ons for portable stereos (where the headphones clip on the outside) to padded day-bags that lovingly cuddle headphones, headphone amplifiers and other “can’t-leave-home-without-em” accessories, as well as a month’s worth of music CDs. The lightweight cases often have straps for over the shoulder wear and a belt loop for waist-hugging accessibility, but may trade durability for convenience. Where the headphones clip on the outside, these cases do nothing to protect headphones. The heavy-duty cases offer a great deal of protection (especially those with foam inserts to cradle the headphones), but are bulky.

Another source of headphone bags is aviation supply stores. Pilots wear headphones all the time, and have their own designer bags (with foam inserts) in which to transport one or two set of cans. Search the internet for the term “headset bag”. For those who are not too particular, video camera bags and computer laptop bags can also become cozy repositories for the headphone entourage. CaseLogic, HeadRoom, Sony and AVCOMM are examples of companies that make cases for headphone systems.

Cord Managers

Long headphone cords are advantageous in home or office audio systems where the headphone jack can be several feet away, but can become a tangled mess for listeners who are travelling. There are headphones with short cords, but they are not popular yet in the US and are less convenient for home and office systems. Long cords are often shortened by coiling and tying the extra length into a bundle. However, a coiled up cord tends to look messy. Belkin’s Cable Ties (part no. F8B024) are lightweight, reusable strips of Veclro for this purpose and come in a variety of colors. The list price is $8 US.

Another type of cord manager is a bobbin for holding the extra length. The black plastic cord manager from Radio Shack (part no. 43-2011) works best with thin headphone cords. It measures about 4″ x 1.25″, has a belt clip and retails for $2.49 US. The Cable Turtle from Clever Line looks like a rubber yo-yo. After the Cable Turtle is opened, the cord slack is wound around the core and then secured by closing the Cable Turtle. It is available several colors and in two sizes: small (2.5″) and large (3.5″). The small ones cost about $8 US.

Display Stands

The well-heeled audiophile (and those who strive to be well-heeled) who likes to see everything in its proper place will blanch at the idea of carelessly tossing headphones on top of an audio cabinet or other piece of furniture. And Heaven forbid that they should end up on the floor! Very expensive headphones come with a storage box, but it can be inconvenient to put headphones away in a box after every use. And owners of more affordable models are equally concerned with good display and storage options.

The easiest solution is hang headphones from a hook installed on the wall next to the audio cabinet or perhaps in the cabinet itself. Hardware and home supply stores sell inexpensive and attractive plastic hooks with an adhesive backing for quick installation. Custom display stands are available, though somewhat difficult to find. For example, Stax makes a headphone stand complete with a dust cover. Houseware and antique shops may sell stands that can serve as headphone displays. Styrofoam and glass dummy heads for displaying hats and wigs are equally good for headphones (search the net for “wig stand” or “display head”). If a dummy head is to serve as a display stand, take care to select a head size that will not force the headband out of shape.

Equalizers

How a person hears is influenced by the shape of the listener’s head and ears, which modifies the amplitude and phase characteristics of sounds before they get to the eardrums. Headphones direct sounds straight to the eardrums without these acoustic modifications and may impart an artificial quality to the reproduction. The graph in figure 1 is an example of what the left and right ears actually hear when listening to a sound source positioned slightly left of center. These curves are called head-related transfer functions (HRTFs). Every listener has a unique set of HRTFs for 3D hearing. For more information on HRTFs, see the 3-D Audio Primer.

The frequency responses of headphones are rarely flat, but what matters is how the flatness is measured. Headphones that are diffuse-field equalized have a frequency response similar to the HRTF curves in figure 1, but are supposed to sound flat because they mimic the frequency contouring of the human head. However, diffuse-field EQ is based on an average response that may not be suitable for every listener. With non-equalized headphones, a little equalization can go a long way in mitigating the artificial quality of the sound. But how should the equalizer be set?

In Taking Audio in Another Direction, John Sunier and Ron Cole suggest following a “biophonic” curve for setting equalizers to correct for ear canal resonance and other differences in the spectrum between speaker listening and headphone listening:

This biophonic curve is not too different from the HRTF curve in figure 1. Experimentation with these EQ settings is highly recommended.

