Judging Headphones For Accuracy.

caliper_head.gif

Evaluating headphones for accuracy can be difficult, because they present an unusual acoustic environment in which human ears were not designed to operate. The perception of 3-dimensional sound depends on the acoustic contouring that occurs when sound interacts with a listener’s head and outer ears. Because headphone transducers are pressed against the ears and because most stereo recordings are designed for playback over loudspeakers, this acoustic contouring is missing in headphone reproduction. The result is that the sound field has a “trapped inside the head” perspective, with a less than smooth frequency response. There are devices that attempt to mimic 3-dimensional hearing called acoustic simulators, but headphones can be judged for “accuracy” on their own terms.

The Language of Accuracy

The acoustic space in headphones is, by its very nature, distorted. However, audio reproduction is more than recreating an acoustic space. It is also the faithful reproduction of vocal and instrument qualities such as timbre, dynamics, transients and musical details. Acoustic simulators (and to some extent, binaural recordings) are becoming more widely available, so any discussion of headphone accuracy can include spatial fidelity, but the performance of acoustic simulators varies according to the listener, simulator circuitry and the headphones used. Without acoustic simulation, accuracy in headphone listening must be a viewed as a different concept from its loudspeaker counterpart.

In order to discuss issues of accuracy, there must be reference points in the language to convey analyses and descriptions. The world of audiophiles is replete with vivid adjectives that attempt to codify a universal audio quality vocabulary. Sound can be “steely” or “piercing” or “ice-cold” or “chocolatey” or “silky” or “grainy” or “yin” or “yang” and the list is endless. Does sound that is “steely” to one person come across as “silky” to another? Given the divergent reviews of audio equipment, these contradictions are a fact of life.

At the very least, a person who reads equipment reviews or gets buying tips from acquaintances should verify these assessments independently. When speaking of the sound of headphones, the following keyboard-frequency chart and list of terms from The Abso!ute Sound’s Guide to High End Audio Components (1994) conveniently divide the audio spectrum into bands:

keyboard.gif

Extreme bottom below 32 Hz
Low bass, bottom octave 20 to 40 Hz
Midbass 40 to 80 Hz
Upper bass 80 to 160 Hz
Lower midrange 160 to 320 Hz
Midrange 320 to 2,560 Hz
Upper midrange 2,560 to 5,120 Hz
Highs, lower highs 5,120 to 10,240 Hz
Extreme highs, top octave 10,240 to 20,000 Hz

Thus, if a review says that the midrange on a pair of headphones is “recessed,” the reader will know where in the audio spectrum to seek out this anomaly. These terms are not hard and fast rules, and perhaps when the reviewer specifies the midrange, it is actually the lower portion of the upper midrange that is under scrutiny.

Accurate Sound vs. Better Sound

 

hrtf.gif
Figure 1: Graph of a head-related transfer function.

Headphones that have built-in equalization to sound “flat” do not necessarily measure flat. Figure 1 shows how the ears hear a flat frequency sweep projected from a loudspeaker positioned slightly left of center. The hills and valleys in the two curves are due to the head-related transfer functions (HRTFs) that shape sounds interacting with the listener’s head and the pinna of the ears. The brain processes the amplitude and phase-shifts from the HRTFs to determine the nature and location of the sound. Diffused-field equalized headphones alter the frequency response of headphones to resemble a curve similar to those in figure 1, thereby restoring some of the HRTF contouring that is normally missing with headphones. Thus, diffused-field equalized headphones are supposed to sound natural and flat.

What about equalization that enhances the sound of headphones – not to make them sound flatter, but “better?” For example, some headphones are equalized with a treble boost. This high emphasis can make headphones sound detailed and balanced. In other models, the sound “sizzles,” which, though artificial, many people find initially pleasing. As with loudspeakers, headphones that appeal to consumer excesses may find their way into homes more quickly, since they sound impressive in fast-paced evaluations. Al Fasoldt, Technofile columnist for the Syracuse Newspapers, commented on headphones that sound better by design:

[I]n this age of fast food and quick desires, accuracy can be dull. Engineers who mix rock albums already know that and they usually add a punchy mid-range even before it gets to your amplifier. So the punch whether it’s added before or after can often add some life to the sound. That’s not bad, but it’s not necessarily good, either.

Take the case of a chef, for example. If the chef added pepper to everything, the food would taste pretty tangy all the time. You’d get tired of it fast…. And of course you’d hardly think the food tasted natural.

But back to headphones. You’ll probably be a lot happier with an accurate-sounding model than one that’s been spiced up. Unless the engineer’s twiddling has been nixed in the mix, you’ll end up with a double boost-once at the studio and once alongside your head.

The old adage “All that glitters is not gold” is equally applicable to aural and visual glitter. Accurate headphones may not make one’s ears prick up in excitement, but in the long run, the two will settle into a comfortable and lasting relationship.

construction.gif

Finding Low-bass Notes in Headphones

How well headphones reproduce bass depends on several factors such as whether the type of phones is closed-ear, open-air or in-ear. The open-air types tend to leak sound, so the bass may not be as defined as with closed-ear types. Canalphones that form an acoustic seal inside the ear canal have excellent clarity, but the close-coupling tends to highlight the lighter quality of headphone bass. Compared to loudspeakers, the best headphones seem to be bass-shy, regardless of how far down the frequency response extends. Headphone manufacturers may incorporate a bass boost to compensate. The overall response of a pair of phones can sound flat, yet the low notes lack heft, so these “mega” boosts appear to be justified.

