The Hum is perceived as a low frequency sound but has nothing to do with sound waves. The illusion of sound is generated inside the brain.
The Hum is caused by gravity waves that are created as a result of an interaction between the power grid and the ionosphere.
Besides giving rise to a constant irritating noise, these gravity waves produce headaches, nausea and chronic insomnia.
The waves also affect the body temperature giving rise to over-heating, this can be dangerous to the very young
question and reply from the European Commission
A recording of the Hum made in Bristol and showing the phase changes responsible for the heaches and nausea experienced by Hum sufferers is available as a video on YouTube Bristol Hum