# UMIK and SPL measurement



## HifiZine (Feb 7, 2013)

I've been playing with a UMIK and the latest beta version of REW. Well nice, well done JohnM, and thank you for making the beta also available for the Mac :clap:

I am wondering if anyone here has verified the UMIK/REW SPL measurement against a calibrated SPL meter?


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

If by calibrated you mean a reference lab-grade meter, I doubt it. Those things are very expensive.

Regards, 
Wayne


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## Dwight Angus (Dec 17, 2007)

Why a calibrated spl meter?


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## AudiocRaver (Jun 6, 2012)

HifiZine said:


> I've been playing with a UMIK and the latest beta version of REW. Well nice, well done JohnM, and thank you for making the beta also available for the Mac :clap:
> 
> I am wondering if anyone here has verified the UMIK/REW SPL measurement against a calibrated SPL meter?


Not that I have heard of. Is there a particular concern you have?

By the way, welcome to Home Theater Shack.


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## JohnM (Apr 11, 2006)

See this thread.


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## HifiZine (Feb 7, 2013)

Wayne A. Pflughaupt said:


> If by calibrated you mean a reference lab-grade meter, I doubt it. Those things are very expensive.


Thanks Wayne, guys. I meant something like the calibrated CM140 from Herb at CSL. It seems that the UMIK could remove the need to get a calibrated SPL meter at all. I'm writing a review of the UMIK and was looking for verification of SPL accuracy.



JohnM said:


> See this thread.


Thanks John, looks very promising. I did get the UMIK running under Windows and will try comparing with it running on the Mac tomorrow.

JohnR


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## JohnM (Apr 11, 2006)

Bear in mind that REW cannot access the volume control on the Mac so it assumes you leave the volume at the unity gain setting (the default when the mic is plugged in). If you alter the input gain the SPL readings will be altered correspondingly.


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

HifiZine said:


> Thanks Wayne, guys. I meant something like the calibrated CM140 from Herb at CSL. It seems that the UMIK could remove the need to get a calibrated SPL meter at all. I'm writing a review of the UMIK and was looking for verification of SPL accuracy.


 I’m not sure which function of the CM-140 meter that you’re referring to. Herb calibrates the CM-140 separately for frequency response and SPL accuracy (read precise measurement of signal level). Those are two entirely separate functions of the meter. It’s common for people to misunderstand these two calibrations as it relates to SLMs. 

Perhaps you can realize that a measurement mic like the UMlK by itself cannot be a substitute for a SLM. Remember, the SLM has a mic, too: The meter’s electronics translate the signal picked up from the mic to a SPL reading. REW does not have the capability to do this.

It’s long established that REW can be used independent of accurate SPL calibration: The graph is still relevant, and there will be an actual 5 dB spread between the graph’s 75 and 80 dB indications, even if 75 dB is actually 69 for example. But if you require that the graph’s SPL indicators be accurate, that still requires an outboard SLM.

Regards, 
Wayne


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## HifiZine (Feb 7, 2013)

Wayne A. Pflughaupt said:


> Perhaps you can realize that a measurement mic like the UMlK by itself cannot be a substitute for a SLM. Remember, the SLM has a mic, too: The meter’s electronics translate the signal picked up from the mic to a SPL reading. REW does not have the capability to do this.


Hi Wayne, I'm not following you, REW has an SPL meter function, as described here. It seems to be in every way the equivalent of a regular SPL meter... in what way is it not functioning as one?



> But if you require that the graph’s SPL indicators be accurate, that still requires an outboard SLM.


The Beta13 of REW reads the level calibration from the UMIK calibration file, so doesn't require an outboard SLM. In fact, it won't let you change the calibration so you can't use an outboard SLM even if you wanted to. The question (for me anyway  ) is how accurate/consistent the number in the UMIK cal file is.


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## HifiZine (Feb 7, 2013)

JohnM said:


> Bear in mind that REW cannot access the volume control on the Mac so it assumes you leave the volume at the unity gain setting (the default when the mic is plugged in). If you alter the input gain the SPL readings will be altered correspondingly.


Hi John, I did a comparison using the UMIK on OSX 10.8.2 with the mic level set to 0.318, and on Windows 7 with the mic level set to 10 (/100). Windows 7 was running as a VM on the Mac. I assume it behaves the same as a regular machine would... The REW output level was set about 6 dB higher on Windows to get the same acoustic level, I assume because the default output levels in Windows is 49 and I had the Mac set at full out. 

I set the acoustic output levels to be the same on both systems using a separate mic on a different computer running an FFT. One thing I noted is that on the Mac, setting the REW signal generator to 1 kHz produces 920 Hz; it needs to be set to 1088 Hz to produce 1 khz.

