The human hearing range is typically between 20 Hz and 20 kHz, determined by the cochlea's structure, which localizes low frequencies at one end and high frequencies at the other. Research indicates that while humans cannot consciously hear frequencies below 20 Hz, the ear's structures respond to infrasonic tones, such as 5 Hz bias tones used in laboratory studies. These tones can manipulate cochlear responses, demonstrating that low-frequency sounds affect ear function even at levels below human hearing capability. Evolution may have optimized our hearing range to balance functionality and biological cost.
PREREQUISITESAcoustics researchers, audiologists, evolutionary biologists, and anyone interested in the physiological mechanisms of hearing and sound perception.
The research performed in our laboratory covers a number of areas related to inner ear function and the physiology of the cochlear fluids (apparent from the rest of the Cochlear Fluids website). Our group has for years been using infrasonic tones to study how the ear works. These are often described as “biasing tones”, because they allow the structures of the ear to be displaced slowly while measurements are made. For almost 10 years we have been using infrasonic 5 Hz bias tones at levels as low as 85 dB SPL (shown as the green diamond in the graph at the right) to manipulate cochlear responses in guinea pigs. The guinea pig is LESS sensitive to low frequencies than the human, so this makes you realize that low frequency infrasonic sounds ARE AFFECTING THE FUNCTION OF THE EAR at levels well below those that are heard by humans.