Also, when listening at low (safe) volumes, the sound may seem to lack a low and high end. As shown in the Fletcher-Munson curves above, loudness perception is both frequency and volume dependent. Although the standard equalizer cannot dynamically compensate for loudness as the Fletcher-Munson curves would suggest, a small treble and bass boost can make headphone sound at lower levels more satisfying. (See Designing A Pocket Equalizer for Headphone Listening and Preventing Hearing Damage When Listening With Headphones for more information about loudness compensation.)

As for the equalizer itself, there are two types: graphic and parametric. A graphic equalizer divides the audio band into a series of fixed center frequencies which are individually adjustable for boost or cut. The octave designation (such as 1/3 octave or 2/3 octave) specifies the spacing between frequencies. The smaller the octave number, the more center frequencies that can be adjusted. If the EQ has a bandwidth control, the effect of each frequency control can be broadened or narrowed in its impact on nearby frequencies.

For headphone listening, a 3-band per channel unit could be sufficient, if it has the center frequencies desired. If possible, try out a 1/3 octave EQ (over 30 frequency controls per channel) with headphones to determine which frequencies require adjustment for the most accurate sound. Then, the budget conscious can buy a smaller EQ with just the desired center frequencies.

A parametric equalizer will have fewer bands than its graphic cousins, but each band is completely customizable – from the choice of center frequency to the shape of the boost or cut. Parametric equalizers tend to be expensive and are a good buy only if the adjustments on a cheaper graphic EQ are not satisfactory. A few models have both graphic and parametric bands. As for fancy features that help tune room acoustics, if the EQ is primarily for headphone listening, only the sound quality matters. For casual listening, Koss Corporation sells a basic, pocket-sized, 3-band equalizer with marginal performance. HeadWize has project plans for several portable equalizers.

Evaluating an equalizer for headphones can be difficult, because headphones are an artificial listening environment. If possible, purchase an audio test CD with test tones that are similar to the band frequencies on the equalizer. If a headphone acoustic simulator (such as Dolby Headphone) is part of the listening system, evalute the equalizer with and without the simulator engaged. See Judging Headphones For Accuracy for more information on using test discs to evaluate headphones. Also, have on hand at least one music CD with strong transients and dynamics to check the equalizer for distortion and overload. The equalizer should have a bypass switch. If the EQ does not have a headphone output, it will have to be installed in the tape monitor loop of the preamp or integrated amp. Plug in the headphones and set the EQ controls for a flat response. Compare the flat EQ settings and the bypassed sound – there should be no difference. Listen to music with lots of transients and dynamics. If there is any distortion or overload, make sure it is not coming from the preamp or CD player.

Using the pink noise tracks on the test CD, try and balance the headphones so that all frequencies are at the same loudness at a comfortable listening volume (remember that loudness is a function of frequency and volume). Also try the biophonic settings described above to see if there is an improvement in the naturalness of the headphone sound. If there is no improvement after trying several equalizer models and settings, then perhaps an equalizer will not benefit this combination of listener and headphones.

Audio retailers have a very small selection of standalone equalizers these days. Instead, equalization functions are integrated into digital signal processors for use in home theater systems. Other sources for equalizers include pro audio dealers and electronics outlets. Also, consider car audio EQs. These can often be converted to home use, but may require extensive auditioning to find a product that sounds good.

Pricing on equalizers is across the board – from about $30 to several thousand dollars. Because equalizers are more complex than preamplifiers, quality in sound and construction should be paramount in the evaluation process. For more information on selecting equalizers, see Signal Processing Fundamentals and Designing a 3-Band Equalizer for Headphone Listening for more information about equalizers and how they work. Those who have a PC with a good soundcard might try the Graphic Equalizer Pro shareware by Anwida Soft. This software can process audio signals in real-time.

Limiters

Sold as pro audio accessories, limiters can enforce safe headphone listening volumes by preventing the audio signal from exceeding a preset threshold. Unlike compressors, which reduce volume but may not actually prevent unsafe levels, limiters function like a brick wall and will clip any signal that exceeds the threshold. The effect can range from subtle to harsh and distorted and, depending on the circuit, may exhibit “breathing” or “pumping.” Multiband limiters divide an audio signal into bands to isolate the clipping and avoid limiting the entire signal. Thus, if the drums are constantly being overdriven, a multiband limiter will leave the vocals clean. Also, compressing a signal before sending it to a limiter helps achieve a clean and natural sound. Personal monitors for musicians sold by several manufacturers such as Garwood Communications, Sennheiser and Shure, have limiter circuitry.

