Ultrasonic 2 : Frequencies above the upper limit of the normal range of hearing at or about 20 kHz. Ultrasonics is the general term for the study and application of ultrasonic sound and vibrations. (Chambers)
H. J. Porter (Chairman - Technical Committee), Dictionary of Fire Technology, The Institution of Fire Engineers, The Victory Press (Leicester) Ltd. 1979 UK. (Fr Tech - Brit) {81}