# Detailed Instructions for SPL Calibration



## subterFUSE (May 10, 2014)

I've been using REW for some time now, and feel like I have a good grasp on things. However, sometimes we often find ourselves moving quickly over some of the basics and then later questioning our techniques. I'm wondering about something like that today.

Topic: Calibrating the SPL with a CM-140 meter and a Dayton EMM6 microphone.


Obviously this is such a fundamental topic, but I honestly don't think I've ever actually seen specific instructions on HOW to do it properly.


Here is what I have been doing:

1. Set soundcard calibration with loopback.
2. Set Input sensitivity via "Levels" in the Prefs window. I usually use the subwoofer for this.
3. Close Prefs, and click on SPL.
4. Click Calibrate.
5. Set my CM-140 to C weighting, Slow, and set range to 100dB max.
6. Adjust volume of my processor until level reads approx 85-90 dB on the CM-140.
7. Enter the SPL from the CM-140 into REW for the calibration.



The reason why I am questioning the process is because when I use the above method to set the SPL in REW, and then I finish tuning my system (in a car, BTW), I always feel like the subwoofer is far too quiet compared to my mains. This is surprising because I always tune with a house curve that has a substantial boost in the bass frequencies, yet it still sounds like not enough subwoofer output when I get finished. And no, it's not because I'm a basshead.  In all of the sound quality competitions I attended this past year my subwoofer performance has been judged as lacking. And no, it's not due to incorrect phase alignment with the midbass. Even with a boost in the crossover region due to proper phase alignment the sub output is still less than expected.

Here is a quick screenshot showing what my latest tune looks like, so you can see how much boost there is to the subwoofer region. My setup is a 3-way. Red line is the house curve.












I have been wondering if I'm calibrating the SPL incorrectly, and maybe that's why I end up with less subwoofer output than I'm expecting based on the graph?



So.... questions to hopefully clarify things for me:



1. When we are calibrating the SPL, should the CM-140 (or whichever SPL meter is being used) be set to C or A weighting?

2. Do we need to select a weighting on the SPL meter in REW before calibrating? I see there are A, C and Z buttons on the SPL meter.

3. Do we need to use a subwoofer signal for setting SPL for the subwoofer measurements, and then re-calibrate with mains signal to measure the mains? Thus far, I have always used the subwoofer signal to set SPL and then left it alone and measured both subs and mains with the same SPL calibration.



Thanks in advance!


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

subterFUSE said:


> Here is what I have been doing:
> 
> 1. Set soundcard calibration with loopback.
> 2. Set Input sensitivity via "Levels" in the Prefs window. I usually use the subwoofer for this.
> ...


Seems to be some confusion here. REW’s SPL calibration routine is simply to “tell” REW where 75 dB is (or 90 dB in your case), in order that the frequency response graphs generated show accurate left-side information. IOW, if the graph shows an 80 dB peak at 65 Hz, you know 65 Hz was hitting an actual 80 dB in the room. 

The SPL calibration routine is actually not needed to get accurate frequency response graphs. You can simply run up the processor’s volume to a comfortable level and enter say, 75 dB in REW. In this case your 65 Hz peak probably isn’t hitting an actual 80 dB, but if the graph shows a 12 dB difference between that and the neighboring trough at 55 Hz, the difference is an actual 12 dB. IOW, the response off the graph is accurate, even if the left-side dB indications are not Make sense? 




> In all of the sound quality competitions I attended this past year my subwoofer performance has been judged as lacking.
> 
> Here is a quick screenshot showing what my latest tune looks like, so you can see how much boost there is to the subwoofer region. My setup is a 3-way. Red line is the house curve.


I have no idea what the judges’ qualifier for “lacking” is, but your graph is not tracking your house curve very well. Seems to me like it would be very weak in the mid bass region.




> 1. When we are calibrating the SPL, should the CM-140 (or whichever SPL meter is being used) be set to C or A weighting?


If you’re calibrating using a subwoofer test tone, use C weighting. A-weighting ignores bass information.




> 2. Do we need to select a weighting on the SPL meter in REW before calibrating? I see there are A, C and Z buttons on the SPL meter.


