# "C" weighting question



## finalrinse (Sep 7, 2007)

I'm a little confused on which setting I should use with my B&K model 2230 SPL Meter, Preamp & Mic.. I have 4 choices, keep in mind these weighting filters are actually processed within the SPL meter and output through the sound card and REW. Maybe all SLP Meters do this, just thought I would mention it FYI. OK, I have "A" "C" "L with 20Hz & 20KHz rolloff, and "L". Should I use "C"? If so should I tick or untick the "C Weighted SPL Meter"" box under "Type" in REW "Mic/Meter" tab under "Settings"? In the help files I read that if I use a preamp & Mic I should uncheck the "C" box. Well I believe I have a preamp & Mic, so I'm confused on what to put my SPL Meter on and whether to check or uncheck the "C" setting.
Thanks,
Tom


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## brucek (Apr 11, 2006)

I'm not familiar with the term "L"-weighting, but I suppose it means flat.

When we use a Radio Shack or similar meter, it has only an A or C weighting capability. We select the C-Weighting choice on the meters. There is no flat output on the Radio Shack or Galaxy meters. So, since it measures with a C-weighting curve on its output, we want to apply the inverse C-weighting curve in REW to end up with a flat response for the meter. So, normally we check the C-Weight box if the meter has a C-Weight output.

But here's the rub. The Radio Shack and just about every other inexpensive meter don't perfectly track a C-Weighting in their measurement. They are slightly off, and increasingly so at the very low end where we like to take readings for subs (such as 10Hz). So, in place of the C-Weight curve, we substitute a calibration file that is a modified C-Weight curve that more closely is the inverse of the response of the meter. We supply these cal files in our download section.

OK, here's the confusing part. REW will apply a C-Weight inverse curve when the box is checked over the entire audio range *except for the area that has a calibration file loaded*. If no calibration file is loaded, then the entire range is a perfect C-Weight spec. If I loaded a cal file that had a start frequency of 20Hz and an end of 200Hz, then the cal file would be substituted within the C-Weight curve from 20Hz to 200Hz. If the C-Weight was unchecked, then the compensation would be flat before and after 20Hz and 200Hz.

The other microphone that we like to recommend is the ECM8000. It is a supposed flat microphone. We certainly uncheck the C-Weight box for this and any flat mic. But again, it isn't perfectly flat, so we provide a calibration file to tweak it a bit to hopefully get the response a bit flatter.

So, for your mic, if the L output is flat, use that, and uncheck the C-Weight box and don't use a cal file (you don't need it).
Or, you could use the C-Weight output, that I assume is equally accurate and select the C-Weight box and no cal file.

brucek


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## finalrinse (Sep 7, 2007)

brucek said:


> I'm not familiar with the term "L"-weighting, but I suppose it means flat.
> 
> When we use a Radio Shack or similar meter, it has only an A or C weighting capability. We select the C-Weighting choice on the meters. There is no flat output on the Radio Shack or Galaxy meters. So, since it measures with a C-weighting curve on its output, we want to apply the inverse C-weighting curve in REW to end up with a flat response for the meter. So, normally we check the C-Weight box if the meter has a C-Weight output.
> 
> ...


L = Linier, like you say, Flat (no filtering). After reading your very good explanation I think I will choose to use "L" on the SPL Meter and make sure "C" is unchecked.
Thank you very much!
Tom


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## Wayne A. Pflughaupt (Apr 13, 2006)

finalrinse said:


> L = Linier, like you say, Flat (no filtering).


Strange terminology indeed. No filtering is more typically referred to as Z-weighted or unweighted. :scratch:

Regards,
Wayne


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## finalrinse (Sep 7, 2007)

Wayne A. Pflughaupt said:


> Strange terminology indeed. No filtering is more typically referred to as Z-weighted or unweighted. :scratch:
> 
> Regards,
> Wayne


Hi Wayne,
Yes, I've been using Bruel & Kjaer stuff for some time, which has always been there terminology, maybe a Denmark think. Here are a couple of scans from the manual explaining the two different Linear modes, which are not completely unweighted. In my previous post I did miss spell Linear.


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