decibels

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From: Mark Lawton (markslawton@hotmail.com)
Date: Wed Jan 15 2003 - 14:10:32 PST 

From: "Mark Lawton" <markslawton@hotmail.com>
Subject: decibels
Date: Wed, 15 Jan 2003 14:10:32 -0800
Message-ID: <F132X3qrB0Xn2Pi1Y7P0002bcc0@hotmail.com>

Here's what I believe is going on with the decibel situation.

There are two uses of decibels:

The first is what one hears and your description holds:

0 = threshold of human hearing.

The other case is a measure of the electrical power of the amplifier output
versus it's input.

The scale is logarithmic:

The number of decibels = 10 log (power output/power input)

So:
10 decibels means that the power out of the amp is 10 times the power of the
input signal (because 10 = 10 * log 10)

20 decibles means that the power out of the amplifies is 100 times the power
of the input signal (because 20 = 10 * log of 100)

negative 60 decibels means that the power out of the amp is 1 million times
smaller than the input (because - 60 = 10 * log of (1/1,000,000 ) this
means that the amp is actually cutting the signal down.

-Mark

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