Although low-frequency signals are barely perceptible to the human ear, they actually trigger measureable responses in the human auditory system, according to research done by neurobiologists at Ludwig Maximilian University of Munich (LMU) in Germany.
Humans have trouble perceiving low-frequency sounds, having only a range of 20 to 20,000 Hz. Hearing threshold, the sound level which a person’s ear is unable to detect any sound, are much higher for frequencies lower than 250 Hz. However, according to LMU neurobiologists in the journal Royal Society Open Science,frequencies below 100 Hz do in fact evoke detectable micromechanical responses by the nerve cells in the inner ear.
Low-frequency signals are very common in everyday life, but the assumption that human ears are simply unresponsive to these signals is actually incorrect. Air-conditioners, wind turbines, and heating pumps, for example, emit these low-frequency sounds, and do actually affect the ear.
Dr. Markus Drexl of LMU has collaborated with researcher Professor Benedikt Grothe and a team based at the Munich University Medical Center. Through their teamwork, they performed a laboratory study that demonstrated how low-frequency sounds, although virtually imperceptible, have a strong effect on sensory cells in the inner ear. The study used 21 experimental subjects with normal hearing, who were exposed to a 30 Hz tone for a minute and a half at 80 dB. To measure the effect on their subjects’ ears, the researchers used spontaneous otoacoustic emissions (SOAEs). SOAEs are barely perceptible acoustic signals made by the inner ear in the absence of stimulation; they were measured with sensitive microphones placed in the inner ear.
The data received from the laboratory study indicate that the stimulus of the cochlea that results from the exposure of the frequency last longer than the exposure itself. The subjects’ SOAEs had slow oscillations for up to 120 seconds, 30 seconds longer than the exposure. This finding brings up the discussion of whether or not low-frequencies contribute to noise-induced auditory damage.
Source: Science Daily