Tests for noise damage usually involve hearing thresholds that measure whether an individual finds it harder to detect faint sounds than expected. Now it seems that long-term exposure to sounds not usually considered a risk could cause damage that these tests miss. During the new study, Xiaoming Zhou from the East China Normal University in Shanghai and Michael Merzenich at the University of California, San Francisco, exposed rats to noise at 65 decibels for 10 hours, followed by 14 hours of silence over two months, to mimic noise at work and a quiet home environment. Then they carried out a number of tests that did not involve traditional hearing thresholds. In one, they found that rats exposed to noise were significantly slower at differentiating between sounds than other rats. In addition, these rats had less activity in their brain’s auditory cortex in response to noise stimuli. “High-level noises damage the hearing threshold, but moderate-level noises do not, which is why the damage may not have been picked up before,” says Zhou.
New Scientist, 15 May 2012 ;http://www.newscientist.com/ ;