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
History of broadcasting in Australia there is just a small change in format. 1920s all broadcast services allocated to a few specific frequencies, MF + LF early 1920s Class A Services low end of AM band & longwave, Class B services upper end of band, Amateurs above 1400? kHz mid 1920s AWA-influenced longwave services commenced to be switched off due to difficulties in achieving radiator efficiency late 1920s Australia commences to allocate AM radio services in the range between 1400 kHz and 1500 kHz 1926 AM restack 1930 Captain Eckersley and Radio Research Board give consideration to widespread deployment of longwave services, but eventually it