Refers to sounds or signals whose frequencies are below the normal human hearing range, generally considered to be 20 Hertz (see DJzone 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.
These are frequently also referred to Subsonic. Frequencies from 1 to 20 Hz which are inaudible but which causes visible oscillation of the speaker woofer cone caused by turntable resonances, floppy floors, acoustic feedback, bad recordings, and aggravated by vented speaker systems cause great unnecessary use of amp power, IM distortion and occasional woofer destruction.