When a person hears bass notes, the experience is both aural and visceral. The feeling of bass notes as they are conducted through the body contributes to the sensation of bass. Headphone listening limits the experience to the ears alone. In that respect, standard headphones will always have less dramatic bass than speakers (canalphones more so, since they radiate bass straight to the ear drums with little or no bone conduction). Therefore, it is important to distinguish between truly anemic bass and bass which feels light from the lack of physical impact.

The lightweight headphones that come with portables are likely to have truly anemic bass. When lightweight headphones have transducers too small to reproduce low bass, a boost in the midbass response can create the illusion of deep bass and a more balanced sound. Al Fasoldt recounts an incident that illustrates how the brain can synthesize missing bass notes:

I watched [a utility crew put in new light poles] as I listened to an old organ recording. The performance was full of extended pedal notes, down to 16 Hz. That’s low enough to qualify as thunder. After two or three minutes I began to realize that I was hearing more than just the usual Walkman-type sounds from my tiny headphones. As the organist stepped down onto the pedals, rumbles of the deepest imaginable bass poured into my ears. I became so excited that I tripped over the headphone cord….

With my headphones dangling near the ground, I was still hearing those bass notes. One of the workers walked over to the diesel generator and switched it off, and my bass notes disappeared. I had been hearing an ordinary engine’s chug-chugs and interpreting them as Bach’s mellifluous pedal-point. Frequency-response checks on my portable player’s headphones showed them to have almost no bass at all. And yet the bass sounded fine when I listened to music….

The solution to this mystery of the missing bass notes is found in the study of psychoacoustics. Music is a complex mix of different frequencies superimposed on each other. The sounds from a voice or an instrument are characterized by a fundamental tone and a series of overtones or harmonics that are mathematically related the fundamental tone. Thus, two different musical instruments are said to have the same pitch, if they are playing the same note. Yet, the sound of a violin is clearly distinguishable from that of a trumpet. They have different timbre because the harmonics from each instrument are not the same.

A fundamental note and its harmonics are mathematically related such that the brain can actually synthesize a missing fundamental note, so long as the harmonics are audible. The exact psychoacoustic process for this synthesis is still a subject of debate, but may be a combination of recognizing the timing intervals and the patterns of harmonics. Thus, low bass sounds from headphones that cannot physically reproduce low bass are nothing more than an illusion. Further, this synthesized bass often has a lighter timbre in comparison with the real thing. Nevertheless, it is this illusion that continues to drive the portable stereo industry. Lightweight headphones that could not create the sensation of low bass would probably have been doomed to commercial failure. For more information about the Missing Fundamental effect, see The Elements of Musical Perception.

vibes.gif

Musicians who monitor with headphones often miss the “tactile” feel of instruments such as thump of drums or the pull of a bow across a violin, which high volume loudspeaker monitors can convey. One of the benefits of headphone monitoring is being able to listen to a mix at safe volumes to preserve hearing. Vibration devices such as subwoofers, “shakers” and other tactile sound transducers can supplement headphone sound to create the physical sensation of high volume sound systems without the risk of hearing damage. Vibration transducers mount on floors and furniture (wherever there is contact with human bodies) and vibrate in synchronicity with the low frequencies in music. The vibrations travel in the body via bone conduction. The brain integrates these vibrations into the listening experience. Some headphones have mini-shaker transducers mounted on the sides, but these do not generate the same levels of deep, body-rumbling bass. See A Quick Guide To Headphone Accessories for more information about vibration transducers.

Tools of the Trade

The best tools for evaluating headphones are music CDs and audio test CDs (skipping those tracks that are specific to loudspeakers, such as speaker-setup tests). A good headphone test CD should have pink noise tracks, an assortment of frequency sweeps and/or chromatic scales as well as binaural tracks (or use separate binaural recordings). The music CDs should contain music that has not been overly processed. Recordings that exaggerate sounds by design are difficult to use as an accuracy reference. When listening to binaural recordings, be aware that closed-ear and circumaural headphones are usually better for binaural playback than other headphone types. Also, diffuse-field equalized headphones work best with binaural recordings that are made with the microphones mounted on sides of the dummy head, but not inside the artificial ears.

test_cds.gif

Listening to audio test CDs can be excruciatingly uninteresting, but are the best means for evaluating the individual nature of headphone performance (and are especially valuable for evaluating equalizers). Of the standard series of test tracks, the pink noise, slow spectrum sweeps and chromatic scales are probably the easiest to use. Listen to the pink noise tracks standing close to a reference-quality loudspeaker to have a baseline for comparing headphone sound. In a balanced system, the pink noise will sound smooth, with no frequencies standing out. Since loudness perception is frequency and volume dependent (see discussion below), listen to the pink noise at different volumes. This test will point out any rough areas in the headphones’ audio spectrum. The warbles and sweeps can identify any mechanical resonances in the headphone earcups.

