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From: wlsmith@valve.heart.rri.uwo.ca (Wayne L. Smith)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: db's?
Message-ID: <3201@ria.ccs.uwo.ca>
Date: 31 May 91 16:02:50 GMT
References: <1991May31.001234.7155@agate.berkeley.edu>
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Organization: The John P. Robarts Research Institute, London, Ontario
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In article <1991May31.001234.7155@agate.berkeley.edu> danny@garnet.berkeley.edu (Danny Lee) writes:
>
>	Ok, stupid question of the day/week/month . . .   Why is the decibal
>scale expressed in negative numbers?  Or should I say, why is 0db louder than
>-45db?  Also, is there  a differance in acoustic db and electrical db's?
>Thanx in advance.

This should be asked in sci.electronics, but...
Db is a log scale of measured intensity versus an absolute reference.
If you send a 1V signal through a box and get 50V output, the output is
10 x LOG (50/1) = 17 db higher than the input.  If the output is 0.003 V, then
the output is 10 x LOG (.003/1) = -25 db.
A negative db measurement indicates a loss or reduction, and a positive db
indicates a gain.
Acoustic db's relate so sound `pressure' (a physical phenomena), while
electrical db's can be signal voltage or power.
I don't know what the 0 db reference is for sound, but 120 db (ie 1x10E12)
above that reference is, I think, either the threshold of pain, or a jet
engine at 10 ft.