Ultrasound independent Advisory Group on Non-ionising Radiation (AGNIR) produced a report in 2010, which was published by the UK Health Protection Agency (HPA). This report recommended an exposure limit for the general public to airborne ultrasound sound pressure levels (SPL) of 70 dB (at 20 kHz), and 100 dB (at 25 kHz and above). Ultrasound Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is not different from "normal" (audible) sound in its physical properties, except that humans cannot hear it. This limit varies from person to person and is approximately 20 kilohertz (20,000
60-meter band bandplan strongly recommends that the WRC-15 frequencies should only be used if other 5 MHz frequencies, allocated under Article 4.4, are not available. It does not require stations to adopt the USB dial frequencies of 5354, 5357, 5360 and 5363 kHz but these frequencies provide a good fit with the American channel on 5357 kHz, and UK stations which can use 5354 and 5363 kHz (but not the middle two). Also, it is only by using these exact frequencies that there will be enough room for four simultaneous SSB conversations without mutual interference, assuming the 2.8 kHz "de facto" standard