# SPL Calibration



## AustinJerry (Apr 2, 2010)

I have been trying to adjust to the enhanced SPL calibration tool in REW 5.0. I have read the Help file, but I an still having a small difficulty. Specifically, when I attempt to calibrate my RS SPL using C weighting and Slow response, I expect the SPL reading to "settle down" after a short period of time. However, my experience has been that the reading never really settles down like it used to, i.e. the readings contine to vary over a 2-3 dB range, whereas I am expecting more like a .3-.5 dB range. This makes it quite difficult to determine what SPL setting to use. I believe the previous SPL calibration tool had a "Very Slow" setting.

Am I missing something?

Edit: A second question--When measuring a test tone generated by my AVR (which is supposed to be close to 75 dB), the analog reading on the front of the SPL is approximately 73 dB, yet the reading showing on the REW SPL is 89 dB. Why the difference?


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## JohnM (Apr 11, 2006)

The REW meter will not produce readings that correspond to the external meter until after going through calibration. The readings should then correspond. Note that if you change the range of the external meter from the range used for calibration the REW meter will no longer read correctly, because that changes the levels of the signals being fed to REW. 

To get longer term average readings use the Leq rather than SPL setting, that gives you an average over the time since "Reset All" was last pressed.


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## AustinJerry (Apr 2, 2010)

Thanks, John, I think I understand how it works now. However, I continue to have a peculiar problem. Here is what I observed:

I set up the SPL with C-weighting, Slow settings, 70 dB range, launched REW 5.0, and opened the SPL tool. I clicked on Leq, the Reset All, and then Calibrate (using the sub tone). I let it run for approximately 30 seconds, and entered the LCSmax reading as the SPL calibration level. I then ran my AVR's internal speaker level test tones, and observed levels of 75 dB +/- .5 dB across all speakers, as expected. So far, so good.

I then started a series of REW measurements to obtain a multiple-location average room response. For each placement of the SPL, I re-ran the SPL calibration prior to taking the measurement, just to be precise. As I took each successive measurement, I noticed that the SPL level was creeping up 1-2 full dB for each calibration. This certainly wasn't expected. I then took several SPL calibrations without moving the SPL or altering the output level of the calibration tone. Sure enough, each successive SPL measurement was 1-2 dB higher! Finally, I re-ran the AVR test tones, and this time the test tones measured 107 dB, instead of 75 dB.

Obviously, something is wrong here. Could I have a faulty SPL meter? Perhaps a failing component that reads successively higher levels as something heats up? Or a failed soundcard (I am using a new SoundBlaster X-Fi USB card)? Any recommendations on how to trouble-shoot and isolate the issue?

Thanks in advance for your guidance.


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## AustinJerry (Apr 2, 2010)

Update:

I ran a log on the SPL readings. With NO TONE being generated, the SPL level is showing 85 dB. When the AVR test tone is invoked, the SPL level increases to 105 dB. This is a tone that should be registering 75 dB, according to the analog needle on the SPL. See attached log. :gulp:


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## JohnM (Apr 11, 2006)

JerryMeeker said:


> I let it run for approximately 30 seconds, and entered the LCSmax reading as the SPL calibration level.


LCSmax is the maximum reading that occurred during the measurement period. The figure you should use is the main display figure (the large digits).



> For each placement of the SPL, I re-ran the SPL calibration prior to taking the measurement, just to be precise.


There is no need to recalibrate and no benefit. If you kept entering LCSmax figures then your readings would keep creeping up, as the maximum is always going to be higher than the average.


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## AustinJerry (Apr 2, 2010)

John, Perhaps I am not doing a good job of communicating the behavior I am observing. Regardless, I am getting quite frustrated, and I appreciate your patience as I work through the issues.

OK, I acknowledge what reading I should enter (i.e. the "large numbers"). However, the "big numbers" read over 90 dB when the volume one the AVR is very low (-30 dB). 

Just to assure you that I am doing things correctly, here are the exact keystrokes I am using:

1. Click Reset All.
2. Click Leq.
3. Click Calibrate.
4. Select "subwoofer cal signal", and click OK.
5. The "Big numbers" read approximately 90 dB.
6. Enter 90 dB in the reading calibration window and click Finished.
7. Run the AVR test tones, which should read 75 dB. Actual reading is 101 dB, clearly not right.

I am at a complete loss.

BTW, as soon as I click "Calibrate", the Leq button turns off and the SPL button is enabled, and the "Big numbers" are jumping all over the place, never settling down. What is up with that?


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## JohnM (Apr 11, 2006)

Ah! The problem is you are running REW around in circles trying to calibrate it against itself  

To calibrate REW's SPL meter you need to enter the reading from your _external_ SPL meter. REW then knows how to relate the input dB FS figures of the signal it is receiving to a real world SPL figure.


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## AustinJerry (Apr 2, 2010)

Well, that would explain it, now wouldn't it? I have been thinking all along that the signal output from the RS SPL was somehow converting the analog needle position into a digital signal that was being displayed by the "big numbers". I guess the RS SPL isn't that sophisticated... :R

Now that I know, it was very simple to read the actual SPL level and input it into REW. Onwards and upwards! 

Thanks again for bearing with me.


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