| The units of frequency are named after the German physicist Heinrich Rudolf Hertz, 1857-1894. One hertz (abbreviation Hz) equals one cycle per second. Sound with a frequency above 20,000Hz is known as ultrasound. Most sonographers use a range of frequencies from about 1MHz (CW transducers) to 13MHz. One megahertz (MHz) is a million hertz. Dolphins are not capable of producing ultrasound within the diagnostic imaging range. They use between 40 - 150 kilohertz to echolocate fish. Low frequency - low intensity ultrasound is thought to be very safe (which is no consolation to the fish). |