Limiters can be custom integrated into headphones, into personal monitors or added as outboard devices. Headphones with built-in limiters may not provide any means for user adjustment of the threshold, so they must be carefully evaluated for the degree of hearing protection offered as well as sound quality. If possible, request a pair of phones be individually adjusted to suit the listener. Canford Audio sells Beyerdynamic and Sennheiser phones that have been modified with internal limiters designed not to exceed 93dB (to ensure that the average exposure level is below 85dB). They also sell inline limiters that attach to the headphone cord. Both are passive circuits and can be customized at additional cost.

A personal monitor (used by musicians onstage) is essentially a an in-ear headphone and a portable headphone amplifier with a built-in limiter. Beyerdynamic, Future Sonics, Garwood Communications, Sennheiser and Shure are a few of the companies that make wired and wireless personal monitors. Separate active limiters, such as those from Rane Corporation and Aphex Systems, have the most features and adjustability. Also look for combo compressor/limiters. For more information about limiters, see Signal Processing Fundamentals and Designing a Limiter for Headphone Amplifiers.

RF/EMI Noise Suppressors

Radio frequency (RF) noise and electromagnetic interference (EMI) can come from radio stations, household appliances, computers, etc. Occasionally a problem in home audio systems, this type of noise is far more noticeable in portable equipment, especially when there is a separate headphone amplifier. Portable audio sources such as CD or MD players are prone to RF interference because they must remain lightweight and avoid metal shielding. Further, long interconnects and headphone cords can act as antennas that channel RF signals into headphone amplifiers.

The easiest way to block RF noise is to add a clip-on ferrite noise suppressor to audio cables. They function as low pass filters to screen out very high frequencies (well above the audio range). A ferrite clip-on consists of a small donut or tube of ferrite material that has been cut in half and both sections dropped into a hinged plastic housing. To use it, the clip-on is placed around the cable and snapped shut.

Ferrite cores should be located on the end of a cable as close as possible to the input and/or output of the headphone amplifier. In cases of severe interference, additional clip-ons will increase the filtering action. If the filtering action is too strong, the clip-ons could affect sound quality, but can be removed when not necessary.

They are sized for different diameter cables and have an impedance rating measured at 1MHz. For audio applications, try ferrite cores of 80 ohms and higher. Get ferrite clip-ons in a size that is as close as possible to the diameter of the cable. If ferrite clip-on fits loosely on the cable, loop the cable one or more times around one of the ferrite sections to anchor it before closing the case.

Ferrite clip-ons are available from electronics outlets like Digikey (stock nos. 240-2070-ND and 240-2066-ND), Hosfelt Electronics (stock no. 80-287) and Mouser Electronics (stock nos. 623-0444164281 and 623-0444167281), from computer suppliers, and from audio accessories retailers. They can cost less than $1.00 US each. Audiophile branded versions cost much more. For example, the Audioquest RF Stoppers start at $10 each. AVLink Inc. makes audio interconnects with integrated ferrite filters.

Spatial Simulators (3D Sound)

The topic of spatial simulators (also called acoustic simulators) is so complex that this Quick Guide will only be able to give a brief overview. Headphones have a distorted “inside-the-head,” “between-the-ears” perspective because the acoustically isolated environment of the earpieces lacks the locational cues of normal hearing. Spatial simulators restore realism to headphone sound by adding crosstalk, delay, equalization, reverberation and other spatial elements. The simplest of the simulators can help mitigate headphone listening fatigue and provide a more natural listening environment, but they do not externalize the soundfield.

The most advanced simulators (called virtualizers) can create realistic 3D soundfields in headphones, and may even offer custom acoustic environments (e.g., simulating a large reverberant concert hall or a small nightclub). Stereo virtualizers project 2 virtual loudspeaker-like sound sources inside headphones. Surround sound virtualizers simulate multi-channel virtual speakers inside headphones. When evaluating spatial simulators, be careful to distinguish between those for headphones and those for loudspeakers. Loudspeaker virtualizers project a 3D soundfield from stereo loudspeakers, and may not be effective in headphones.

