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Low noise levels can damage the hearing centres of the brain
London (dapd). Even relatively low-level continuous noise exposure can damage the hearing centres of the brain and thereby impair hearing. Alternating sounds with an intensity of just 65 dB suffice, corresponding to the volume of a television set or a loud ventilation fan. This was shown in experiments on rats by an inter-national research team. They subjected the animals for two months to irregularly varying noise levels of 65 dB. The brains of the rats subsequently showed clearly recognizable changes in the processing of auditory stimuli. As a result, the animals had difficulties in distinguishing different rapid tonal sequences. These effects arose not only after a 24-hour exposure but also after only 10 hours of noise per day, as reported by the researchers in Nature Communications (doi:10.1038/ ncomms1849).
“Environmental noise exposure degrades normal listening processes” write Xiaoming Zhou of the East China Normal University in Shanghai and Michael Merzenich of the University of California in San Francisco. This applies even when the sounds lie in noise-level ranges previously accepted as harmless.
Until now it was known that noise above 85 dB, through damage to the inner ear, can permanently damage hearing capacity. Continuous low-level noise was previously considered to be without danger, since it did not affect the ear itself. One had
previously only observed effects of noise below 85 dB in the unborn, the researchers say. Now, it appears that the less sensitive adult brain also reacts to these low noise levels.
Humans are continually bombarded with environmental noise in the workplace and at home, the researchers observe. In their opinion, the risks of these noise burdens must be more accurately documented and noise protection measures must be appropriately adjusted.
Using three-month-old (adult) rats, the scientists exposed the animals for two months to so-called structured noise. This consists of sounds of various tones and volumes which change at irregular intervals. This broad-band burden realistically duplicates the typical soundscape of industrial workplaces and other modern workplace environments, the scientists say. The maximal volume of this exposure was 65 dB. One group of rats was exposed to this noise for 24 hours, a second group was only exposed for 10 hours, followed by a 14-hour rest period.
After the end of the two-month noise phase, the researchers analysed the brain activity of the rats in the hearing centres of the brain and compared these with control animals which had been kept without noise. In a supplementary behavioural test they also examined how well the rats could distinguish tonal sequences of 20 tones per second from slower or faster sequences. The brain activity as well as the hearing tests both indicated that the noise burden had left systemic damage in the hearing centres, the researchers report. Even after more than six weeks after the end of the noise period, this effect had not abated.
A human is a creature that bangs, makes bad music and lets its dog bark. Sometimes it is also quiet, but then it is dead.- Kurt Tucholsky