Cotton Balls & Delicate Precision Instruments Page 2

But the example of the Peruvian Indians, if true, is suggestive. So is the well-known gender difference. The numbers cited above were only for males. Adult women apparently hear high frequencies more acutely than most men do. Is this due to genetic factors? Or is it actually an effect of long-term noise exposure? On the average, men spend much more time in noisy environments than women do. Whether inborn or due to socialization, this disparity begins in childhood. Boys are more likely to engage in active games involving a lot of shouting and screaming, while girls play quietly with dolls. Adolescent boys gravitate toward guns, loud cars, and boomboxes, while girls chat with friends. Statistically, men drive twice as many miles as women, make longer trips, and drive noisier vehicles (motorcycles, sporty cars, pickups, heavy trucks). Men become soldiers, fly airplanes, and work with loud machinery in auto repair shops and industrial factories.

Now that women are sharing more equally in these activities, will they start losing their highs at the same rate as men? Housework has also become noisy in the modern era as a result of washing machines, blenders, garbage disposals, vacuum cleaners, et al. Our grandmothers did their housework with less noise and a lot more effort. Nevertheless, the evidence to date indicates that the gender disparity may be getting worse rather than better. In a study reported last November at the annual meeting of the Gerontological Society of America, tests for hearing losses in adults between 30 and 80 revealed that men of every age had impairments twice as severe as women in the same age group. And at the mid-treble frequencies involved in speech perception, men's impairments became worse with age at a faster rate than women's.

I'm presenting questions and hypotheses here, not definite answers. It may be that top-octave loss is not related to long-term noise exposure. Whatever the case, top-octave rolloff probably involves a different biological mechanism than impairments caused by intense sound. It's well known that hearing losses due to extremely high sound levels occur first and most importantly at the frequencies where the ear is most sensitive, between 2 and 4kHz. This may be because the tube resonances in the ear canal cause the actual sound-pressure levels at the eardrum to be highest at those frequencies. Those, unfortunately, are also the frequencies most critically needed for speech comprehension. If you spend too much time shooting guns, driving sports cars, occupying a front-row seat at heavy-metal rock concerts, or playing oboe in a symphony orchestra while sitting directly in front of the first trumpet, the first sign of permanent damage is that you start asking everyone to repeat what they just said.

At any rate, having hypothesized at age 19 that long-term noise exposure might have some effect on hearing, I started the cotton earplug habit. By cutting my subjective noise exposure in half, I hoped to reduce or deter whatever losses might inevitably befall me.

Has the experiment succeeded? I have no rigorous scientific proof. But my high-frequency limit, which was 18kHz when I turned 20, was still at 18kHz when I was 30. At 40 I was still bothered by the TV flyback whistle that my friends had become oblivious to. I no longer hear it today (at 49), but my ears are still good to 14kHz. Clearly my top-octave loss has progressed more slowly than the statistical average for American males. In most other respects I have the usual symptoms of my age: aching joints, gray hair, and an inability to read (or see anything clearly) without eyeglasses. I've never regarded myself as a particularly "golden-eared" audio critic; perhaps my ears have just been lucky.

One more bit of evidence is suggestive. Several years ago I had an audiometric exam, which measures the threshold of hearing at various frequencies (footnote 1). Both ears were within the "normal" range, but they were not identical. The left ear was about 10dB less sensitive than the right, particularly in the 2–4kHz region. This disparity may be only a biological accident, but I have another theory. For 20 years I habitually drove with my driver's-side window rolled down during the warmer half of the year. Even with earplugs, my left ear was exposed to substantially higher levels of noise—wind turbulence, engine noise, and the roar of trucks and buses in the next lane. Could two decades of such exposure be the reason for the permanent threshold shift in my left ear? I don't know, but after learning the result of that hearing test I changed my driving habits. Now I keep the window rolled up and use the air conditioner.

The brain automatically compensates for modest changes in threshold sensitivity. It's like the automatic-level control in a tape recorder, adjusting its "gain" to maintain a constant recording level. From the inside of my head I'm not aware of any difference between my left and right ears. And when you experience a temporary threshold shift due to excessively loud sound, you may be unaware of it unless the shift is so great that familiar sounds (like your wife's voice) are altered. By then it may be too late; you may have stressed your inner ear beyond its ability to recover.

It may be that the capacity of the hearing system is a finite resource. If we don't use it up by constantly assaulting it with loud noise, it will still be available, unimpaired, when we want it most—for music and other important pleasures. I've become so conscious of this issue that I am even bothered by constant noises that are not loud, such as the whirring power-supply fan in the personal computer that I sit in front of all day. I've been using PCs since 1981, and the first thing I do when I buy a new one is to slow down its excessively loud fan. My next desktop PC will have no fan at all, thanks to low-power chips that were developed for laptop portables. Even if ten years of fan noise has no effect at all on the hearing system, its suppression lets me enjoy music while I work.

Whether long-term exposure to moderate noise levels has any permanent effect remains an open question. My experience doesn't prove anything, but it is sufficiently suggestive that I'm going to continue wearing earplugs just in case, whenever I'm not at home or at a concert. If you value the pleasure you experience from subtle aspects of high-end sound reproduction, you might want to do the same. For a simple test, try wearing cotton plugs for an hour or two before a critical listening session, especially while driving to a concert or audio store. Take them out when you arrive, and see whether the resulting freedom of TTS enhances your perception.

