Monday, Jan 23, 2012 at 08:19
My apologies Bill - I thought you might be having a lend of us suggesting a 5kva with those noise readings ! :-)
In these situations, noise is usually measured in dbA - a scale that has the high and low freqencies removed as they are beyond the capabilities of the human ear.
It is a logorithmic scale (as are the others) and, as some have corrrectly said, an increase of 10 dbA from, for example, 70 to 80, represents a 10 fold increase in perceived noise (with 3 being an approx doubling). Thus your reading of 68 (possibly not dbA - depending on the instrument you have and the settings) at 15m is some 10x the 55 to 60 you mentioned as acceptable.
A couple of other factors might be useful for your considerations.
As a general rule, sound reduces as a function of the square of the distance, that is, double the distance, a quarter of the sound.
Some frequencies can be heard over the greater distance - hence being able to hear the low frequency drums in dorf dorf "music" a long way away.
Sound is more noticeable and more annoying at night when the ambient noise level is lower and (most) people are trying to sleep.
Sound carries a long distance over water, so if you are on the opposite side of a lake (for example) to a noise source, it will be much more noticeable than the same distance over land.
Andrew
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