The speaker is an Aurasound NS3 3" fullrange driver in a sealed box. Mics were placed 5cm from the cone (and not moved during the measurements).

Numbers in square brackets are the RMS values generated by the FFT on the other computer. Here's what I got:

*Win7*

1 kHz sine wave: 94.0 [-4.6]. Pink Noise Speaker Cal: 95.9 [-2.8]. Pink PN: 93.7 [-5.3].

Running a sweep, with 1/48 smoothing, readings at 500/1k/2k = 95.9/94.5/96.7.

*Mac 10.8*

1 kHz sine wave: 94.1 [-4.5]. Pink Noise speaker cal : 95.6 [-3.0]. Pink PN: 93.6 [-5.6].

Running a sweep, with 1/48 smoothing, readings at 500/1k/2k = 96.3/94.5/96.7.


That's probably more than I needed to do, but I satisfied myself that the results between Win and Mac are essentially identical.

Another note: miniDSP's supplier seems to have changed the calibrator they use for these mics, judging by the file format.


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

HifiZine said:


> Hi Wayne, I'm not following you, REW has an SPL meter function, as described here. It seems to be in every way the equivalent of a regular SPL meter... in what way is it not functioning as one?


 I did notice after making my post that apparently the UMIK can replace calibration from an external SLM (yes, I know you said as much, but it didn’t register!), so my apologies for “muddying the waters,” as they say. Nevertheless, REW’s SPL meter still requires calibration from an outboard source, as it always has. 




> The Beta13 of REW reads the level calibration from the UMIK calibration file, so doesn't require an outboard SLM. In fact, it won't let you change the calibration so you can't use an outboard SLM even if you wanted to. The question (for me anyway  ) is how accurate/consistent the number in the UMIK cal file is.


 Really, for our purposes its accuracy is of minor relevance. We’ve typically calibrated REW with a Radio Shack SLM, which is a substandard Class 3 device. Class 3 meters are supposed to be accurate +/- 1.5 dB; it’s more like +/- 2 dB with the Radio Shack meter. So the standard for miniDSP to meet or exceed is pretty low. 

The point is, we don’t need laboratory-grade SPL accuracy for REW. It isn’t going to “make or break” the relevance of any frequency response waterfall, impulse response etc. measurements we end up with. Reference-grade accuracy is the stuff of professional noise surveys made by acousticians. And really, if you want accuracy on that level, the professional protocol requires a fresh calibration for the meter at either 94 dB or 114 dB from a separate (read outboard) calibration device, both before and after each use, to account for the effects of temperature and humidity on the mic. Thus, for pinpoint accurate SPL, REW must be calibrated with a SLM that’s been calibrated itself, on-site right before it’s used!

I understand that you’re merely trying to establish the accuracy of the UMIK’s SPL calibration for the purposes of your review. Maybe you could just compare it against a decent-quality SLM, like take a pink noise reading with both the stand-alone meter and REW as calibrated from the UMIK, and see how close they are. If you don’t have a meter, you could purchase one from a retailer, then return it for a refund after your evaluation.

Regards, 
Wayne


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## AudiocRaver (Jun 6, 2012)

> Perhaps you can realize that a measurement mic like the UMlK by itself cannot be a substitute for a SLM. Remember, the SLM has a mic, too: The meter’s electronics translate the signal picked up from the mic to a SPL reading. REW does not have the capability to do this.


Hey, good news on this front. The UMIK-1 calibration file includes a sensitivity value provided by the factory as part of the individual mic calibration. That sensitivity number is read by Room EQ Wizard when the calibration file is imported, and then that gives REW the calibration information it needs to be able to take SPL measurements with the UMIK-1 as its input device. The normal SPL calibration step is not needed with the UMIK-1. This is a recent capability added in the latest beta versions of REW (not sure exactly when it was added, just grab the latest).:T

Edit: Bag pardon, Wayne, looks like you posted just as I was finishing up mine.


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## HifiZine (Feb 7, 2013)

Wayne A. Pflughaupt said:


> The point is, we don’t need laboratory-grade SPL accuracy for REW. It isn’t going to “make or break” the relevance of any frequency response waterfall, impulse response etc. measurements we end up with. Reference-grade accuracy is the stuff of professional noise surveys made by acousticians. And really, if you want accuracy on that level, the professional protocol requires a fresh calibration for the meter at either 94 dB or 114 dB from a separate (read outboard) calibration device, both before and after each use, to account for the effects of temperature and humidity on the mic. Thus, for pinpoint accurate SPL, REW must be calibrated with a SLM that’s been calibrated itself, on-site right before it’s used!


Thank you Wayne - nice explanation


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