The division of acoustics that studies inaudible sounds, i.e., those with frequencies greater than 20,000 cycles/sec (20,000 Hz or 20 kHz). Biological effects may result, depending on the intensity of the beams. Heating effects are produced by beams of low intensity, paralytic effects by those of moderate intensity, and lethal effects by those of high intensity. The lethal action of ultra-sonics is primarily the result, either directly or indirectly, of cavitation of tissues. Ultra-sonics is used clinically for therapeutic and diagnostic purposes; diagnostic ultrasound uses transducers that emit in the range from 2-15 MHz. In dentistry, instruments producing 29 kHz are used in periodontal surgery, curettage, and root planing.