Ultrasonic hearing is a recognised auditory effect which allows humans to perceive sounds of a much higher frequency than would ordinarily be audible using the physical inner ear, usually by stimulation of the base of the cochlea through bone conduction. Human hearing is recognised as having an upper bound around 17–20 kHz, depending on the person, but ultrasonic sinusoids as high as 120 kHz have been reported as successfully perceived.
Two competing theories are proposed to explain this effect. The first asserts that ultrasonic sounds excite the inner hair cells of the cochlea basal turn, which are responsive to high frequency sounds. The second proposes that ultrasonic signals resonate the brain and are modulated down to frequencies that the cochlea can then detect.
By modulating speech signals onto an ultrasonic carrier, intelligible speech has also been perceived with a high degree of clarity, especially in areas of high ambient noise. Deatherage states that what humans experience as ultrasonic perception may have been a necessary precursor in the evolution of echolocation in marine mammals.
- Nishimura, T.; Nakagawa, S.; Sakaguchi, T. (January 2003). "Ultrasonic masker clarifies ultrasonic perception in man". Hearing Research. 175 (1–2): 171–177. doi:10.1016/S0378-5955(02)00735-9. PMID 12527135.
- Lenhardt, M. (2003). "Ultrasonic hearing in humans: applications for tinnitus treatment" (PDF). Int. Tinnitus J. 9 (2): 69–75. PMID 15106276.
- T. Oohashi, E. Nishina, M. Honda, Y. Yonekura, Y. Fuwamoto, N. Kawai, T. Maekawa, S. Nakamura, H. Fukuyama, and H. Shibasaki. Inaudible high-frequency sounds affect brain activity: Hypersonic effect. Journal of Neurophysiology, 83(6):3548–3558, 2000.
- Deatherage, B.; Jeffress, L.; Blodgett, H. (1954). "A Note on the Audibility of Intense Ultrasonic Sound". J. Acoust. Soc. Am. 26 (582): 582. Bibcode:1954ASAJ...26..582D. doi:10.1121/1.1907379.
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