Cotton Balls & Delicate Precision Instruments
In the New Hampshire countryside where I grew up, the loudest noises on an average day were crickets, cows, and an occasional rifle shot. When I moved to Boston to attend college the levels of ambient noise were much higher, and I had to ride subway trains every day. The clatter and screech of steel wheels on rails, strengthened by reflections off nearby tunnel walls, was painfully loud. In self-defense I started to wear earplugs.
When I drove my old car 200 miles to New York on weekends, I found that if I wore earplugs during the four-hour drive, I arrived at my destination feeling much fresher and less fatigued. And when I skipped Art History class on Fridays to stand in line for cheap "rush" tickets to Boston Symphony concerts, I discovered that wearing my earplugs on the way to the concert made a remarkable difference to the sound I heard. Entering Symphony Hall with fresh ears, I noticed that the smallest details of musical timbre and hall ambience were wonderfully vivid. Those faint sounds were obscured if I left the earplugs at home and exposed my ears to the raucous din of city traffic, sirens, bus engines, and subway trains while traveling to the hall. As I later learned, audiologists call this "temporary threshold shift" (TTS): the noise exposure altered my threshold of hearing.
Most people experience some TTS every day. Any exposure to loud sound will do it. The higher the spl and the longer the exposure, the greater the threshold shift. You may not be aware of it, because the perception of ordinary sounds such as conversation and TV doesn't change. A 10dB shift in your threshold of hearing won't be obvious unless you're trying to focus on faint sounds that are barely detectableand perhaps not even then. After a night's sleep, or just a few hours in a less noisy environment, the ears recover from their stressunless the noise was extremely loud (over 110dB). If the noise is loud enough, or your exposure long enough, the change in your hearing may become a permanent loss and may affect your perception of all sounds, not just faint ones.
After 30 years I still wear earplugs whenever I spend more than a few minutes in any environment with sustained noise levels above 80dB spl. That includes all vehiclescars, buses, trains, and airplanes. In places like mid-Manhattan and downtown Chicago, with their constant din of sirens, taxi horns, and buses, I wear plugs even when I walk down the street.
Am I thus cut off from the world? Not at all. My regular earplugs don't obstruct sound, they just reduce its volume a little. Ordinary conversation involves sound levels around 60dB spl. Cutting back to 45 or 50dB spl doesn't make conversation any more difficult to understand, since the surrounding background noise is reduced as well. Wearing earplugs has never impaired my ability to hear normal speech. (But I sometimes have to take one out when trying to hear a faint voice on the telephone.) During the years when I worked in an office, with the constant rushing noise from airconditioning vents in the ceiling and the clacking of typewriters in the adjacent office, I used to leave cotton plugs in my ear canals all day long, removing them only at night when I got home to my quiet basement apartment. Most of my co-workers never knew I was wearing them.
My earplugs are simple wads of sterile cotton fiber. I buy a bag of 300 cotton balls at a drugstore, and each ball contains enough cotton for three or four earplugs. The cost is low enough that I can start each day with a fresh pair of plugs. Unlike most commercial earplugs, which are designed to attenuate sounds by 20 to 40dB, a small wad of cotton stuffed into the ear canal provides only about 10dB of attenuation. The absorption varies smoothly with frequency from about 5dB in the bass up to a maximum of 15dB in the highs.
With such relatively modest attenuation I can even listen to voices and music from the car radio while driving. I simply crank up the volume and treble to compensate for the plug's slight dulling of the sound, and thus achieve a satisfying ratio of desired signal to unwanted ambient noise. I suppose the resulting sound might be too loud and bright for fellow passengers in a car pool, but I usually drive alone. Of course, I could avoid this if I owned a quiet-riding Cadillac or $40,000 Lexus, but I'd rather buy a cheap car and spend my money on recordings, concert tickets, and stereo gear.
