It is not an absolute rule, but most people begin to hear the Hum around the age of 50, this is because of changes taking place in the body.
The Hum will appear quite suddenly and at first be intermittent, high levels at night and low during the day. It may also come and go for days and even weeks at a time.
During this initial stage the Hum level will be much stronger indoors than outside, in most cases it will be sufficient just to stand under the lintel of a door or window to observe a marked change.
During the next year or so the Hum level will slowly increase and eventually become permanent, also the difference in Hum level between indoors and outdoors will become blurred as the sensitivity reaches saturation and the sufferer will hear the Hum everywhere.
The search, which has been followed by many thousands over the past forty years, usually has two phases. The first phase involves looking for a low frequency sound around the house with central heating, fridge's and freezers, air conditioning and water pumps. The search then extends to the neighbours, local factories and super markets.
This procedure lasts about a year and often involves the local environmental health department, acoustic experts,doctors and the local press. The result is inevitably negative and no low frequency source is found.
The second phase concentrates on radio waves, particularly those of extremely low frequency used by the military, and at the other end of the spectrum, the mobile phones. This phase is more difficult and likely to be expensive in terms of equipment, however, after another long search it will become apparent that there are no radio waves which can account for the Hum.
The Hum sufferer, sadder but wiser, will now be back to square one and to make any progress it will be necessary to accept the following facts.
The Hum is not low frequency sound, the illusion of noise comes from a completely different form of energy.
The Hum cannot be recorded on a tape recorder or detected by any othe type of acoustic equipment, for the Hum has nothing to do with acoustics.
The Hum has been in existence for over 40 years and when a person starts to "hear" the noise it is not because of a new factory or transmitter or something the neighbours have just installed, it is the result of changes taking place within the person's own body
What started as a search for a noise around the house will be found to cover at least three continents and is most probably a global phenomena.
Failure to come to terms with these facts will result in years, or even a lifetime of wasted effort.