The setting in REW’s meter has nothing to do with the calibration you’re talking about. It’s merely for using REW itself as a SPL meter.




> 3. Do we need to use a subwoofer signal for setting SPL for the subwoofer measurements, and then re-calibrate with mains signal to measure the mains? Thus far, I have always used the subwoofer signal to set SPL and then left it alone and measured both subs and mains with the same SPL calibration.


Calibrating REW with C-weighting is valid for both subs and mains measurements

Having said all this, your issue (aside from the possible EQ issue I mentioned) is properly setting the sub’s level in relation to the mains. REW can only give you measurements from which to see what the system response is. It can’t tell you if the subs are too loud or too soft, as those figures are entirely dependent on the room and personal taste. Those factors cannot be entered into REW.

Regards,
Wayne


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## subterFUSE (May 10, 2014)

Wayne A. Pflughaupt said:


> The SPL calibration routine is actually not needed to get accurate frequency response graphs. You can simply run up the processor’s volume to a comfortable level and enter say, 75 dB in REW. In this case your 65 Hz peak probably isn’t hitting an actual 80 dB, but if the graph shows a 12 dB difference between that and the neighboring trough at 55 Hz, the difference is an actual 12 dB. IOW, the response off the graph is accurate, even if the left-side dB indications are not Make sense?


Glad to have it confirmed. 





> I have no idea what the judges’ qualifier for “lacking” is, but your graph is not tracking your house curve very well. Seems to me like it would be very weak in the mid bass region.


I failed to mention those measurements are individual driver measurements. If I run a full system measurement with all speakers running then the summed response follows the curve more closely.




> If you’re calibrating using a subwoofer test tone, use C weighting. A-weighting ignores bass information.
> 
> 
> Calibrating REW with C-weighting is valid for both subs and mains measurements


Perfect, thanks!


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## Kix_N_Grins (Feb 14, 2015)

Awesome thread. My house-curve I'm targeting looks a little similar to yours...

Wayne, if a person has subs, is there a time a person would use the "Use Main Speaker to Set/Check Levels" setting?

SubterFUSE, I'm sorry if the next questions doesn't pertain to SPL, but they do pertain to your chart above and it would seem silly to start another thread with your chart... How are you displaying your Target on your SPL screen? I've only been able to display it on the Filters screen.

Wayne, when you referred to having a weak mid-bass, are your referring to 60-120Hz or 200-400Hz as not tracking well (or both)? I understand these are individual drivers, just calibrating myself.

Thanks,
Kix


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## subterFUSE (May 10, 2014)

I imported the target curve as Frequency Response, and then adjusted the Offset until it matched my other measurements.


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

Kix_N_Grins said:


> Wayne, if a person has subs, is there a time a person would use the "Use Main Speaker to Set/Check Levels" setting?


JohnM can probably give a better-informed answer, but I don’t see why it should matter either way.



> Wayne, when you referred to having a weak mid-bass, are your referring to 60-120Hz or 200-400Hz as not tracking well (or both)? I understand these are individual drivers, just calibrating myself.


 I assume you’re talking about the graph that was posted? I was referring to the 60-120 Hz region.

Regards, 
Wayne


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## JohnM (Apr 11, 2006)

Kix_N_Grins said:


> if a person has subs, is there a time a person would use the "Use Main Speaker to Set/Check Levels" setting?


The option isn't very well named, it is really offering a choice between a band of pink noise centred at 50 Hz (so suitable for a sub) and one centred at 1 kHz (so suitable for a main speaker). I'll change the wording.


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## subterFUSE (May 10, 2014)

JohnM said:


> The option isn't very well named, it is really offering a choice between a band of pink noise centred at 50 Hz (so suitable for a sub) and one centred at 1 kHz (so suitable for a main speaker). I'll change the wording.



But once the SPL has been calibrated, whether by sub or main signal, then it can be left alone so long as the input gain is not changed on the sound card?
It's not necessary to calibrate once for main speakers, and a second time for subs, correct?


Sent from my iPhone using Tapatalk


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## JohnM (Apr 11, 2006)

Correct.


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