Plane & Pilot
Tuesday, November 8, 2011

The Dangers Of Noise Fatigue

Noise fatigue may be an often-overlooked cause of aircraft accidents in general aviation

The Power Flow tuned exhaust results in two to five knots more speed on an older Mooney, but also costs higher fuel burn and a slightly higher noise level.
It seems noise suppression, like weight reduction, is one of the industry's most impossible missions. Jets position the pilots far out on the pointy end, with engine(s) 80 feet behind and nothing out front to beat airflow against the windshield. Single-engine, tractor-configured piston singles put the motive force directly in front of the pilot, and the exhaust noise directly below.

As the owner of an older Mooney, I know exactly how loud a four-seat retractable in the 150- to 160-knot range can be. My airplane is a comfortable, efficient machine, wider than a Bonanza and nearly as fast, but it's also one of the noisiest airplanes in the sky, partially because it's old, but also because the state of the noise-attenuation art wasn't very advanced 40 years ago.

Engineers simply didn't know how to build a quiet cockpit. Unlike automobiles, where weight isn't a critical concern and you can simply stuff in all the soundproofing you want, airplanes must bow to reasonable weight limitations in order to preserve some semblance of payload.

Today, we have the benefit of some truly amazing headsets with active noise reduction (ANR). I have two Lightspeed Zulu headsets in the front seats of my Mooney, and two Bose headsets for passengers in back to keep the decibels at bay. Without them, the din would be about the same as an AA fuel dragster on burnout, enough to jar the wax from your ears. (Ok, maybe it's not quite that bad, but it's still definitely worth suppressing.)

Passive headsets have been around for decades, and they typically reduce noise levels by 10-20 dB in the critical vocal range below 3,000 Hz. These products rely on the design itself to restrict noise, utilizing such features as gel ear seals and clamp force to prevent noise from penetrating the headset.

Active noise reduction takes the science several steps further, actually treating the noise that does enter the headset. It's not a new technology. Dr. Amar Bose began research on noise cancellation in 1978, and introduced his first ANR headset for the U.S. Air Force a decade later.

Active noise attenuation simply monitors the frequencies produced inside the headset, inverts it to an "antinoise" signal, and transmits that to effectively cancel most of the offending decibels. Headsets produced by Bose, David Clark, Lightspeed, Sennheiser and several other companies produce excellent ANR headsets that may reduce noise level by 30 dB or more.

Good headsets help enormously, but the noise level in many airplanes remains a problem. Every once in a while, I'll remove my headset to check the ambient noise level, and I'm always amazed at how loud it is.

Recently, out of curiosity, I purchased an inexpensive sound meter and recorded a maximum noise level at climb power of 114 dB. That's especially bad news, as the threshold of hearing damage is considered to be 85 dB, maintained for longer than eight hours. Raise the sound level to 100 dB, and you have only 15 minutes before there's permanent damage. Above that, you're rolling dice at any level.

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