Hearing Affected by Sounds You Cannot Hear
Researchers led by neurobiologist Markus Drexl at the Ludwig Maximilian University have found evidence that inaudible sounds could cause hearing loss. While many people know that exposure to loud, audible sounds for long periods of time can cause hearing damage, previously, research had yet to show evidence that inaudible sounds could do the same (1).
Humans can hear sounds at frequencies ranging from 20 to 20,000 hertz (Hz), or vibrations per second, but this range shrinks due to hearing loss and aging. Sounds at the lower end of the range are often barely heard, however, so prolonged exposure to low-frequency sounds can be hard to detect (1).
In the study, researchers had 21 volunteers listen to a 30 Hz sound for 90 seconds in an otherwise sound-proof room. They monitored the activity of the ears after the end of the sound, looking for a phenomenon known as spontaneous otoacoustic emissions (SOAEs). During SOAEs, the ear emits a very faint whistling sound that can be picked up by microphones the researchers placed in the volunteers’ ears (1).
Normally, SOAEs are stable, but they can change along with changes in one’s hearing, and they can disappear during hearing loss. In the experiment, the researchers found that the SOAEs did not remain stable. Instead, the SOAEs started to alternately increase and decrease in strength (1).
These changes may not necessarily indicate hearing loss, but they suggest that the ears may be susceptible to hearing loss shortly after exposure to low-frequency sounds. Drexl believes that prolonged exposure to these low-frequency sounds could ultimately lead to some permanent damage (1).
Another researcher, M. Charles Liberman, believes that further experiments about the effects of low-frequency sounds should be done. He suggests that researchers should look into how specific sounds affect hearing, like the low hums of controversial wind turbines, or how the ears would react to other sounds presented after the initial prolonged exposure to low-frequency tones (1).
Sources:
1. Williams, S. “Sounds you can’t hear can still hurt your ears.” Science/AAAS | News – Up to the minute news and features from Science. American Association for the Advancement of Science, n.d. Web. 6 Oct. 2014. <http://news.sciencemag.org/>.