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." 
- . Insect sounds. London : Witherby; 1961.
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