Guidelines for the Safe Use of Ultrasound:
Part II - Industrial & Commercial Applications - Safety Code 24

Appendix 2 - Glossary

The purpose of this glossary is to explain terminology in the document which may be unfamiliar to some readers. The explanatory paragraphs are based on standard definitions given in the cited references. In some cases, definitions may have been shortened or paraphrased for explanatory purposes. The reader is referred to the cited standards for more rigorous definitions.

Cavitation - Broadly defined, any sonically generated activity of highly compressible bodies (bubbles) of gas and/or vapour in the propagation medium. An example of cavitation is the formation of bubbles in the liquid surrounding the active element of a cell disintegrator. For high-power ultrasound the bubble activity is usually unpredictable and violent (NCRP 83).

Free field - The propagation condition for the acoustic wave in a homogeneous, isotropic medium (such as air) where the boundaries of the medium (such as the walls of a room) exert a negligible effect on the acoustic wave (IEC 84).

Free field response - The response of a microphone to an incident acoustic wave which, with the microphone removed, would be propagating under free field conditions (IEC 84).

High-power ultrasound - There is no standard definition for high-power ultrasound. For the purposes of this document, it is ultrasound of sufficient power or intensity to expose a workpiece to enough vibratory energy to bring about some permanent physical change (Sh 75).

Microstreaming - Circulatory fluid flow on the scale of micrometers. Often it occurs in the vicinity of vibrating bubbles which have been set into motion due to cavitation by high-power ultrasound (NCRP 83).

1/3-octave band - The interval between two frequencies whose ratio is 2^1/3:1. These frequencies define the edges of the band. Normally, the band is referred to by its band centre frequency which is the geometric mean of the two edge frequencies. For upper sonic and ultrasonic frequencies the bands have been designated as the 10, 12.5, 16, 20, 25, 31.5, 40 and 50 kHz bands (CSA 75).

Random incidence - The propagation condition for acoustic waves that create an acoustic field that has statistically uniform energy density and for which the directions of propagation at any point are randomly distributed (IEC 84).

Random incidence response - The response of a microphone to an incident acoustic wave which, with the microphone removed, would be propagating at random incidence (IEC 84).

Response - The output of a device that results from a stimulus under specified conditions. For example, the response of a microphone is the output voltage resulting from an acoustic wave incident on the microphone (IEC 84).

Sound pressure level (SPL) - Given in decibels (dB), it is 20 times the logarithm to the base 10 of the ratio of a given sound pressure to the reference sound pressure, which is 20 micropascals (mPa) for airborne sound. The sound pressure is the root mean square value of the instantaneous sound pressure over a given time interval (IEC 84).

Temporary threshold shift (TTS) - The temporary hearing loss suffered as the result of exposure to an acoustic wave. All or part of the loss is recovered during a given period of time after the exposure has been stopped (O1 75).

Threshold of hearing - For a given listener, the minimum sound pressure level of a specified sound that is capable of evoking an auditory sensation. The sound reaching the ears from other sources is assumed to be negligible (IEC 84).

Transducer - A device capable of converting electrical energy to mechanical energy and reciprocally converting mechanical energy to electrical energy (IEC 86).

Ultrasonic - Utilizing, produced by or relating to ultrasonic waves or vibrations. For the purposes of this document, ultrasonic waves are acoustic waves having a frequency above 18 kHz. The limit value of 18 kHz was chosen as a compromise between the two values most commonly found in the literature, 16 and 20 kHz.

Upper sonic - Utilizing, produced by or relating to upper sonic waves or vibrations. For the purposes of this document upper sonic waves are acoustic waves in the frequency range between 10 and 18 kHz (Mi 74).