Ultrasonic surveillance monitoring, to check the operational integrity of Public Address (PA) and Emergency Communication Systems, has been in existence for over 40 years—particularly in Europe. Since its inception, there has been debate as to the potential audibility that these systems may have. As the vast majority of sound systems engineers and designers have not heard or experienced any effects, it has generally been assumed that the general public do not either. Recently, however, concern has been raised and claims of ill effects have been reported with respect to ultrasonic emissions. There is, however, little or no data as to the ultrasonic sound levels that sound systems actually emit. This paper discusses the results of an initial survey of ultrasound radiated by a sample of some 50 sound systems and compares the results with a number of international standards—there currently being little or no specific guidance. This paper reviews the technology involved, typical emission levels, and concludes by making a number of recommendations to assist with the control of ultrasonic emissions to help mitigate unintended side effects. It is shown that most PA systems generate ultrasonic levels of under 70 dB at normal listener locations.

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