Sunday, August 14, 2011

Lowering the Volume for Motorcyclists

(ISNS)—Before hopping on his motorcycle, Michael Carley puts on earplugs, followed by his helmet. It's a step many riders take. After accelerating, most of the sound that a rider can hear isn't from the bike engine or other vehicles on the road, but from the air rushing over and around his helmet.
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The helmet Carley wears is designed and tested for comfort and impact protection—but not for sound protection. Noise inside the helmet can reach rock-concert levels when traveling at highway speeds.
Carley, a mechanical engineer, has gathered a group of engineers and psychologists to study how to minimize helmet noise. The group includes researchers from two U.K. universities, the University of Bath, where Carley works, and nearby Bath Spa University. They are studying how to protect riders from hearing damage and reduce the potential distraction that noise poses to riders.
"Riding a motorcycle is a very noisy endeavor," said Rick Korchak, editor of webBikeWorld, a popular motorcycle website that carries detailed helmet reviews. "There are no quiet motorcycles and there are no quiet helmets."
While riders and others recognize that noise can be a problem, many U.S. jurisdictions prohibit the use of earplugs or other noise-reducinging devices. One long road trip could take a rider through numerous changes in local laws.
"A lot of people, they just do what they're comfortable with, irrespective of what the jurisdiction says is the law," said Charles Brown, a psychologist specializing in sound perception from the University of South Alabama in Mobile.
"We strongly advocate the use of high-quality, correctly inserted ear plugs when riding a motorcycle," said Korchak. He added that good ear plugs will not eliminate all noises, allowing sirens and traffic to be heard.
Sound Research
Riders experience noise from multiple sources.
First is the engine noise, which is a relatively insignificant factor once riders accelerate to highway speeds. Audible wind noise can reach volumes as high as 115 decibels or more, roughly equivalent to what power saw operators without ear protection would hear. The Occupational Safety and Health Administration recommends limiting such exposure to 15 minutes or less per day.
Most initial academic research in motorcycle sound focused on long term hearing damage. The group from Bath began researching the topic by establishing reliable measurements of what happens to air passing over a helmet in a wind tunnel. Then, on roads and test tracks, they studied the relative placement of the rider and the motorcycle. They studied how all the important variables interact, such as the height of the windshield, the location of the helmet and the size of the rider.
Turbulent air buffeting off the motorcycle windshield is one problem. The researchers found that small differences in air flow patterns can cause large variations in sound level. Subsequently, the Bath group took their experiments to wind tunnels to measure in detail what riders experience.
The Bath group's most recent paper, accepted for publication by The Journal of the Acoustical Society of America, details the way airflow around the helmet creates noise. They found that for the type of helmets with a full shield and visor covering the face, the largest component of noise comes from air rushing around the chin bar, which wraps around the jaw.
There's another type of sound experienced by riders, but it's more difficult to quantify, and not affected by earplugs. It's called body conducted sound. Aboard a motorcycle, this process transmits engine vibrations and the percussive force of the wind through the flesh and bone to the ear. Earplugs don't stop it, and it can be significant.
"It's why your voice sounds different when you hear a recording of your voice versus what you experience as your own voice," said Carley.
Limiting Sound's Impact
Reducing the amount of sound that reaches riders can benefit more than motorcyclists' hearing. Riders have to process the sound of riding, the sound conducted through their bones, and a wide variety of visual stimuli as well, all while responding to the situation around them.

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