A German Hum researcher was given access to two insulated chambers, the first shields acoustic, electrical and magnetic signals.
The EM shielding works excellently for frequencies above 1kHz and the acoustic shielding is very good at all frequencies.
The second chamber was not built with acoustic shielding but does block most ambient noise and it shields all magnetic and electrical fields.
It has the best shielding standards in the world, no chamber has better data, neither the Earth's magnetic field or any EM is detectable within this chamber.
These chambers are most probably the quietest places on Earth and yet the Hum was "heard" in both.
This adds to the mounting evidence that the Hum is neither acoustic or electromagnetic and therefore must have some other cause.
Further Evidence.
As is well known, the majority of Hum sufferers can only hear the noise inside of a building, this is an important fact which as yet, has not received sufficient attention or a satisfactory explanation.
However, for those who can hear the Hum outside, a simple but very important observation can be made.
Go to an open field where the Hum can be hear, and then lay flat upon the ground.
It will be found that after a delay of a few seconds the Hum will disappear, however, upon the observer returning to a standing position the Hum will reappear.This is an important fact because the same sequence does not take place inside a building where the Hum can be heard.