When awake, human brainwaves cycle between 15 & 18 Hz
you cannot hear this, at all

You can, however, custom build a burglar alarm that cycles at these frequencies
when tripped, sending out infa-sound pulses at these frequencies
rendering the potential intruder unconscious

The Nazi's were (for all the wrong reasons, mind) instrumental in exploring the human reactions to low frequency sound

Night clubs use a sneaky form of infa-sound in their speaker setup
I forget the frequencies involved, but the stomach resonants at a set frequency range
hitting these frequencies will induce hunger

Rumour has it, but I have no actual proof, that the clitorus resonates at about 33Hz

And so ends another episode of Frequency Fun with Tony
tune in next week to learn how to make someone shit themselves involuntarily