# New Guy question - SLP calibration files



## Denali (Mar 29, 2014)

I’m new to REW and trying to use the calibration file for the Radio Shack 33-2050 SPL meter, but I’m confused with the results.

First I calibrated my PC soundcard with REW and the REW SPL meter, when I use the correct cal file for the 33-2050 I get the red response shown in the attachment. When I don’t use a cal file I get the response shown in blue. Both are using the C Weighted SPL input type. I’m calibrating my two front speakers only (Martin Logan Montis), without a sub, center or surrounds at 75 db. The purple graph can’t be correct showing plus 10 db @20 Hz and plus 16 db @ 30 Hz, the speakers low end spec is 29 Hz + - 3 db. The red response (no SPL cal file) is far more believable. What’s up, I don’t get it?


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## Denali (Mar 29, 2014)

Opps, my bad the red sweep is using the calibration file and the blue is not.
Sorry for the confusion.


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

Not sure I get what your concern is?



Denali said:


> The purple graph can’t be correct showing plus 10 db @20 Hz and plus 16 db @ 30 Hz


In relation to what? What is your 0 dB reference?

Regards, 
Wayne


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## Denali (Mar 29, 2014)

Wayne,

Thanks for your help, I hope I correctly understand your question, I would say my 0 db reference is 75 db. I'm using the REW SPL meter test tone and adjusting the volume on my preamp to get 75 db on the RS meter then setting the REW SPL meter at 75 db also.


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

Got it. Well, with 75 dB as a 0 reference it’s the red trace showing +10 dB at 20 Hz and +60 at 30 Hz, not the purple one. :scratch:

As you probably know, the C-weighting curve rolls out the low frequencies. When you check the “C Weighted SPL Meter” box on the “Mic/Meter” preferences page, REW will automatically compensate for the C-weighting curve. However, if you use a calibration file along with the “C Weighting” box checked, REW will only compensate for the difference between the calibration file and the C-weighted curve.

With that background, your graphs look like the program is somehow stacking the C-weighting curve on top of the calibration file, giving an unnatural low-end boost. I’d suggest just unchecking the “C Weighting” box if you’re using the calibration file.

It should also be mentioned that a manufacturer’s frequency response measurement often has little bearing on what you’ll get with an in-room measurement. It’s very much room dependant. For instance, a tiny room will give a natural low frequency boost, while a really huge one will show reduced bass extension and output.

Regards, 
Wayne


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## Denali (Mar 29, 2014)

Wayne,
Yes, you are correct the red trace is with the C weighted SPL Meter type selected and the RS 33-2050 calibration file and the purple trace is with the C weighted SPL Meter type selected and no calibration file.

You suggested:

"Your graphs look like the program is somehow stacking the C-weighting curve on top of the calibration file, giving an unnatural low-end boost. I’d suggest just unchecking the “C Weighting” box if you’re using the calibration file."

This is exactly my impression also. REW is somehow using both curves on top of each other at the same time. You suggested not using C weighted SPL Meter type and just the 33-2050 cal file, but that is not an option. Whether using a cal file or not, you either have to select the “C Weighted SPL Meter” type or the “Z Weighted SPL Meter” type, there is no other option, you must use one or the other.

I have also re-verified that I am using the correct calibration file for my RS 33-2050 meter.

What else do you suggest I try to figure out what’s going on?


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## JohnM (Apr 11, 2006)

REW does not combine cal file and C weighting corrections, compensation for C weighting (if selected) is only applied outside the span of the cal file. 

Your plots show exactly what would be expected from applying the cal file to an uncorrected measurement, note that the mdat file for your blue curve does not have C weighting correction enabled. The differences between the red and blue curves reflect the cal file entries (e.g. 10.44 dB correction at 31.5 Hz). Those corrections are based on measurements of examples of the various RS meter types, they are typically larger than the equivalent C weighting curve corrections at the low end (i.e. the meters that were measured did not track the C weighting curve very well but in fact rolled off much faster). Meters vary, however, your meter might follow the C weighting curve more closely than the one that was measured to generate the corrections so you might be better off just using C weighting compensation without a cal file.


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

Denali said:


> Wayne,
> Yes, you are correct the red trace is with the C weighted SPL Meter type selected and the RS 33-2050 calibration file and the purple trace is with the C weighted SPL Meter type selected and no calibration file.


John is correct, your purple trace indeed did not include any calibration file, nor was C weighting engaged. (I didn’t take the time to download your .mdat file before commenting, and I probably wouldn’t have caught that detail anyway.) So both your measurements are correct, as the red has calibration compensation and the purple has none.




> You suggested not using C weighted SPL Meter type and just the 33-2050 cal file, but that is not an option. Whether using a cal file or not, you either have to select the “C Weighted SPL Meter” type or the “Z Weighted SPL Meter” type, there is no other option, you must use one or the other.


Looks like you’re mistaking the SPL meter with the Mic/Meter calibration tab from the Preferences box. The latter is where you apply calibration files, and as you can see from the picture below, it only has a “C” option that can be turned off.



















Regards, 
Wayne


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## Denali (Mar 29, 2014)

*Re: New Guy question - SPL calibration files*

John and Wayne,

Thanks to both of your for your replies. In the screen shot Wayne provided of the preferences applet it looks different than mine. I believe Wayne’s screen shot may be from a earlier version of REW. I am using the latest REW version 5.01 Beta 19 Build 3293, my preferences applet is shown below. Again with the radio buttons in my version, I must select either select “C” or “Z” rating. No weighting is not an option. With Wayne’s version there is a check box to turn the weighting on or off.

So, John with my version is the “C Weighting” automatically turned off when a SPL meter cal fine is selected? If not, then all my measurements are with “C Weighting” enabled including the blue trace, which is not using the RS 33-2050 cal file.


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

Curious, I thought I was using the latest version. Regardless, Z weighting is flat response, which is the equivalent of no weighting (as explained in the instructions in the “Input Type” section of the Preferences applet shown). So checking that box will remove C weighting. :T

Regards, 
Wayne


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## JohnM (Apr 11, 2006)

*Re: New Guy question - SPL calibration files*



Denali said:


> So, John with my version is the “C Weighting” automatically turned off when a SPL meter cal fine is selected?


No, correction for C weighting is applied outside the range of the cal file (i.e. below the cal file's lowest frequency and above its highest frequency).



> If not, then all my measurements are with “C Weighting” enabled including the blue trace, which is not using the RS 33-2050 cal file.


Not the one you posted, that has no correction applied (click the "Change Cal..." button and you will see the C weighting box is not selected for that measurement). If C weighting correction is applied without a cal file the mic/meter cal trace shows a C weighting curve.


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## Denali (Mar 29, 2014)

John,

Sorry I haven’t replied sooner, way too much to do and not enough time.

You’re exactly right about the “C Weighing” not being enabled, my bad although I don’t know what happened. Anyway I’ve taken all new measurements and checked “Change Cal” on each measurement to make sure I was not using a SPL calibration file and that “C Weighting” is enabled.

Since my Radio Shack 33-2050 SPL meter seems to deviate a lot from the 33-2050 calibration file I downloaded, I’m very concerned about its accuracy. I’m mainly interested in the low frequencies between 35 Hz and 250 Hz but would purchase a UMIK-1 microphone if the result would be more accurate. I would not be using the HDMI output from my PC soundcard as my preamp\processor does not have HDMI inputs. What are your thoughts, should I purchase a UMIK-1?


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## JohnM (Apr 11, 2006)

A UMIK-1 (or another calibrated mic) would be more accurate. It doesn't matter what output you are using.


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