Decibel level calculations

[Greybeard]

Unfortunately, it's looking like a gas well drilling company is about to get a variance to be able to drill the first of who-knows-how-many wells about 600' from our "outdoor classroom" at the range. Their "sound management plan" predicts approximately 77 dB(a) when measured 300' from the rig. (Coincidently, the max supposedly allowed by the little town ordinance is 78.) Somewhere in my notes from one of the meetings, I have "each doubling of distance results in a reduction of 6 decibels". Truth or trash?

Once their pad site is cleared, there may not be much between us and the pad site except a (perpendicular) asphalt road, our gravel parking lot and about 150' of grass. Any SWAGs on how loud it's gonna be?

[seamusTX]

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

[Greybeard]

I wish it was 600 yards, but it's to be less than 600 feet.

Gun show table at Market Hall is unmanned just, now, so I best get on down that way. Thanks for the "calcs".

[jimlongley]

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.

[Greybeard]

Thanks for the replies, guys.

The following table (from a musician's web site) helped simply things a bit for me.

Perceptions of Increases in Decibel Level
Imperceptible Change 1dB
Barely Perceptible Change 3dB
Clearly Noticeable Change 5dB
About Twice as Loud 10dB
About Four Times as Loud 20dB

There are some residential back yards even closer to the proposed well site than our classroom, so those folks could be even more unhappy campers once they get cranking. What the drilling company has done at another current site in the area is stack 8' X 8' X 40' shipping containers 2 high around the perimenter of the pad site ...

[flintknapper]

The noise is not really that bad. There are five wells on our property...and one was drilled fairly close to my home.

The drilling goes on non-stop 24/7 for about 21-25 days on average....so the "constant" noise is kind of irritating at times. Once in awhile they will bang some drill stem together...you'll definitely notice that...but otherwise its a combination of a constant drone with the occasional high pitched noise thrown in.

The lower frequency sound is not too objectionable. The "fingernails on a chalk board" (high pitched) sounds are bothersome....but infrequent.


Good luck to you,

Flint.