Plane & Pilot
Tuesday, October 5, 2010

Hearing Loss: Could It Happen to You?


The right headset and a little knowledge can save your hearing




A 2008 study by hearing expert Sergei Kochkin found that 35 million Americans have permanent hearing impairment. Based on current trends, that number is expected to grow to 40 million by 2025, representing nearly 12% of the U.S. population. While statistics on hearing loss among pilots aren’t easy to compile (because most pilots try to hide their hearing problems), both the FAA and the U.S. Military have launched their own studies, and the results are both surprising and disturbing: Permanent hearing loss occurs in about 30% of aviators. As a professional musician for many years, hearing is a keen interest of mine, and the effects of long-term exposure to harmful sound is something I’ve examined carefully, especially because I also fly.

Some form of hearing impairment is almost universal among pilots, and is especially evident in those who began flying before the advent of headsets. The subject is serious enough that even NASA has embarked on studies that examine hearing damage among pilots and ways to prevent it. And the issue of pilots flying with impaired hearing is something the Flight Safety Foundation—an independent international group—has focused on in terms of the risk of aviation accidents.

It’s true that some hearing loss is a by-product of aging. But several formal studies—including one conducted by the British Defense Research Agency and another by the U.S. Army—found that aging doesn’t account solely for the increased hearing loss among aviators. In comparison to nonaviators, pilots were found to have a greater decrease in hearing ability in the high-frequency range of 2-6 kHz. While the entire spectrum of the human voice covers from 250 Hz to 3 kHz, sounds like warning announcements and subtle changes in engine sounds or airflow can be missed due to high-frequency hearing loss, and ATC communications become difficult to discern.

How Loud Can Aviation Be?
Our sense of hearing is quite complex, while the mechanics of sound are relatively simple. In essence, sound starts out as a wave of pressure that emanates from the center of a source and radiates outward, like the waves formed when a pebble is tossed into a still pond. The “pressure” is made of air molecules; the louder the sound, the more sound pressure is generated. This pressure—in the form of air—acts upon our eardrum. We perceive a shout as being louder than a whisper because more air molecules strike our eardrum from a shout than a whisper. This pressure is measured in units called “decibels,” noted by the abbreviation “dB.”

Sounds (pressure waves) are “collected” and directed by the outer ear toward the ear canal and cochlea (named for its resemblance to a snail shell). The waves of air pressure move tiny hair-like cells inside the cochlea—much the same way as a gentle breeze blows across a field full of long grass. Those hairs transmit signals to the brain via the auditory nerve. Hearing damage occurs when the sound is loud enough to break the fragile hair-like cells in the cochlea. The length of time we’re exposed to sound and the frequency of the sound determine the extent of damage. Once those tiny cells are broken, there’s no repair.

To give some sense to all this, it’s helpful to know that a human whisper measures about 30 dB, while a normal conversation happens at around 60 dB. A jackhammer pounds the concrete at some 120 dB, and you tool along in your car on the highway at 70 dB. Physical pain begins at about 125 dB, and your eardrum will burst with exposure to 140 dB.



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