Mr. Dick Bowdler:

There is some confusion about low-frequency noise and infrasound. Vibration gets muddled into it sometimes. When we talk about vibration, we are talking about vibration transmitted through the ground. It has nothing to do with what we hear with our ears but with what we feel with our bodies when we are lying in bed or sitting down, for example. Infrasound is generally thought to be any sound below 20 Hz or, in old terminology, 20 cycles per second. It is quite a lot lower than the hum from a piece of electrical machinery. It is generally thought to be the lowest level at which sound can be heard.

Anything below 20 hertz is call infrasound. Between 20 hertz and 200 hertz, about the A below middle C on the piano, is generally called low frequency noise.