There are a couple of great articles at How Stuff Works on the subject of dB.

They are: What is a decibel, and what is the loudest sound I can listen to before it hurts my ears? and Decibels.

The second article explains when to use 10log and 20log...

-Trevor

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Mk 2, Green 12GB 080000349
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-Trevor

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Mk 2, Green 12GB, Tuner, 2.0b11, 080000349