What is known beyond doubt is that prolonged exposure to high-intensity sound causes permanent hearing loss. If you're lucky, you may get a warning. A buzzing or ringing sensation in the ear, particularly one that continues for hours after the loud exposure, is a clear indication of inner-ear overload. But just as you can burn a permanent scar into your retina by staring at the sun through thin clouds, you can permanently damage your hearing with high spls and not notice anything wrong until afterward. So if you experience ringing and don't notice a permanent loss the next day, consider yourself well warned: don't expose your unreplaceable ears to such loud sounds again.

When I took my teenage nephew to a Van Halen heavy-metal rock concert a few years ago, the amplified sound was so loud—even in a relatively distant upper-balcony seat—that it was impossible to converse at all, even by yelling directly into his ear. I kept my earplugs in, except for a few brief listening tests. Even so, I noticed some TTS in my hearing after the concert. My nephew, and the other kids nearby, experienced severe TTS with ringing in the ears and some difficulty understanding conversation on the way home.

High-intensity sound is thrilling. It produces a "rush" that is fun to experience—like sex. But since you can get that orgasmic thrill from a brief burst of intense sound, is there really a significant added benefit from continued exposure lasting longer than an hour? And is it worth the risk of permanent hearing loss? Injured knees can be replaced, but you only get one pair of ears, and they have to last for the rest of your life.

High sound levels are not the exclusive property of rock music. A jazz combo can produce some pretty intense sound in a small club, even without the amplification that most groups use these days (even on brass instruments! footnote 2). A symphony orchestra also can produce an impressively big sound for listeners in the first 20 rows. I've measured sustained levels above 100dB spl, and transient peaks up to 115dB, during the finale of Mahler's Symphony 2 ("Resurrection"), the Turangalîla Symphony of Messiaen, and in a Wagner aria sung by soprano Jessye Norman. But such grandiose levels are produced in classical music for only brief periods, a few seconds or a few minutes at most. It would be too much of a strain on the musicians to continue playing fortissimo for longer. Acoustic music usually doesn't endanger its listeners.

In playback, it's easy to overload your ears by playing loud passages constantly. I once spent a day at CES demonstrating a new surround-sound system to visitors, using (among other things) a tape of the end of the "Resurrection." We used big speakers and a 700W amp in order to match the levels that would be heard in the best seats in the hall. At the beginning of the day it sounded wonderful, building up to a gloriously big, spacious, powerful climax. The highest levels produced some TTS, as indeed the live sound would. Normally that wouldn't matter; after hearing that sound at a concert I probably wouldn't be exposed to high spls again until the next day, perhaps not until the next week. But as the demonstrations continued I played the same tape over and over without giving my ears a chance to recover, and my TTS became progressively more severe. I inadvertently began turning the volume up in order to produce the same subjective impression. By the end of the day I was playing the system so loud that the woofer voice-coils in the surround speakers were banging against their magnets. Did you ever go into a hi-fi showroom and wonder why the salesman was playing the system so loud? He, like me, probably was temporarily deaf from hearing loud music all day.

Most living-room stereo systems can't generate +110dB sound levels without producing pretty obvious distortion. If it can, your family and neighbors may serve as effective regulators to keep you from habitually blasting your ears. But while testing car stereo components, I have measured peak spls as high as 130dB. And headphones can generate dangerous levels without even straining.

Koss, the American headphone company, has become concerned about the propensity of some listeners to fry their ears without knowing the risks. Several years ago a Koss headphone radio featured a yellow warning light that flashed whenever the sound level in the headphones exceeded 95dB. Since significant risks are presented by the combination of high-powered PA systems and thousands of people cheering in an enclosed stadium, Koss has embarked on a program of giving away compressible-foam earplugs at major rock concerts and indoor sporting events. Koss distributed 10,000 pairs of plugs at a Metallica concert last November (during which sound levels up to 116dB were measured in the audience), and 35,000 pairs at the Super Bowl in January.

High-intensity sound can be great fun in small doses. But like chocolate candy and free sex, excess may have serious and long-lasting consequences. Music offers subtle pleasures too, and it would be a shame to lose our ability to enjoy them.—Peter W. Mitchell

Footnote 1: I encourage Stereophile writers to have regular audiograms—I had one last summer, for example. (At age 43, my hearing appears to be better than normal between 500Hz and 2kHz, though I have some slight threshold shift—still within the region classified as "normal," however—in both ears above 4kHz.) But note that audiograms, which only extend up to 8kHz, do not test your upper-frequency hearing limit. You can use the test tones on our Test CD 2—track 27, index points 30 to 37, which cover frequencies of 8kHz, 10kHz, 12.5kHz, 14kHz, 15kHz, 16kHz, 18kHz, and 20kHz—to check for yourself where you can no longer hear the tone. I can hear the 16kHz tone, for example, but not the ones above that. Don't play these tones too loud, however. We don't want you either to deafen yourself or to blow out your tweeters by increasing the volume of a tone above your hearing range.—John Atkinson

Footnote 2: Before joining Hi-Fi News & Record Review in 1976 as a lowly editorial assistant, I worked full-time as a musician, playing bass guitar. While all my professional work was with electric ensembles, the loudest group I ever experienced was an all-acoustic amateur big band. Until you've sat next to the drums in front of full saxophone and brass sections, you don't know what the words "dynamic range" really mean.—John Atkinson

Article Contents
Share | |
Site Map / Direct Links