The human ear cannot detect sound frequencies of less than 20 vibrations per second i.e. 20 Hz.
Human beings are normally able to detect sounds in the range of 20-20,000 Hz and it is well known that sounds within this range can damage hearing.
However, sounds under the frequency of 20 Hz can also affect our ear even though we are unable to hear them
A young ontologically healthy person in the workplace can detect a signal sound over the frequency range of 20 to 20000 Hz, but the level of the signal sound required for detection depends on such variables as the frequency of the sound, the duration of the sound, and the nature of any other sound that may be present at or near the same time as the signal sound that may mask the signal sound.