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Discussion Starter #1
Hi Folks,

Using the Radio Shack 33-2050 SPL meter with associated *.cal file, what frequency response can be expected in the 'C' weighted position?

Thanks.

JPC
 

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Discussion Starter #5 (Edited)
Hi Folks,

Thanks for the replies but now I'm confused. 'C' weighting corresponds to 5dB down at 20Hz and 5dB down at 10kHz. So, Bruce, you say limit the upper end to 3kHz. Does that mean that the 33-2050 is not providing the correct 'C' weighting characteristic? Interestingly, the link provided by joetama would imply that the 33-2050 is usable up to 20kHz with the appropriate correction. It's also usable down to 10Hz albeit with +20dB correction according to this link. This same link refers to a -1dB correction at 10kHz but, with 'C' weighting, the response should be 5dB down at 10kHz, as stated earlier. So, are the correction figures (shown in this table) supposed to ensure that the 33-2050 complies with the 'C' weighting curve or ensure a flat response from 10Hz to 20kHz? Surely, what we want from the measurement system is a flat response from a few Hz to at least 20kHz.

All feedback greatly appreciated.

JPC
 

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Does that mean that the 33-2050 is not providing the correct 'C' weighting characteristic?
There are many examples of tests between calibrated measurement mics and the Radio Shack meters. They tend to be inaccurate above the 3KHz area. You're far better off with an SPL meter such as the Galaxy CM-140 (we supply the calibration file for it also).

So, are the correction figures (shown in this table) supposed to ensure that the 33-2050 complies with the 'C' weighting curve or ensure a flat response from 10Hz to 20kHz?
If the Radio Shack meter complied with the C-Weight curve it would not require a calibration file to be used with REW, since REW has a C-Weight checkbox that would render the meter flat. Unforunately, this isn't the case. So, we developed a calibration file that replaces the C-Weight checkbox to render the Radio Shack meter flat over its limits. The limits of the file are from 7hz to 200Hz. After 200Hz the RS meters are fairly accurate up to ~3KHz. The file extends to 7Hz, but seriously we only offered that low value after repeated requests to provide a rough approximation. We tested mutiple RS meters in the development of the cal files and in my opinion they should only be used to about 15Hz. Below that the correction is usually down in the noise level and the results can't be trusted.
If anyone is concerned about accuracy, they shouldn't be using a Radio Shack meter - get a decent meter of measurement mic.

brucek
 

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Should I should have the "C Weighted meter" checkbox UNchecked when using the cal file for a RS meter?
The meter.cal file over-rides the portion of the C-Weight curve (if checked) over the limits of the meter file. So, if the file extended from 10Hz-200Hz and the C-Weight box was checked, then those values would be over-riden and the C-Weight curve would extend out from there. But since the C-Weight correction is flat above 200Hz, and the fact that the Radio Shack meter shouldn't be used above ~3KHz, this is a long story to say that the answer to your question is, it doesn't matter.

brucek
 
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