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Re: [Nature Recordists] Measuring ambient sound levels

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Subject: Re: [Nature Recordists] Measuring ambient sound levels
From: Rob Danielson <>
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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