A wind turbine, a howling mob at a soccer game, a jet engine operating at full capacity: Each these things produces sound waves that are far below the frequency humans can hear. But just because you can not hear the low frequency components of these sounds does not mean they have no effect on your ears. Listening just 0 seconds of low frequency sound can change the way your inner ear works for a few minutes after the noise, a new study shows.
"low frequency sound exposure has long been considered harmless, and this study suggests that it is not," said audiology researcher Jeffery Lichtenhan of medical school at Washington University in St. Louis who was not involved in the new work.
humans can usually sense the sounds at frequencies between 20 and 20,000 cycles per second, or hertz (Hz) -although this range narrows as a person ages. Prolonged exposure to loud noise in the audible range have long been known to cause hearing loss over time. But establish the effect sounds with frequencies below about 250 Hz was more difficult. Even if they are above the lower limit of 20 Hz, these low-frequency sounds tend to be either inaudible or barely audible, and people do not always know when they are exposed.
For the new study, neurobiologist Markus Drexl and his colleagues at the Ludwig Maximilian University in Munich, Germany, asked 21 volunteers with normal hearing to stay inside soundproof booths and played a sound of 30 Hz for 0 seconds. The sound of deep vibration, Drexl said, is what you might hear "if you open the windows of your car while driving fast on a highway." Then they used sensors to record the natural activity of the ear after the noise ended, enjoying a spontaneous phenomenon dubbed otoacoustic emissions (SOAEs) in which the healthy human ear itself emits low hissing sound . "Usually they are too weak to be heard, but with a microphone which is more sensitive than the human ear, we can detect," says Drexl. Researchers know that SOAEs change if human hearing change and disappear along with hearing loss.
The SOAEs people are normally stable over short periods. But in the study, after 0 seconds of low-frequency sounds, SOAEs participants began to oscillate, becoming alternately strong and weak. Fluctuations lasted about 3 minutes, the reports of the team today Open Science Royal Society it. The changes are not a direct indication of hearing loss, but they are only the ear can temporarily be more prone to damage after being exposed to low frequency sounds, Drexl said. "Although we have not yet shown, there is a real possibility that if you are exposed to low frequency sounds for a longer time, it could have a permanent effect," adds Drexl.
"the misfortune of our ears is that we can do terrible things to them with sounds that are not necessarily painful," says researcher hearing loss M. Charles Liberman of Harvard Medical School in Boston. to explore the potential prejudice specific sounds, such as the question hotly debated the effect of wind on hearing, Liberman said the same experiment could be repeated with conditions mimicking the sound of the wind. He would also see the larger study to see how ears react to noise rather than silence within minutes after low frequency sound exposure.
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