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
Aspidistra (transmitter) seven bomb-free zones in central and southern Germany. All these announcements were false. German radio stations tried announcing "The enemy is broadcasting counterfeit instructions on our frequencies. Do not be misled by them. Here is an official announcement of the Reich authority." However, "Aspidistra" broadcasts included similar announcements, leaving the listeners confused. Although mainly intended for the military and propaganda transmissions described above, "Aspidistra" was also used during the war for some BBC European Service broadcasts to Europe on 804 kHz. After 1945, "Aspidistra" continued to be used by the BBC. Frequencies used included 1122 kHz (1945–1950), 1340 kHz (1950–1962),