The generally accepted range is 20 to 20,000 Hertz (Hz)(Cycles per
second), although there does of course have to be a certain volume of
sound.

A common low frequency sound is the AC hum coming from electrical
equipment such as transformers and fluorescent lights. This is the mains
AC frequency of 50 Hz (60 Hz for you turbocharged Americans). This was
the original speed at which 2 Pole AC alternators could be reasonably
driven (3000 RPM - 3600 in America) by the old steam engines -this
frequency is very accurate for anyone that wants to carry out a
frequency check, since the utility companies are mandated to maintain
accuracy in order to keep our electric clocks accurate.
Alan Riach
Bathgate,Edinburgh
Scotland