Decibel level question

[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-know-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?

[AmbulanceDriver]

Greybeard, sound intensity follows what's called the "inverse square law".  Which means that for every doubling of the distance, the sound intensity is just one quarter of what it was.  This of course, applies only to perfect conditions, such as a point source and measurements taken as a sphere with no reflections/reverberations.  But even in "real life" scenarios, we can use the same formula for estimates.  If at 300 feet, the sound level is 77dB, then according to the inverse square law, at 600 feet you're looking at approximately 71dB, and at 1200 feet, 65 dB.  So their estimate of losing 6 dB every time the distance doubles is pretty accurate. 

[mgdavis]

That's about the same level as a normal conversation.

http://www.gcaudio.com/resources/howtos/loudness.html

[Greybeard]

Thanks to all. This was especially helpful:

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

[280plus]

What? 

[AJ Dual]

Yes, and as greybeard noted, Decibels are a logarithmic unit of measurement. A drop of just one Decibel is an order of magnitude, which is why conversations, TV's etc are 70-odd decibels, a shout is like 80, a rock concert 90, and a jet engine 100+...

[AmbulanceDriver]

Actually, it's not quite an order of magnitude.  The scale is logarithmic though.  One Bel (10 deciBels) increase represents a doubling of the sound intensity.  So 70 dB is twice as loud as 60 dB, and 80dB is 4 times as loud as 60dB.  Which is why when you get into sound intensities of 90, 100, 110+ dB, the hearing damage occurs so much more rapidly.