There are two broad categories of image processing for headphones: Audio Enhancement (AE) and Head-Related Transfer Function (HRTF) types. Audio Enhancement simulators employ a variety of psychoacoustic effects processing, such as ambience recovery, to create a 3D soundfield. They are available in analog and digital versions. Less sophisticated than the HRTF types, audio enhancement simulators are more affordable and effective for most people without the need for careful fine-tuning.

HRTF simulators (also called positional audio) attempt to recreate locational cues in headphone sound. These simulators are the result of many experiments using dummy heads with microphones installed in the ear canals to analyze how sound is affected by the shape of a listener’s head. HRTF simulators can be implemented with analog filters or digital processing. The best of them can generate very realistic soundfields in headphones with strong localization.

When buying a virtualizer, a careful audition is a must with stereo and surround sources. Virtualizers with an array of customization options can be difficult to set up. If the virtualizer has a built-in headphone amplifier, then any extraneous noise or sonic coloration introduced by the amplifier (with the virtualizer bypassed) must be judged carefully as to acceptability. For more information about headphone-based surround sound technologies, see Technologies for Presentation of Surround Sound in Headphones and The Art of Monitoring and Mixing with Headphones.

Audio Enhancement Simulators

Both audio enhancement and HRTF simulators for headphones have been on the market for years. The simplest of the audio enhancement effects is ambience recovery (also called stereo expanders. Ambience recovery processing produces a spacious sound in headphones by emphasizing the ambient information in the audio signal. Ambience recovery (AR) circuits can widen loudspeaker imaging, because the processing cancels much of the crosstalk between speakers. Basic AR circuits are small and inexpensive. For example, Koss Corporation has a line of professional and home-stereo headphones with a “Phase” accessory built into the headphone cord. The Phase circuit produces a modest increase in spaciousness in the headphones. SPL Vitalizer‘s Psychoacoustic Enhancer and QSound‘s Ultra-Q are two examples of more sophisticated AR simulators.

Audio Enhancement-type virtualizers includes additional equalization and reverberation processing based on psychoacoustic research to improve the sense of realism in headphones. Spatializer Labs and SRS Labs sell plugins for popular PC music players as well as hardware simulators that generate soundfields convincing enough to be called virtualizers. The Environmental Audio Extensions (EAX) 3D headphone technology in SoundBlaster Live soundcards from Creative Labs are AE-type virtualizers. When auditioning AE simulators, listen for any changes in balance, tonal quality, added noise, pumping and breathing effects and blurring of localization cues. The HeadWize FAQs page has several schematics of spatial expanders for use with headphones and loudspeakers.

HRTF Simulators

HRTF simulators mimic the effect of the listener’s head on sound before it reaches the ear drums. The shape of the listener’s head and the shape of the earlobes provide locational cues by subtly altering the frequency response depending on the position of the sound source and adding multiple reflections and delays. The simplest HRTF simulators are called crossfeed filters and introduce crosstalk into the stereo signal to compensate for the acoustic isolation of headphones. Because high frequencies in crosstalk are absorbed and deflected by facial features, and because crosstalk arrives at the ear a few milliseconds after the direct signal reaches the other ear, the crossfeed signal is usually bandwidth-limited and time-delayed.

Crossfeed filters lend a natural quality to headphone sound by seeming to position the soundfield slightly in front of the listener and by centering low frequencies. Currently, two companies manufacture commercial headphone crossfeed filters. HeadRoom Corporation‘s amplifiers have an active-filter crossfeed. Sonic Frontiers (and the recently defunct Audio Alchemy) incorporate variations of HeadRoom’s circuit into their preamplifiers. The Corda amplifiers have an adjustable passive-filter crossfeed. The HeadWize Projects Library has schematics for several crossfeed circuits and headphone amplifiers with crossfeed. When auditioning a crossfeed filter, listen for any phasey effects and the softening of high frequencies.

Like AE-type virtualizers, HRTF virtualizers also use equalization and reverberation processing to improve the realism of basic crossfeed. Whereas AE-type virtualizers rely on psychoacoustic effects, HRTF virtualizers try to mimic the mechanisms of human hearing by intricately reshaping the frequency response and other aspects of the audio signal. The sound of HRTF virtualizers is characterized by strong localization and a very natural 3D presentation. However, they may be more complex to use, more expensive and may not work well for all listeners (although the latest HRTF virtualizers claim universal effectiveness).

