Hi Mike--
Thanks for the suggestions and particulars about
your solutions. If the options are to hack and
then re-calibrate a SLM with a 94dB calibrator
instrument, it seems like any means using very
low noise mics and very low noise pre is worth
mental exploration-- especially for settings
under 20 dBA. Two record level reference
settings, one for natural ambience and another
about 20-25 dB less would probably cover almost
every setting I record. Using speaker playback
in the process might be impractical or horribly
inaccurate for all I know. Rob D.
=3D =3D =3D
At 12:15 PM +0000 11/24/10, Mike Rooke wrote:
> Hi,
>
>1. Buy a dedicated SPL meter, 32-130 dB - or if
>< 32 dB is needed hack a lower noise
>mic into one and re-calibrate (and hope it works)
>
>2. As above high quality B&K or similar 20 - x dB =3D expensive.
>
>3. Use an existing mobile device, phone or ipod.
>
>iPod touch + signal scope pro works for me. ipod
>touch can be calibrated down to
>a lower limit of 18-24dB if using the
>headphone/mic input. Theres a few dedicated
>preamps for such setups now probably lower spl.
>Problem here is to measure low levels you need a
>sensitive mic, which will then clip during
>calibration if using a simple mic calibrator
>(94dB 1Khz tone). Calibrating with a speaker at
>a lower level is also possible but not as easy.
>
><http://www.faberacoustical.com/products/iphone/signalscope_pro/>http://ww=
w.faberacoustical.com/products/iphone/signalscope_pro/
>
>Still that doesnt beat a simple spl meter which
>you can also hack and install a lower noise mic
>to and then recalibrate it (94dB calibrator
>required) or make a note of offset. This method
>is probably +-3dB or worse.
>
>Center 325 SPL meters are decent enough to carry
>around, they cover 32 dB - 130 dB
>also cheap. < 100 Euro in scandinavia.
>
>For SPL levels in recordings, either call out
>the spl from a meter or if automating a plugin
>is required that can calibrate at the start of
>the recording and also include the correct
>weighting (if A is being used) - I wrote a
>reaper plugin that will calibrate against a
>known level and then display the spl for the
>rest of the recording, weighting filters are
>missing so its "flat" - I didnt find it of much
>use.
>
>-Mike.
>
>--- In
><naturerecordists%40yahoogroups.com>=
m,
>Gregory O'Drobinak <> wrote:
>>
>> Rob:
>>
>> I don't think so.
>>
>> In the first case, during the comparison of
>>your sound level meter (SLM) level
>> (dBA) to the meter on your recorder (dB), you
>>are comparing apples to oranges,
>> 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 energy
>> from the reference speaker. Thus your '0 dB' recorder meter reading wou=
ld
>> actually be much less after applying the same A-weighting filter to tha=
t
>> 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.=
You
>> cannot use onlythe plain old level meter on your recorder to set any ki=
nd 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/wi=
ki/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/Pri=
mers.aspx
> >
>> My own opinion is that A-weighting does not
>>necessarily relate to how my ears
>> 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 valid
>> 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, things
>> 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 think,
>> 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, s=
o how
>> 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. W=
hen 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 mea=
sure.
>>
>> IMHO, if you want a quick & dirty perspective
>>of the SPL in a natural setting,
>> grab your trusty SLM and write down the dBA and dBC readings for each t=
ime
>> 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. Calibr=
ating
>> your recorder & mics means you can never
>>deviate from that one configuration,
>> 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 <>
>> To:
>><naturerecordists%40yahoogroups.com>=
om
>> 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 weightin=
g.
> >
>> 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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