Tuesday, October 5, 2010
Hearing Loss: Could It Happen to You?
The right headset and a little knowledge can save your hearing
The Occupational Safety and Health Organization (OSHA) has established that the maximum level of “safe” exposure to loud sounds is 90 dB for up to eight hours, or 100 dB for up to two hours. Meanwhile, the Environmental Protection Agency (EPA) has established their own exposure-level standards that are more conservative, with a maximum of 85 dB for up to eight hours and 92 dB for one hour. How does general aviation measure in comparison?
NASA conducted experiments by placing microphones inside a Cessna 182 to measure sound during different phases of flight (run-up, cruise, climb, etc.). NASA found maximum sound levels of between 105 and 109 dB. Another more recent study by OSHA measured noise levels in a Cessna 172S and found an average maximum sound level of just over 101 dB. The main thing to remember here is that any noise over 90 dB means hearing loss unless you wear ear protection. In both studies, the only factor that reduced noise consistently was wearing a headset.
ANR Versus PNR
By now, most pilots understand the difference between passive headsets (PNR—Passive Noise Reduction) and active headsets (ANR—Active Noise Reduction). Aside from the price (active is more expensive), both types offer good hearing protection, though they go about it differently and are for different environments. It’s important to understand the differences.
ANR headsets work by sampling the damaging noise coming into the ear cup with a miniature microphone and then “canceling” that noise by generating a counter-noise through a tiny speaker, also inside the ear cup. The effect is like hitting a pool of water with your hand. The left hand can be the “bad” noise. When you strike your fist onto the water, waves radiate from it. Then, thinking of your right fist as the “good” noise, pounding it on the water with the same force creates another set of waves. As the two waves meet they cancel each other out and dissipate, just like sound. ANR proponents say it’s the best solution for protecting your hearing.
Mostly, ANR attenuates frequencies in the lower spectrum, from about 20 Hz to around 300 Hz, with peak reduction at 70-150 Hz. Studies have shown that peak noise levels generated by the propeller, engine and exhaust all combine around the 100 Hz point—precisely the area that ANR attenuates best. Higher-frequency noise is caused mostly by air flowing over the cockpit and fuselage. Most ANR headsets don’t attenuate high frequencies. Because each airplane’s noise signature is different, a headset that’s right for one aircraft owner isn’t necessarily right for another.
Headset manufacturers aren’t keen on showing you their frequency response charts. They figure most consumers don’t understand them, so they just give an overall attenuation rating of, say, 40 dB. The key is finding out which harmful sound frequency the headset attacks at that rating. For example, the “40 dB” claim may reduce only 100 Hz by that much, while 2000 Hz (in the human voice spectrum) isn’t attenuated at all and is more harmful. Consumers should examine the breadth of the attenuation (span of frequencies reduced) and the depth (by how much). It’s also important to see charts of what the passive attenuation is like if the batteries fail.
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