Hi Adrian,

“dB” is a ratio expressed logarithmically. 0dB means it is the original value or an agreed standard.

Start with:
http:/en.wikipedia.org/wiki/Decibel

Very good is:
http://www.sengpielaudio.com/
If you don’t mind a few German words thrown in with the English

also:
http://www.sengpielaudio.com/Calculations03.htm
and
http://www.sengpielaudio.com/calculator-db-volt.htm


Best wishes,

Herbert