seamusTX wrote:That is correct. Sound pressure (that absolute measure of intensity of sound) decreases by the square of distance. So if you double the distance, you cut the sound pressure to 1/4 of its value. A reduction of 3/4 in sound pressure is equivalent to 6 dB.
So 77 dB at 300 yards will give you 71 dB at 600 yards.
That doesn't tell you how irritating or annoying the sound will be. Some frequencies and patterns of sound are more irritating than others.
- Jim
I spent a lot of time working with sound for the phone company and still have audio frequency spectrum analyzer software on my laptop.
One of the major problems with such "measurements" and predictions is that they can depend a great deal on the spectrum measured, and you can bet that the company sponsored measurements will be the most favorable to the company, and each different spectrum is arguably as valid as any other.
One case in point: When the city of McKinney, TX was trying very hard to shut down the Collin County Gun Range, one of the things they did after annexing the land on which the range was situated, was pass a noise ordinance. The noise ordinance, as written, specified a "dB level" of noise detectable at the property lines and such - the details are just a little hazy now, I wasn't more than peripherally involved. The problem was that the dB level was ill defined, no spectrum, and initially not even a reference level, and without a reference level, the "a" in dBa, then "dB' means absolutely nothing.
dB is not an absolute measurement, it is a logarithmic comparison of two levels, thus one compared to two is (simply put anyway) a three dB difference. It doesn't matter whether the units are volts, watts, newtons, phons, sones, angstroms, or miles per hours, a doubling or halving is a three dB change.
http://www.phys.unsw.edu.au/jw/dB.html
It also doesn't matter what the quantity of units is, one compared to two is a three dB change, and one million compared to two million is still a three dB change.
The "a" in dBa is an artificially but scientifically established level of sound equal to no sound at all, a minimum level that dB measurements are compared to - and then there is the spectrum selected to take the measurement in. The human ear does not hear all sounds equally. We are said, if we have perfect hearing, to be able to hear all sounds between 20Hz and 20,000 Hz, but we do not hear all of them equally, thus some spectra chosen for measurement emphasize different frequencies in order to mimic what the human ears hear, and measuring in a specific, and perfectly valid, spectrum will yield different overall dB results than measuring in another.
So, the gas drilling company's results will be the most favorable to them, and if you go to the effort and expense to hire someone to counter them, your results can be what you want them to be, and the only people who understand the difference will be a subset of a subset, and none if them will be on the town board or city council, as we ran into in McKinney.