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
500 kHz 1 MHz). International standards for the use of 500 kHz were expanded by the Third International Radiotelegraphic Convention, which was held after the sinking of the RMS "Titanic". This Convention, meeting in London, produced an agreement which was signed on July 5, 1912, and became effective July 1, 1913. The Service Regulations, affixed to the 1912 Convention, established 500 kHz as the primary frequency for seagoing communication, and the standard ship frequency was changed from 1,000 kHz to 500 kHz, to match the coastal station standard. Communication was generally conducted in Morse code, initially using spark-gap transmitters. Most two-way radio