Sound beyond the upper frequency threshold of human hearing. The distinction between sound and ultrasound has no importance biologically except in relation to human hearing.
"Thus the terms 'sonic' and 'ultrasonic' apply to sounds of a frequency audible and inaudible, respectively, to the human ear; but the hearing organs of many insects respond to sound of both types." 
"Sound above 20kHz in frequency, and thus inaudible to human ears." 
- . Insect sounds. London : Witherby; 1961.
- . The Songs of the Grasshoppers and Crickets of Western Europe. Colchester, Essex: Harley Books; 1998.
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