09-14-11: Almarg
Paperw8, the fundamental misconception you have, which ultimately leads you to incorrectly assert that first order filters roll off at 3db/octave, is the notion that the numerical db value describing the ratio of two signals applied to a given resistive load will be different depending on whether voltage or power is being considered.
while you have posted many informative comments on this forum, one of my criticisms of you is that you post references that you seem to have only half-read, and then draw conclusions that don't stand up to a closer reading of the reference.
as to the first reference you cited, the writer presented a first order RC filter. he also showed the transfer function for the filter. so if you really do want to know the behavior of the circuit, don't take my word for it, just look at the equation and figure it out for yourself.
if you look at the *asymtotic* behavior of that circuit shown (and by "asymtotic" i mean frequency>>RC), the transfer function is:
v_out=(C/f)*v_in
where C is a constant and f is frequency.
let me first establish that an "octave" means a doubling of frequency. given that, for each doubling in frequncy, the output voltage v_out is reduced by half relative to v_in. so if you want to compute the decibel change in voltage for each octave:
v_dB=10*log(v_out(f)/v_in(f))
v_dB=10*log(1/2)/octave
v_dB=10*(-0.3)/octave
v_dB=-3dB/octave
that's the result; it's not subject to your opinion or my opinion, that's just what it is.
09-14-11: Almarg
I'll mention, btw, that early in my career as an electrical design engineer, a great many years ago, I had the exact same misconception, until my boss enlightened me.
The value of that number is one and same, whether voltage or power is being considered. For a given resistive load, reducing the applied voltage by a factor of 2 reduces power by a factor of 4 (as you agree), and the change in signal level is 6db. Period.
what "signal level" are you referring to?
i've got a deal for you almarg, if you're so convinced that i am wrong, then rip an *unedited* copy of my postings and send them to henry ort and ask for his comments on them.
that will be interesting...