| Originally posted by ulysses |
A voltage or SPL level can be expressed in terms of RMS. A ratio between two RMS levels can be expressed in decibels, which is a logarithmic calculation.
which is where all those different db-expressions (and misunderstandings) come in...
logarithmic conversion basically goes like
L = A*lg(X/Y)
where X,Y - compared rms values; A - scaling factor.
when inserting spl (sound pressure level), A equals 10.
when inserting volts, X equals 20 (proof of which would be too confusing here i guess)
for relative comparison of two measured values, insert them for X & Y, and get dBr (r - relative).
for absolute levels, you want to take one measured value and relate it to a commonly known base value. so it's like
Lp = 10*lg(P/Po) or Lu = 20*lg(U/Uo) respectively.
Po is 2*10^-5 Pa (pascal) and relates to the human hearing threshold @ 1kHz. this gives you dB SPL.
Uo can either be 1V (for dBV) or .775V (for dBu or dBm). dBV is easier to handle without a calculator and is found more in the consumer field i believe. the others are standard in pro audio, the odd value being due to historical reasons. the conversion is pretty easy though, it gets pretty close to dBV - 2,2dB = dBu.
there's a ton of other dB-expressions, like e.g. dB(A), which is a frequency-weighted (i.e. filtered) curve, approximating the frequency-dependent human ear. or, in the digital world, you'll come across dBfs (full scale). where 0dBfs = maximum, any positive value will mean overload.
hope this helps