Noise-induced hearing loss is one of the most common auditory conditions, resulting from over stimulation of the cochlea- a spiral shaped cavity in the inner ear. Until now the accepted perception has been that what we can’t hear doesn’t hurt us but a study published in Royal Society Open Science today reveals that this perception might be wrong. Intense low-frequency sounds, with frequencies below 250 Hz, might be beyond the limits of human hearing but could still be affecting our ears.

The team behind this study measured faint sounds which are emitted by healthy human ears to get an idea of how processes in the inner ear are working. These faint sounds, called otoacoustic emissions, are naturally emitted as a part of the hearing process and originate from the inner ear. In this study researchers from the University of Munich recorded the sounds emitted from the ears of 17 of 21 hearing volunteers and investigated how these faint sounds changed when the volunteer was exposed to low frequency sounds.

Otoacoustic sounds normally stay at the same frequency but when volunteers listened to low frequency noises the sounds their ears emitted began to slowly oscillate in frequency. The researchers say this is an indication that low frequencies were altering the mechanisms at work in the inner ear. The oscillation lasted for a couple of minutes after the low frequency sound was played to the volunteers. ‘The recovery process significantly exceeds the exposure duration,’ say the researchers, even when the low frequency sound is not perceived as uncomfortably loud.

The range of frequencies we can perceive reflects the frequencies the inner hair cells, which send information to the brain, can pick up. Their sensitivity to very low frequency sounds is poor so we either hear low frequency sounds very faintly or do not perceive them at all. The scientists warn that this doesn’t necessarily mean low frequency sounds aren’t affecting other parts of the ear.

Outer hair cells, which are responsible for amplifying sounds waves in the ear, are more sensitive to low frequency sounds than inner hair cells. They are thought to be responsible for otoacoustic emissions and these results show that they could be affected when exposed to low frequency sound waves.

The team say the results could have repercussions in assessments of risk potential of exposure to low frequency sounds, for example those produced by wind turbines, block-type thermal power stations, and air-conditioning systems.