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 contacts were to be initiated on this frequency, although once established the participating stations could shift to another frequency to avoid the congestion on 500 kHz. To facilitate communication between operators speaking different languages, standardized abbreviations were used including a set of "Q codes" specified by the 1912 Service Regulations. Article XXI of the Service Regulations required that whenever an SOS distress call was heard, all transmissions unrelated to the emergency had to immediately cease until the emergency was declared over. There was a potential problem if a ship transmitted a distress call: The use of 500 kHz as a