Hi Greg--
Thanks for chewing on the question and the useful links. So I'd need
to have A-weighting metering in the recorder to establish the 0dB
reference,.. Could one possibly apply A-weighting to the pink noise
stimulus and bracket many speaker playback levels until one came out
close to producing o dB as measured in the recording post with
A-weighting? That would require a a nearly anechoic setting for the
speakers and mics I bet.
There is an absolute measurement vs. perception debate embedded in
the use of metering. I'd guess that Gordon and others started
bringing meters along to produce more scientific evidence-- not to
appreciably magnify their awareness.
If I could buy a SLM that measures under 20 dB for $200-300, I'd own
one out of sheer curiosity. I agree that there are few connections
between understanding perceptual noise and broadband amplitude
measurements even with the best curves we can dream up.
Since so many recordists use the same very low noise mics, recorder
and gain when recording natural settings, it seems like an idea worth
exploring. The numbers might apply to past recordings and others with
the same gear. Rob D.
At 12:04 AM -0800 11/24/10, Gregory O'Drobinak wrote:
>Rob:
>
>I don't think so.
>
>In the first case, during the comparison of your sound level meter (SLM) l=
evel
>(dBA) to the meter on your recorder (dB), you are comparing apples to oran=
ges,
>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 en=
ergy
>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 would 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. Yo=
u
>cannot use onlythe plain old level meter on your recorder to set any kind =
of
>reference level relative to dBA. I am sure that the whole
>calibration process is
>a bit more involved than this, but I will leave it to Dan or someone else =
that
>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>http://www.rane.com/par-=
w.html#weighting_filters
>and
><http://en.wikipedia.org/wiki/Weighting_filter>http://en.wikipedia.org/wik=
i/Weighting_filter
>
>If you want to dig deeper, try B&K's website (they make the best
>level meters -
>I've used them!):
><http://www.bksv.com/Library/Primers.aspx>http://www.bksv.com/Library/Prim=
ers.aspx
>
>My own opinion is that A-weighting does not necessarily relate to how my e=
ars
>actually perceive "noise level" or environmental sound level. In
>essence, my low
>frequency hearing is quite good and my perception of the level of
>the LF part of
>the sound spectrum may not be in line with what A-weighted filtering
>is telling
>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 v=
alid
>approximation of a "generally perceived" sound pressure level at only one =
SPL
>value!! What about SPLs greater or less than 30 dB? I'll stick with
>Z-weighting,
>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, thi=
ngs
>are different than they were in the field (not that the mics had
>anything to do
>with it! :>}).
>
>I really do think that the whole idea of making an "absolute"
>measurement of the
>SPL (sound pressure level - sorry if I didn't define this term earlier)
>accurately in an arbitrary environment is more complicated than people thi=
nk,
>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 h=
ow
>does one translate that into an actual measurement? Also, over what
>time period
>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 meters
>depending on what type of signal (speech, noise, etc.) we needed to measur=
e.
>
>IMHO, if you want a quick & dirty perspective of the SPL in a natural sett=
ing,
>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 marker 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 =
may
>not be strictly and closely related to the recorded SLM numbers. At least =
you
>have some constant (yet relative) reference for your recordings. Calibrati=
ng
>your recorder & mics means you can never deviate from that one configurati=
on,
>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
>
>________________________________
>From: Rob Danielson <<type%40uwm.edu>>
>To:
><naturerecordists%40yahoogroups.com>=
m
>Sent: Tue, November 23, 2010 11:20:10 PM
>Subject: Re: [Nature Recordists] Measuring ambient sound levels
>
>At 9:09 PM -0500 11/23/10, Dan Dugan wrote:
>>
>>
>> > Does the meter's use of A-weighting present a problem for the
>>> low-noise mics/recorder extrapolation method--with no matching
>>> weighting in the recorder? Rob D.
>>
>>I've just been through a calibration process for several of my
>>systems, and A-weighting does matter. One must measure the recorded
>>tracks with A-weighting. Pro Tools has an A-weighted meter in the
>>PAZ meters supplied with the software, but they are difficult to
>>read and require a correction factor (they measure combined stereo
>>level -6 dB. Spectrafoo doesn't have A-weighting. My best bet so far
>>is a set of parameters for three equalizer plug-ins in series to
>>create A-weighting, the result measured with Spectrafoo. That
>>requires a level adjustment to calibrate to unity gain at 1K, but it
>>can be recorded in the setting of one of the equalizers.
>>
>>There's also an interesting "true peak" meter in Pro Tools 8 but I'm
>>remote right now and can't remember its name.
>>
>>-Dan
>>
>
>Makes sense. Elemental Audio's Inspector XL plug has A, B, & C weighting.
>
>I'm looking at evening presence from very distant traffic right now.
>Using A weighting changes -24 dB RMS to -50 dB RMS.
>
>For discussion purposes, might a extrapolation method this crude
>work? (1) Set my recorder's gain at the level I normally use. Low cut
>filtering OFF. (2) Outdoors, point my low noise mics and my
>inexpensive SPL meter, side by side, at a good, full range speaker 1
>meter away. (3) Play pink noise and adjust the playback level until
>I get a meter reading on the recorder of 0dB. (4) Note the A weighted
>reading on the SPL meter generated by the pink noise.
>
>If the A weighted SPL reading of the pink noise at 1 meter is 70 dB
>and a field recording made with the same mics/gain is -50 dB RMS
>measured in post with A weighting, would the ambient sound level of
>the location be around 20 dB SPL (A)? If it was something simple
>like this, it would provide a useful reference for one's recordings
>in general, right? Rob D.
>--
--
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