Original
Message 1/7
Date: 20-Aug-04 @ 03:22 PM -
RMS dB?
Can anyone give a SUCCINT definition of RMS dB? Maybe with a diagram.
Progs such as Cubase give the option of Peak or RMS meters, but what do the
little lights really mean?
I understand Peak dB ok, as a relative difference in power equal to ten times the
common logarithm of the ratio of two signal levels (or voltages). For power
(voltage) dB=10log10(V/Vref).
For amplitude (sound pressure) dB=20log10 (A/Aref) because squaring a value
doubles its logarithm.
Appending the suffix 'U' denotes that the ratio is relative to 0.775V RMS and 'V'
expresses the ratio relative to 1Volt RMS.
Simplifying the problem: If I take a 'pure' sine wave, my bad maths leads me to
think that any Root Mean Square dBV value over a complete cycle must equal
zero. I'm just bright enough to know I must have this wrong! But why is it wrong?
I checked several sites but can't find an explanation that makes me go "Oh of
course!" In fact, the more I've read the more I'm sure I've missed a vital clue.
This is what comes of having a virus and not making music!