Rob:
I don't think so.
In the first case, during the comparison of your sound level meter (SLM) le=
vel
(dBA) to the meter on your recorder (dB), you are comparing apples to orang=
es,
i.e., the SLM has the proper A-weighting filter to roll off the low end of =
the
pink noise spectrum, but the recorder is really measuring all of the LF ene=
rgy
from the reference speaker. Thus your '0 dB' recorder meter reading would
actually be much less after applying the same A-weighting filter to that
recorder's "reference" recording, let's say about '-8 dBA'.
Then when you play back "a field recording made with the same mics/gain is =
-50
dB RMS measured in post with A weighting", the actual A-weighted level woul=
d be
70 dBA- [-8 - (-50)] dBA =3D 28 dBA. So you can see that you must use
the A-weighting filter at all times to peg the levels on your recorder. You=
cannot use onlythe plain old level meter on your recorder to set any kind o=
f
reference level relative to dBA. I am sure that the whole calibration proce=
ss is
a bit more involved than this, but I will leave it to Dan or someone else t=
hat
has gone through the rigors of setting up gear to do such measurements.
For some good & simple A-weighting references try
these: http://www.rane.com/par-w.html#weighting_filters
and http://en.wikipedia.org/wiki/Weighting_filter
If you want to dig deeper, try B&K's website (they make the best level mete=
rs -
I've used them!): http://www.bksv.com/Library/Primers.aspx
My own opinion is that A-weighting does not necessarily relate to how my ea=
rs
actually perceive "noise level" or environmental sound level. In essence, m=
y low
frequency hearing is quite good and my perception of the level of the LF pa=
rt of
the sound spectrum may not be in line with what A-weighted filtering is tel=
ling
me. Also note that the A-weighting filter curve is the inverse of the 30 dB=
-SPL
equal-loudness curve of Fletcher-Munson. So A-weighting is really only a va=
lid
approximation of a "generally perceived" sound pressure level at only one S=
PL
value!! What about SPLs greater or less than 30 dB? I'll stick with Z-weigh=
ting,
so I'm just going to go with what my recorder's level meter says since
everything is relative anyway when you start adjusting gains to grab a
recording, you may be using different mics, they may not be matched, etc. =
Let's
not forget that whatever you do in post can also change things profoundly. =
As
soon as you start filtering out the LF haze, or adding mid or high EQ, thin=
gs
are different than they were in the field (not that the mics had anything t=
o do
with it! :>}).
I really do think that the whole idea of making an "absolute" measurement o=
f the
SPL (sound pressure level - sorry if I didn't define this term earlier)
accurately in an arbitrary environment is more complicated than people thin=
k,
especially in natural spaces. I believe that one's hearing & perception can=
vary
widely depending on the type of sounds that are being heard/recorded, so ho=
w
does one translate that into an actual measurement? Also, over what time pe=
riod
does one integrate the energy in order to produce a valid number? This can =
vary
depending on whether the source is impulsive/short-lived or constant. When =
I
worked in telecom, we used different weighting filters and other types of m=
eters
depending on what type of signal (speech, noise, etc.) we needed to measure=
.
IMHO, if you want a quick & dirty perspective of the SPL in a natural setti=
ng,
grab your trusty SLM and write down the dBA and dBC readings for each time=
weighting that is available on the unit and tag the measurement with a mark=
er on
the recording. You can then compare these readings over a period of time to=
what
you have recorded to get a feel for what the perceived SPL was. It may or m=
ay
not be strictly and closely related to the recorded SLM numbers. At least y=
ou
have some constant (yet relative) reference for your recordings. Calibratin=
g
your recorder & mics means you can never deviate from that one configuratio=
n,
change wind protection, etc. Too much work for me.
I'd be curious to know Gordon Hempton's opinions on this subject.
Take care,
Greg
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