iPod touch + signal scope pro works for me. ipod touch can be calibrated do=
wn to
a lower limit of 18-24dB if using the headphone/mic input. Theres a few ded=
icated 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 ca=
libration if using a simple mic calibrator (94dB 1Khz tone). Calibrating wi=
th a speaker at a lower level is also possible but not as easy.
http://www.faberacoustical.com/products/iphone/signalscope_pro/
Still that doesnt beat a simple spl meter which you can also hack and insta=
ll 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 au=
tomating a plugin is required that can calibrate at the start of the record=
ing 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 t=
he spl for the rest of the recording, weighting filters are missing so its =
"flat" - I didnt find it of much use.
-Mike.
--- In 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 ora=
nges,
> i.e., the SLM has the proper A-weighting filter to roll off the low end o=
f the
> pink noise spectrum, but the recorder is really measuring all of the LF e=
nergy
> 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 i=
s -50
> dB RMS measured in post with A weighting", the actual A-weighted level wo=
uld 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. Y=
ou
> 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 pro=
cess 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
> and http://en.wikipedia.org/wiki/Weighting_filter
>
> If you want to dig deeper, try B&K's website (they make the best level me=
ters -
> 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 =
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 t=
elling
> 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-wei=
ghting,
> 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, th=
ings
> 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 th=
ink,
> especially in natural spaces. I believe that one's hearing & perception c=
an vary
> widely depending on the type of sounds that are being heard/recorded, so =
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 ca=
n vary
> depending on whether the source is impulsive/short-lived or constant. Whe=
n 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 measu=
re.
>
> IMHO, if you want a quick & dirty perspective of the SPL in a natural set=
ting,
> grab your trusty SLM and write down the dBA and dBC readings for each tim=
e
> weighting that is available on the unit and tag the measurement with a ma=
rker 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. Calibrat=
ing
> your recorder & mics means you can never deviate from that one configurat=
ion,
> 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:
> 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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