Refers to sounds or signals whose frequencies are below the normal human hearing range, generally considered to be 20 Hertz (see WFTD archive Hertz). The lowest audible frequency is not easy to absolutely define as it depends strongly on level. Some experiments have found that hearing can extend down to 10 Hz at very high levels. Sometimes the term “subsonic” is wrongly used to mean infrasonic. Subsonic actually refers to the speed of sound propagation through a medium and has noting to do with frequency or pitch.