The A-law issue is actually more complex than just 'levels'.
One issue here is that there are quite possibly an infinite number of shape=
noise spectra that can register as the same A-law level. So if the self noi=
the mic is measured as being 16 dBA, the noise spectrum of that mic could f=
the low, the mids or the highs and still be read by the sound level meter a=
The Fletcher-Munson study that gave rise to the 40 phon (in level) A-law cu=
was done a long time ago and was formulated to account for the perception o=
individual tones, not random noise, back in 1933. Go to the audiologist
and that's what you get: tones to test your hearing, not even band-passed
In particular, A-law weighting of low-level background noise (such as the d=
of the city) does NOT correlate at all well with what most people hear, it=
devalues the low end perception of the spectrum severely. And A-law weighti=
does not account for our enhanced perception of low-level noise at around 6=
it devalues it by 10 - 12 dB. That is why the BBC and others did the resear=
account for this perception and formulated the ITU-R 468 spec. Note that th=
spec specifies not only the weighting curve, but also the precise method fo=
metering (measuring) the level of the noise.
ITU-R 468 is better (and much newer!) science, not an attempt to hammer
something into a 'compartment' in which it does not belong.
Here are some links:
http://www.rane.com/par-w.html#weighting_filters (look towards the end for=
different weighting filters)
I can see why Jez has an issue with A-law specs for mic self-noise. But is =
468 the be-all and end-all spec? For some people, maybe not, for their own=
perception may be heightened or stunted in ways different from the 'norm' o=
human population. But I do believe that applying A-law curve, which was des=
for the perception of tones, is not applicable to measuring noise in
any reasonable way. Why folks continue to apply it to such purposes is quit=
puzzling. Perhaps they didn't know before ( in the '60s) what we know now a=
there is just too much inertia to move away from A-law. It's sad that the m=
manufacturers haven't kept up with current perception theory and measuremen=
Human perception, especially hearing, is quite complex. Pick up a contempor=
psychoacoustics text and you'll see.
ITU-R 468 is much better than A-law for self-noise measurement, but it's st=
an approximation to what really happens in our minds when we listen to nois=
make our own judgements about it.
So what do we do now? :>}
Numbers can lie. I suggest we listen and make more comparison recordings of=
different mics on a common (good) recorder under identical extremely quiet=
conditions. And by identical, I mean not only the same background level, bu=
with the preamp gain calibrated to give the same output level for each mic=
according to a properly chosen calibration tone. Then we can make our own
judgements independently from the vendor's 'spec' sheets.
Let's move forward.