Low frequency noises can be very difficult to find the source of as that are not directional. I have had low frequency sounds that have been driving
me crazy, cause poor or no sleep, and as someone else mentioned, I leave my bedside radio on, or even my computer, in order to mask the noise. In my
case though, I know the source, but the authorities would do nothing to help me. It was from a neighbour, two actually, who had permits to grow
medicinal MJ, and the low frequencies were from their ventillation fans. Becuase the noise was not 'loud' it only registered 60db or so on a meter,
about the same as an air conditioner at 100 ft, but because I live in a quiet rural area, at night it seemed a lot louder. The local by-law
authorities would do nothing.
I showed them research on how low frequency sound can affect the mind and body, and has even been used as a torture device by some governments, but
still, "it's not loud" was the answer I got. I finally went to the police, who said I could file charges of "assault by noise", and he must have
talked to the neighbor who made some changes, and now it is better, still not the silence I should be able to have, but I can sleep without having to
use a masking sound.
Many odd occurences can take place, one for instance is due to wave mixing, so the sound of a truck in the distance would mix with the LF noises from
my neighbour and create a "beat wave", due to the mixing of the soundwaves. The same also happened when the neighbours at the other side of me
started their own legal grow-op, as I was inbetween two noise sources, each quiet by themselves, but at times my windows would vibrate from the
combined sound waves. Luckily the landlady in the second house was sympathetic to my complaints and had the tennants make changes, which illiminated
I sympathise totally with the people who are suffering from wind turbine noise, but until it is accepted that these quiet, but low frequncy noises can
have some very nasty effects on us, they will keep building them. They have even found that animals near wind turbines will exhibit sometimes violent
behaviour, like killing their young or spontaneous fighting, so it's not just affecting humans.
So the larger scale hums could be from combinations of noise, and from long distances away, as the LF waves can travel far. A combination of natural
and man made sources I'd guess might be at work in many cases.
Acoustic Trauma : Bioeffects of Sound