Companies that make HRTF simulator plugins for popular PC music players include Lake Technologies (the inventor of Dolby Headphone) and Wave Arts, Inc.. Lake and Wave Arts also sell powerful acoustic modeling applications for audio workstations. MGI Software makes the SoftDVDMax DVD player application for PCs with a Dolby Headphone decoder.

AKG Acoustics and Sennheiser are two companies that make hardware HRTF simulators. The hardware simulators may have some form of surround sound decoder built in, and will accept stereo and multichannel audio sources. Both AKG and Sennheiser make wireless headphones with HRTF simulators and Dolby decoding. PC soundcards now incorporate a variety of 3D headphone technologies; some of these are HRTF-type algorithms, such as Aureal‘s A3D and Sensaura‘s Virtual Ear. They may also have onboard surround sound decoding.

Finally, there are headphones that have spatial imaging by design, such as the AKG K1000, Precide Ergo and the Ultrasone and Vivanco IFL (In-Front Localization) models. The AKG and Precide have a floating earpieces, which are held slightly off the ears by small cushions on the headband. Then each ear can hear sound coming from the transducer playing to the other ear, and the direct sound from each transducer and interaural crossfeed can interact with ear pinna (the outer ear) to generate location cues. The AKG K1000 earpieces are even pivoted to let the listener adjust them for best spatial effect. The Vivanco IFL headphones look like regular headphones, but the transducers are mounted off axis in the earcups, so that sound bounces off the outer ears before entering the ear canal. These headphones image by engaging the normal hearing mechanisms. Based on reviews, all of these designs can have sonic tradeoffs, so careful auditioning is strongly recommended.

Vibration Transducers For Bass Enhancement

Low frequencies are both heard and felt as vibrations conducted through the body. Bass notes in headphones lack impact because the physical sensation (or vibration) of bass is missing. Musicians monitoring with headphones can add a solid foundation to the sound with “bass shakers” or subwoofers that generate intense low frequency vibrations without the dangerous SPLs of full-range loudspeakers. Subwoofers are loudspeakers, but shakers are compact-sized, vibration devices. Tactile transducers vibrate over a broader range of frequencies than shakers. Bass Shakers and tactile transducers may be mounted on floors or furniture such as drum stools and chairs.

Bass shakers and tactile transducers are available from companies that specialize in pro-applications for earphones, such as Sensaphonics. Vibration transducers are being incorporated into PC gaming accessories, such as gaming seats and vests, and into headphones. For example, Panasonic’s Virtual Motion System and Brain-Shaker phones have tiny shaker actuators mounted on the sides. For more information about how vibration transducers work, see Judging Headphones For Accuracy.

Volume Controls and Remote Mixer Stations

In general, headphones do not need external volume controls because they are usually run from amplifiers or portable stereos that already have volume controls. That said, there are headphones with volume controls built into the cord and accessory add-on volume controls. Such inline controls may have a single “thumbwheel” that affects both channels simultaneously or dual controls for independent channel adjustment. Sources for accessory inline controls include Radio Shack (part no. 42-2459), Koss Corporation (part no. VC20) and Sennheiser (part no. HZR6). For the home headphone listener, there is a wall-mounted volume control with headphone jack and loudspeaker on/off switch from Smarthome.Com. Professional monitoring systems can have remote volume modules or complete remote mixing consoles to give performers complete control over their mix. See the FAQs for information on using a potentiometer as a volume control.

Addendum

12/14/98: Updated these sections: Adapters and Converters; Equalizers, Spatial Simulators.

5/18/99: Updated Spatial Simulators section with information about software-based crossfeed processors.

6/12/99: Added section on headphone display stands.

12/7/99: Updated section on spatial simulators.

10/03/00: Replaced most images with new higher resolution images. Updated sections: Audio Cables, Equalizers, Limiters, Spatial Simulators, Volume Controls. Added section: RF/EMI Noise Suppressors.

3/26/01: Added section on cord managers.

c. 1998, 1999, 2000, 2001 Chu Moy.

Advertisement