Tan A., Tanaka N.

It is categorically convened that industrial noise involves the human-audible sounds from machineries especially those which are generated at high decibels. However, nowadays, new ultrasonic applications have seen much widespread prevalence in some electrical equipment; ultrasonic toothbrushes, ultrasonic contact lens cleansers, car parking sensors and medical sonar-devices. Although the present diminutive occupational and domestic ultrasonic devices do not overly emit intense airborne ultrasounds, they could signify a starting trend of newer and more powerful ultrasound radiating devices to come. Particularly, for an example, is the growing application of an innovative ultrasonic speaker; parametric array loudspeaker (PAL). The PAL is a unique loudspeaker in the sense that it emits finite-amplitude ultrasounds (~50kHz, ~120dB) so as to exploit their nonlinear interaction in the atmospheric air, hence producing a highly directional insonified column of audible sound. The mechanism of the PAL in unbounded medium is discussed since its operation is closely related to the propagation of ultrasound in air. Presently, most applications concerning PAL thus far utilize its far-field region for directional audio listening purposes. Hence, safety issues regarding intense airborne ultrasonic waves impinging human become imperative. Although human exposure of intense ultrasound can be a subjective matter, many past research in this non-ionising radiation have hinted indicative evidences to suggest potentially real undesirable effects, both auditory and bodily. These can be ascertained by some primary dominating factors, namely sound pressure level of the ultrasound, frequency of the ultrasound and the lesser reported ultrasound exposure time, and secondary factors such as thermal index and mechanical index. It is intended to provide an account of existing global guidelines and standards on the airborne ultrasound exposure limits to indicate their safety relevance governing the PAL. Finally, a brief review on the status of airborne ultrasound on human beings in Japan is being discussed.