db meters on amps..whats it all mean??? (1 Viewer)

[bob_andrew]

Have a quick question. i've seen for years those meters that some amps have that show its relative db output.
my question is how come the meters go from say -20db or so to +20db or so ? how can you have a negative db rating if theoretically 0 db is complete silence. i must be missing something here.So if any one could shed somelight on the subject and let me pick your brain on this i would be if nothing else alittle wiser anyway
Thanks.
Bob

 

[Chris PC]

The meters are not displaying audio dB. Instead, they are displaying electrical dB and in comparison to a reference of some kind, thats the 0. Correct me if I'm wrong.

 

[TomCW]

Chris is right... It's 0 dB = some reference level like 1 milliWatt (dBm), 1 Watt (dBW), 1 Volt (dBV) etc.
Tom

 

[Mark Tranchant]

The 0dB on most equipment is not even to a common reference. It is just a starting point for other levels to be based on.
The reason most equipment goes negative is that most preamp sections are actually attenuators: feeding the input directly to the power amp stage would result in huge output.
In acoustic terms, 0dB is not "complete silence" - it is the accepted threshold of human hearing, about 20 micro-Pascals of pressure variation.
Where the dB figure is against a defined reference, the dB is usually followed by a symbol to indicate that reference. So 0dB SPL is 20 micro-Pascals, 0dBm is 1mW into 600ohms, 0dBu is 775mV rms (same as dBm, but "unterminated"), 0dBV is 1Vrms.
6dB increase is a doubling of voltage; 3dB increase is a doubling of power. See the Decibels section at Link Removed.
So to answer your question, -20dB is 20dB below the reference value; if this is against dBu, for example, -20dB would be 77.5mV rms at some arbitrary point in the amplification chain.