Motorcycle helmets, while protecting bikers' brains, may also be contributing to hearing loss. Scientists mapped the airflow and noise patterns to find out why.
The distinctive roar of a Harley's engine is loud, but studies have revealed the biggest source of noise for motorcyclists is actually generated by air whooshing over the riders' helmets. Even at legal speeds, the sound can exceed safe levels. Now, scientists have identified a key source of the rushing din.
Researchers Dr. Michael Carley, of the Dept. of Mechanical Engineering and Dr. Michael Carley at the University of Bath in Bath, England and Dr. Nigel Holt from the Dept. of Psychology at Bath Spa University, also in England, placed motorcycle helmets atop mannequin heads, mounted them in a wind tunnel, and turned on the fans.
By placing microphones at different locations around the helmet and at the mannequin's ear, the researchers found that an area underneath the helmet and near the chin bar is a significant source of the noise that reaches riders' sensitive eardrums. The team also investigated how helmet angle and wind speed affected the loudness. Future tests will move beyond the wind tunnel to real-life riders on the open road.
The findings, described in the Journal of the Acoustical Society of America, may one day be used to design quieter helmets, saving riders' ears for the enjoyment of hard biker rock, the researchers say.
Motorcycle helmets and the frequency dependence of temporary hearing threshold shift
Acoustical Society of America http://www.aip.org/asa_laypapers2011/Holt.html
Most people have been to clubs, festivals and concerts. Many have fought their way to the front and ended up standing close to the stacks where it is possible to feel the breeze coming off the speakers as the band turn the volume up to 11. This level of noise exposure can have a surprisingly significant affect on your hearing in the short term. Evidence your hearing has changed is often experienced as a ringing in the ears the following morning, an experience also familiar to those who ride motorcycles. Most people are unaware that these types of exposure to noise will change their hearing sensitivity. This sort of change to our hearing is temporary – we recover over time. Repeatedly experiencing these types of sounds, particularly before fully recovering can mean that the temporary shift may develop into a permanent hearing loss.
We have measured the type of threshold shift people experience at different frequencies when exposed to noise in normal listening conditions. Wearing a helmet changes the sounds motorcyclists would otherwise be exposed to and because of this the resulting threshold shifts are at different frequencies than would otherwise be expected. We looked at the way the helmet filters sound and found that the effect was to block out certain frequencies of the sound but amplify others. After listening to noise while wearing the helmet, riders become less sensitive to frequencies amplified by the helmet, as expected. Counterintuitively, they actually experience a temporary improvement in their hearing at frequencies blocked out by the helmet.
The type of hearing protection currently available to riders is of a very basic nature, such as ear-plugs and neck-shields. It is important to note that silence is not ideal either, as portions of the sound provide important rider feedback during driving. Hearing protection for motorcycle riders needs to be tailored towards those frequencies that are most detrimental to rider safety, health and comfort. The information revealed in our research, is a first step in setting design goals for developing these noise reduction technologies.
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