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Re: Responses to ambient sound recording

Subject: Re: Responses to ambient sound recording
From: "Gregory O'Drobinak" gmo_dunes2
Date: Sat Nov 27, 2010 1:03 pm ((PST))
Hi!

I find it rather peculiar that folks are worried about a 2 dB gain error du=
e to
complications using a single 1 KHz tone for doing the calibration as Raimun=
d
suggested and yet are not worried about the huge calibration error due to u=
sing
the band-limited pink noise (BLPN) as a stimulus!

By using a single 1 KHz tone for doing the calibration, both your recorder =
and
the SLM (sound level meter) are reading the same energy and so the calibrat=
ion
is valid.

But by using the BLPN, the SLM is integrating that sound energy *after* the=

A-weighting filter has been applied. That means that energy around 200 HZ i=
s
attenuated by ~10 dB, so all of the energy gets progressively "discounted" =
below
1KHz by the weighting filter.

When you use that same BLPN stimulus with your recorder, it does not have t=
hat
same A-weighting filter built in, so its level meter integrates the sound l=
evel
energy *flat* down to the 200 Hz edge of the stimulus. So your "calibration=
" of
that recorder is off relative to the energy at 1 KHz, given A-weighting, an=
d
this calibration point is only valid for only *one* stimulus spectrum, name=
ly
that of a 200 Hz - 1KHz BLPN. I can come up with an infinite number of
band-limited noise stimuli that have different spectra, but will read exact=
ly
the same as a 200 Hz - 1KHz BLWN stimulus, on the SLM using dBA. This is a =
very
important concept and is one reason why other systems for measuring loudnes=
s
have been developed. I don't believe that A-weighting is the holy grail of=

loudness measurement.

Anyway, the point here is that your recorder meters level as a linear funct=
ion
across the whole frequency band (assuming it really has flat response), but=
 the
SLM meters level according to a non-linear frequency curve that attenuates=

energy quickly below the 1 KHz 0 dB point on the A-weighting curve. Therefo=
re
using any stimulus other than a pure tone at 1 KHz, which is where both
units/systems are at the same 0 dB point, will lead to inaccurate calibrati=
on of
your recorder.

I suggest comparing the two methodologies. Put the SLM on a tripod and do t=
he
measurement using a 1KHz one and get your body well away. Use A-weighting o=
n the
SLM. Do the same thing for your mics/recorder using its built-in meter. Not=
e the
calibration point for your recorder.

Repeat the process with the SLM and you recorder using the 200 Hz - 1KHz BL=
PN.
Note the results and compare them to the single-tone measurements.
Then do the measurement using the same BLPN stimulus with the SLM switched =
to
C-weighting. Note that difference. Now bring the stimulus level down until =
the
C-weighted SLM measurement is the same as the A-weighted SLM measurement. N=
ote
how much you have attenuated the stimulus to get this last reading. It shou=
ld be
indicative of the gain difference of your recorder settings produced by the=
 two
methodologies.

Please re-read Raimund's response form today @ 4:44 AM. There is much wisdo=
m in
his response. I especially like his comment about how animals "don't care a=
bout
A-weighting at all". The point here, IMHO, is that other creatures hear in =
ways
much different from our own ears. So if we apply a single artificial loudne=
ss
measurement level to a natural soundspace, what relevance does it really ha=
ve?
I believe that the spectral distribution of energy is much more important a=
nd
that there are many spectral distributions that will give the same A-weight=
ed
level measurement number. For a given number, which is more loud to your ow=
n
ears? You may be surprised.

Check out this paper when you have
time: http://www.envisys.com/us/en/Impact_Sound_Pressure.pdf

There is nothing wrong with calibrating your recording gear, providing it i=
s
done in a careful manner. That way you have a benchmark for the level in an=
y
given setting. Perhaps Bernie can outline his procedure for calibrating his=
 mics
& recording equipment.

Take care,

Greg



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