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"A number of Damage Risk Criteria and Maximum Permissible Levels were first recommended by individual researchers in the 1960s. These tentative recommendations, supported by limited experimental and survey data, were then taken up by national and international bodies.
For the very high frequencies, 10-20 kHz, the limits were given as one-third-octave band Sound Pressure Levels in the range 75-85 dB, to avoid unpleasant subjective effects in exposed persons; higher noise levels were found to cause annoyance, tinnitus, headaches, fatigue and even nausea.
For ultrasonic components above 20 kHz, the limits were set to avoid hearing damage in the audible (lower) frequencies. One-third-octave band levels of 105-115 dB were observed to produce no temporary hearing loss, and were therefore judged non-hazardous in respect of permanent hearing damage.
Since the introduction of these recommended limits, there have been no reports showing systematic hearing loss trends associated with occupational exposure to very high frequency noise."
