Barely perceptible low-frequency signals nevertheless activate measurable responses in our auditory circuits. Neurobiologists at Ludwig-Maximilians-Universitaet (LMU) in Munich have now characterized the remarkable impact of low-frequency sounds on the inner ear.
The human auditory system appears to be poorly adapted to the perception of low-frequency sound waves, as hearing thresholds become markedly higher for frequencies lower than about 250 Hz. Yet sensory cells do react to pressure waves with frequencies below 100 Hz, as revealed by the fact that such signals actually evoke detectable micromechanical responses in nerve cells in the inner ear, as LMU neurobiologists now report in the journal Royal Society Open Science.
Sources of low-frequency signals are a prominent feature of technologically advanced societies like our own. Wind turbines, air-conditioning systems and heat pumps, for instance, can generate such sounds. Hearing thresholds in this region of the acoustic spectrum vary from one person to the next. "But the assumption that the ear is unresponsive to low-frequency sounds because these are seldom consciously perceived is actually quite false. The ear indeed reacts to very low-frequency signals," says Dr. Markus Drexl of LMU. In collaboration with researchers led by Professor Benedikt Grothe (Head of the Division of Neurobiology in LMU's Department of Biology II) and a team based at Munich University Medical Center, Drexl has carried out a laboratory study which shows that low-frequency sounds, though virtually imperceptible, actually have a surprisingly strong effect on sensory cells in the inner ear.
Low-frequency hum stimulates the cochlea
The new study is based on data collected from 21 experimental subjects with normal hearing, whose ears were exposed to a 30-Hz tone for 90 seconds at a sound-pressure level equivalent to 80 decibels. To determine how the inner ear responded to the signal, the researchers took advantage of a phenomenon referred to as spontaneous otoacoustic emissions (SOAEs). SOAEs are scarcely perceptible acoustic signals which are produced by the inner ear in the absence of overt stimulation, and can be detected with a sensitive microphone inserted in the ear canal.
"It turns out that low-frequency sounds have a clearly definable modulatory influence on spontaneous otoacoustic emissions," says Drexl. Following exposure to the 30-Hz signal for 90 seconds, the subjects' SOAEs exhibited slow oscillations in frequency and level, which persisted for up to 120 seconds. "Strikingly, the effect of the low-frequency stimulus on the cochlea persists for longer than the duration of the stimulus itself," Drexl points out. Further experiments will probe the possibility that this phenomenon may be linked to noise-induced auditory damage, one of the most common causes of hearing impairment in industrialized countries.