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
Non-line-of-sight propagation is an example of an active repeater that does change frequency. Communications satellites, in most cases, are in geosynchronous orbit at an altitude of 22,300 miles (35,000 km) above the Equator. Application of the Poynting Vector to vertically polarized plane waves at LF (30 kHz to 300 kHz) and VLF (3 kHz to 30 kHz) indicates that a component of the field is propagated a few metres into the surface of the Earth. The propagation is very low loss and communications over thousands of miles over NLOS links is possible. However, such low frequencies by definition (Nyquist–Shannon sampling theorem) are