Professionals in the hearing-protection field sneer at cotton earplugs because they don't provide enough blockage to protect against dangerously loud sound. True enough. When I want to doze during a cross-country plane trip, I switch to compressible-foam earplugs made by the E-A-R division of the Cabot Safety Corp. They provide 20 to 30dB of attenuation, enough that I can't hear the stewardess asking if I want a drink. And when I take my favorite Remington rifle to the local shooting range I wear David Clarke ear defenders. They look like old-style headphones, with big cups that seal tightly around the ears and make it difficult to hear any speech at all.
Those alternatives are great for occasions when you really don't want to hear the sounds around you. Cotton plugs, on the other hand, have the uniquely valuable property of providing a useful degree of ambient noise reduction while not obstructing the normal activities of everyday life. I call them my "Dolby B" plugs: everything is slightly softer but still well within the normal range of perception. Before long, the ear/brain system adapts to living in a quieter world and you forget about them. Many's the time I've come home at the end of the day, stepped into the shower, and suddenly become aware of wads of waterlogged cotton falling out of my ears.
Is there a point to this story? We're almost there. When I began wearing earplugs, my only purpose was to reduce the annoyance of subway noise. But since the purpose of riding the subway to Boston University was to study science, I began to learn something about acoustics and physiology as well as astrophysics. It is well known that severe noise exposure produces not just temporary threshold shift, but permanent hearing loss. The louder the noise, the less time it takes to stress the inner ear beyond its ability to recover.
In the extreme, if you stand next to the 140dB blast of a 105mm army howitzer, a single bang may leave you with a permanent impairment. If you listen to a heavy-metal rock concert at 115dB for an hour, you'll certainly experience TTS or some ringing in your ears, but you'll probably recover the next day. But if you expose yourself to those levels for several hours every day, your loss may be severe and permanent. Some of the most famous rock musicians have learned this lesson the hard way.
This relationship is codified in OSHA regulations for factory workers. People may be exposed to an ambient noise level of 85dB spl for an 8hour shift, but the permissible exposure time is halved for every 6dB increase in spl. This is a model of a cumulative effect: damage occurs when the product of duration and intensity exceeds a certain threshold. (For the average person the subjective intensity, or perceived "loudness," doubles with each 6dB increase in spl. But this is only an average; some people need only 3dB, while others require more than 10dB to perceive a subjective doubling.)
Since the rules were created for industrial situations, and were devised to prevent severe impairments that affect a worker's ability to understand normal speech, they don't extend below 85dB or above eight hours. But what about subtler impairmentsfor example, a loss of threshold sensitivity that doesn't alter your perception of speech but reduces your ability to hear low-level ambience and detail? Is there any reason to assume that the cumulative effects of noise exposure cease below 85dB? Might 16 hours of exposure at 80dB have an effector several years at 60dB?
I became interested in the effects of long-term noise exposure shortly after I started using earplugs. I read about remote tribes of Peruvian Indians whose high-frequency hearing was reported to be as good at age 70 as a child's. A baby's hearing may extend to nearly 25kHz, but by the time most people are old enough to vote they no longer hear tones above 18 or 20kHz. When you're thirtysomething you cease being annoyed by the 15.75kHz whistle emitted by the horizontal flyback transformer in every TV set. And now that I'm fortysomething, many of my compatriots no longer hear much above 10 or 12kHz.
Is this progressive top-end rolloff in our ears a purely age-related phenomenon? Or might it be partly a result of long-term noise exposure in an industrial society? Pre-industrial societies, like those Indians in the Peruvian Andes, were rarely exposed to noise levels above 60dB spland then only in brief bursts (twigs cracking, a dog's bark, occasional thunderstorms). For much of the day the average noise level in the rural countryside where I grew up (away from highways, farm tractors, and other engines) is no higher than 40dB spl.
Is anything known for certain about the effects of long-term exposure to moderate noise levels? I haven't seen any good statistics to answer this question. Much of the available data on top-octave loss is based on 20th-century European and American populations that have lived with noise-making machines all their lives. I don't know whether people in the 17th century retained their top octave better than we do today. A comparison of hearing data for modern urban vs rural populations probably wouldn't prove much; since 1940, most farmers have spent much of their time atop tractors, 6' from an un-mufflered engine.