If a headphone acoustic simulator or virtualizer (such as Dolby Headphone) is part of the audio system, listen to test CDs with and without the simulator engaged. However, be prepared for the result that the simulator may perform better with music than with test signals. The signal processing in acoustic simulators (and especially virtualizers) is sophisticated and based on psychoacoustic theory, and might be designed to work with complex and dynamic audio sources such as music. Thus, the best way to evaluate headphone acoustic simulators may be to compare their sound with loudspeaker playback.

diffuse.gif

Since measuring the frequency response of headphones by ear is difficult, those who are truly serious about evaluations might try measuring the response using an acoustic coupler (or better yet, an artificial ear). Headphone couplers (also available for earphones) are sold by such audiometric supply companies as Digital Recordings and must be fitted with a microphone connected to a real-time spectrum analyser (most equalizers have a built-in RTA and there are computer programs that turn PCs into spectrum analyzers). If a RTA is not available, the response curve could be manually plotted with a sound level meter from Radio Shack (33-2050 or 33-2053). Be sure to correct for any resonances from the coupler itself or the readings will be inaccurate. Diffuse-field equalized headphones will measure flat only from inside the ear canal of a model dummy head or an artifical ear. The procedure for measuring a diffuse-field equalized headphone is defined under the IEC 60268-7:1996 standard.

fletcher.gif

If test equipment is not an option for determining the frequency response (or as a follow-up to measurements), listening to test tones can still provide a useful assessment of how “flat” headphones sound. The Fletcher-Munson curves above indicate that loudness perception is a function of frequency and sound pressure levels. Unfortunately, the curve is flattest when the loudness is at the threshold of pain. Yet, there is a band of perception that fluctuates only about 5dB over a range of safe listening levels: 200Hz to 5kHz. Keeping in mind that loudness comparisons between frequencies are prone to subjectivity, tone tests between 200Hz and 5kHz can help narrow down irregularities in the response after a pink noise check. As each tone is played, check for variations in volume and character between tones. Again, if a headphone acoustic simulator is part of the listening system, check the frequency response of headphones with and without the simulator engaged.

Musical passages on test discs are usually selected to demonstrate a particular aspect of sound reproduction, and are more revealing of how headphones will perform with complex audio signals. Uncompressed vocals and instrumentals and special effects tracks can stress the dynamic range of headphones, but take care with the volume setting to avoid hearing damage. One safer way to test the headphones’ ability to play loud transients is to wear foam ear plugs to protect the ears while listening to the headphones. Get the kind that are rated to reduce noise by at least 29dB and absorb frequencies evenly across the audio spectrum. Another option is to hold the headphones off the ears at a comfortable distance and listen for distortion. The sound will be tinny and devoid of bass, but if the headphone transducers are poorly damped or overload, the distortion artifacts may be distinguishable. Do NOT pump too much volume or the phones could self-destruct (not to mention the potential damage to one’s hearing)!

The imaging on the binaural tracks depends on how closely the shape of the listener’s head matches that of the dummy head used to make the recording and on the positioning of the microphones on the dummy head. Further, some listeners localize sound with the help of head movement, which is not possible with standard binaural playback. Therefore, if a headphone does not reproduce binaural sound to stunning effect, the accuracy of the headphones may not the cause. Audition binaural tracks on many different headphones to get the best spatial reference. Acoustic simulators should be turned off when listening to binaural recordings. Also, closed-ear and in-ear phones tend to be better for binaural playback than open-air types.

Judging headphones for accuracy is not an intuitive process, but unlike loudspeakers that have to be moved around and positioned for optimal sound, can be done with hardly any exertion. The most important preparation is understanding the psychoacoustics of headphones, so that the listener does not misjudge the unique characteristics of headphone sound fields. Listening to headphones is an experience that can be as satisfying as listening to loudspeakers, if the listener evaluates the phones carefully before purchasing.

Addendum

12/14/98: Added discussion of using acoustic simulators during headphone evaluation.

12/18/99: Updated discussion of using acoustic simulators during headphone evaluation.

References:
__, Sensaphonics Product Literature: Bass Shakers, Tactile Sound Transducers, c. 1997, Sensaphonics Hearing Conservation.
Darwin, Chris, Perception, c. University of Sussex at Brighton.
Fasoldt, Al, “How (and why) to choose good headphones,” c.1988, The Syracuse Newspapers.
Fasoldt, Al, “Generating low-bass notes in my headphones and in my head,” c.1988, The Syracuse Newspapers.
McCale, Steven E., “Earphone Monitoring,” Mix, May 1996.
Murthy, V.S.Madhuri and Sethi, Simer Singh, Basic Acoustics and Psychoacoustics, c. 1996.
Pearson, H., The Abso!ute Sound’s Guide to High-End Audio Components, c. 1994.
Welsh, Norma, Demonstrations in Auditory Perception, McGill Univerity c. 1996.

c. 1998, 1999 Chu Moy.

 

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

This site uses Akismet to reduce spam. Learn how your comment data is processed.