# RS Digital SPL meter and RCA output with REW



## Dent (May 6, 2006)

Hi all,

I had posted the following question to Sonnie in a thread in the AVS forum located here: http://www.avsforum.com/avs-vb/showthread.php?p=7614868#post7614868 and Sonnie suggested I post it here also. I wanted others to verify if my logic is correct or not. It also appears in Sonnie's answer to me over at that forum that he is going to test a couple of RS digital SPL meters so we can once and for all determine the correct answer.
Thanks again, Sonnie, for all the help and I am looking forward to your results of the RS digital SPL meter testing.

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Hi again, Sonnie,

Thanks for the reply again. I know that you had responded to my previous post but I still felt that the question about whether the RCA output of the RS digital SPL meter was c-weighted compensated or uncompensated was still unresolved, that is, not tested and verified by anyone else other than the forum member, Ethan Winer, of whose results that Ilkka posted which suggested that the RCA output was uncompensated unlike the RS analog meter output which is C-weighted compensated.

I believe this would make a big difference when using the REW program and the RCA output of the RS digital SPL meter because according to Ilkka on post #16 of this thread, it says that the value of +6.2 dB at 20 Hz (as taken from a previous compensation table that he had come up with which is referenced in post #10 of this same thread) comes from the general c-weighting compensation. Note this value he is talking about is not referring to any SPL meter inaccuracy or otherwise but is referring to the general c-weighting compensation definition. So I take it to mean that if one had a perfectly flat microphone at 20 Hz but one took the output of that mic at 20 Hz and applied the c-weighting compensation to the output (like the RCA output of the analog RS SPL meter does, that is, it applies a c-weighting compensation), then the output would decrease by 6.2 dB so one would have to add back 6.2 dB to the reading to get it back to normal.

Now the correction values you have come up with were done with the RCA output of the analog RS SPL meter which is known and verified to have a c-weighting compensation done to it so your values corrects for the both the c-weighting compensation plus the inaccuracy of the RS SPL meter mic. So using your value of +11.5 dB for 20 Hz, I would take this to mean that the microphone was inaccurate by -5.3 dB and then the c-weighting compensation of -6.2 dB was added to it by the c-weighting compensation circuits of the RS analog SPL meter so if you add these two numbers together you get your -11.5 dB deviation that you came up with.

Now with the digital RS SPL meter, if it is the case, as Ethan Winer had supposedly verified, does not have the c-weighting compensation applied to the RCA output, then even if the microphone from your analog RS SPL meter was moved into the digital RS SPL meter, since the RCA output does not supposedly have the c-weighting compensation, the correction value at 20 Hz would only be +5.3 dB using your findings.

This is why I think it is important that someone else test another one or two or more digital RS SPL meter RCA outputs and see if Ethan Winer is correct or not.

P.S. I found a webpage here: http://www.diracdelta.co.uk/science/source/c/w/cweighting/source.html that gives the formula for c-weighting compensation plus has a calculator that you can input any frequency and these values follow exactly the compensation table of Ilkka's that is referenced in post #10 of this thread.


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## Sonnie (Apr 11, 2006)

Hey Dent and welcome to the Shack!


To confirm... earlier today I ordered two of the RS digital meters.

This will give me an older analog RS meter, two new analogs, and two new digitals to test against my calibrated ECM.



Now... maybe some of our tech minded will oblige us with more detail on your comments and questions.


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## brucek (Apr 11, 2006)

dent says:


> I believe this would make a big difference when using the REW program and the RCA output of the RS digital SPL meter


Of course it would, and a new digital RS meter calibration file would need to be loaded. This assumes the contention regarding the digital meters RCA output lacking C-weight is true. My feeling is that the C-weight is indeed applied to the jack output on the digital meter, just as it is with the analog version. Sonnie can test this. Either way, he has a calibrated mic to test against it and can come up with a new compensation file if necessary.

REW has the ability of reverse compensating for C-weight to theoretically create a flat response. If you used a mic/meter that produced an output that perfectly tracked a C-weight, then the REW reverse compensating for C-weight would be suffice. But it just ain't so with the RS meter, so we use our own calibration file to create a flat response. If you load a calibration file, the REW C-Weighting compensation is ignored except for areas above and below the calibration files frequency range.

Having said all that, my point is that we don't really care about the weighting once we decide to use our own calibration file. It's simply an inverse compensation for the inadequacies of the mics/meters output . If the digital meter is different from the analog one - no problem, we need to make a new file. But if Sonnie tests a digital one against an analog one and they are the same, we'll just use the present newrs.cal. :wave: 

brucek


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## Dent (May 6, 2006)

Fair enough. Thanks for the answer.


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