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
High Frequency Global Communications System frequencies are in use. The primary HFGCS voice frequencies are 4724.0 kHz, 6739.0 kHz, 8992.0 kHz, 11175.0 kHz, 13200.0 kHz and 15016.0 kHz. In addition to the HFGCS, U.S. aircraft frequently use Military Affiliate Radio System (MARS) HF stations (13927.0 kHz) and Canadian Forces HF stations (11232.0 kHz) to relay messages. One common use for the HFGCS is to place telephone calls from the aircraft in flight by means of the Defense Switched Network (DSN) to an Air Force base to obtain local weather conditions, to arrange for refueling, and to inform the base of the number of passengers and