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View Full Version : Decibels and sound amplification

Tog
2010-Apr-03, 01:36 PM
This might actually be a general science question.

First. Is there a simple way to determine the amount of reduction a sound might get over distance. Say I have a sound source that is about 120 dB 20 meters away, in 50 degree (f) air at about 75% humidity. What would that be by the time it reached my position 20 M away?

The next part is using a parabolic microphone that can amplify the sound about 30 times. How do I figure how loud that would be compared to the non-amplified sound at the same range?

Also, 30 times louder than 100 dB is about 113, right? Not 130?

My set up is that I have a guy doing audio surveillance when one of the people suddenly shoots the other. I know it will suck, and probably blow out the headphones, I'm just wondering if I need to worry about blown eardrums.

Thanks

grapes
2010-Apr-03, 02:53 PM
The next part is using a parabolic microphone that can amplify the sound about 30 times. How do I figure how loud that would be compared to the non-amplified sound at the same range?

Also, 30 times louder than 100 dB is about 113, right? Not 130?30 times 100 dB would be about 115 dB, the logarithm of 30 is 1.477. Is that what you mean?

My set up is that I have a guy doing audio surveillance when one of the people suddenly shoots the other. I know it will suck, and probably blow out the headphones, I'm just wondering if I need to worry about blown eardrums.Depends upon the system, I suppose. Don't most have automatic cutoffs, for that reason?

Tog
2010-Apr-03, 03:13 PM
30 times 100 dB would be about 115 dB, the logarithm of 30 is 1.477. Is that what you mean?
Depends upon the system, I suppose. Don't most have automatic cutoffs, for that reason?

Thanks. That was what I mean, yes. I was at work and just guessed on about what it would be.

Some of the ones I looked at online did, but they were kind of expensive. The cheaper ones didn't seem to mention one.

Donnie B.
2010-Apr-03, 06:11 PM
Unfortunately, sound in air is rather complex and there are many variables -- temperature, humidity, wind, and so on -- other than mere distance.

If all else is equal, sound pressure follows an inverse square law. But seldom is all else equal.

Another complication is that sound propagation distance varies according to the frequency of the sound. Distant thunder will produce only low-frequency rumbling; the same thunder from a nearby strike will include (loud!) high-frequency components.

Your scenario reminds me of The Conversation -- have you seen it? Great film.

Kaptain K
2010-Apr-03, 07:41 PM
As for the eardrums, although heaadphones can get painfully loud, most headphone amps are one watt (or less) so the odds of actually puncturing them are slim.

FTW: I have actually had an eardrum punctured, but it took an under water kick to the ear to do it!