Infrasonic sounds are audio frequencies that can not be heard by the human ear. I am talmbout audio material consisting of sounds that range from anywhere below 20 Hz to 0 Hz; however, ultrasound is anything above the upper limit of human hearing, id est 20 kHz.

In music production, infrasonic sounds are in most cases unwanted, and also in audio production jargon, they are referred to as either: muddy or low-end rumble. BTW, one of the best ways to get rid of all that nasty undesirable subsonic mayhem is to either use a high-pass filter (aka low-cut filter) or a low-shelf filter (aka low-cut shelf) when mixing or mastering music.

For example, cutting somewhere around 20 Hz, 30 Hz, 50 Hz, etc., of course depending on whether that's being done to an entire song or different individual instruments in a music project.
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