# SPL Calibration



## Phillips (Aug 12, 2011)

Hi how do i calibrate the SPL without a SPL meter?

I have a Steinberg C1 and Behringer ECM8000 (calibrated).

Thanks in advance


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## HifiZine (Feb 7, 2013)

If a) your mic calibration includes its sensitivity figure (e.g. -39 dBV wrt 94 dB SPL), and you know b) the gain setting of your mic preamp and c) the full scale sensitivity of your A/D convertor, you can work it out.

You also need a way to see what signal with respect to full scale is coming out of your convertor - I confess that I'm not sure if REW will show this to you.

It's a lot easier to use an SPL meter, at least until you get everything set up to do the above repeatedly.


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## Phillips (Aug 12, 2011)

Thank you for your reply.



> If a) your mic calibration includes its sensitivity figure (e.g. -39 dBV wrt 94 dB SPL), and you know


Yes i do the calibration was done by Cross-spectrum.



> b) the gain setting of your mic preamp


Sorry not sure what you mean?



> c) the full scale sensitivity of your A/D convertor, you can work it out.


Sorry not sure what you mean?



> You also need a way to see what signal with respect to full scale is coming out of your convertor - I confess that I'm not sure if REW will show this to you.


Would a Digital Volt Meter help with any of this?

Thanks again


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## HifiZine (Feb 7, 2013)

Yes, the DVM will be very handy.

The preamp gain and full-scale input voltage are so that you can "translate" the voltage from the mic (related to SPL by its sensitivity) into the value that the computer sees. However, I'm thinking there's a way to combine the two and make it all work in REW - with the help of that DVM (and a loopback cable) . I'll work on it tonight.


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## HifiZine (Feb 7, 2013)

Oh. Actually, thinking about it, it should be a lot simpler than I thought. Something like:


Connect your loopback cable to the output of your soundcard
Use the REW signal generator to generate a 1 kHz sinewave. Set the output level to say -20 dB.
Using the DVM, measure the voltage of the generated signal (needs to be across the balanced connections on the XLR, a TRS-TRS cable is actually easier for this, or use clipleads).
Adjust the output level until the reading is 10x the mic sensitivity in mV. So if the sensitivity is 11 mV/Pa, adjust the level until the DVM reads 110 mV (RMS)
Plug the loopback cable into the mic input (NB turn off phantom power first)
Adjust the input gain on your mic preamp so the REW level meter reads fairly high, say -2 dB
In the REW SPL meter, use the calibration window and set it to *114*.
Turn off the signal generator, unplug loopback cable, turn on phantom power, plug in mic. You should be set to go  You can change the output levels to suit, but don't change the input gain on the mic pre.

I _think_ this will work. I'll give it a try tonight.


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## jtalden (Mar 12, 2009)

That is the way I have calibrated my REW SPL meter on some occasions. It works fine.

Just some notes for those who want to try this to achieve higher SPL accuracy:
> It is necessary to have a individual mic sensitivity calibration rather than a generic spec. 
> It is best to have a DMM that that is designed for accurate measurements at 1000 Hz. The least expensive ones are often designed for 60 Hz and start to lose accuracy as the freq changes.
> I’m expecting that it may be advantageous to use a voltage divider on the loopback so that the total output voltage can be greater while keeping the input at the very low value needed. This should provide a lower noise signal at the mic
input. I haven’t actually taken this step or checked out this expectation however.

I would expect that if the above care is not taken, the results will be no better that an inexpensive SLM will provide.

Other comments:
> If you are measuring at the recommended 75 dB with REW, I would recommend a using sensitivity x 2 and 100 dB rather than sensitivity x 10 and 114 dB. This will avoid the warnings in REW that the signal level is too low whenever a measurement is taken. If you are testing at higher levels then the calibration can be adjusted up accordingly. 
> I checked my calibration vs my 30 yr old RS SLM using Pink noise from my Pre/Pro and the difference was only 0.5 dB so it confirmed my RS SLM cal was still reasonable.
> Most of us are only looking at relative values anyway so there is no practical need for SPL cal accuracy. If you have need for SPL accuracy and have a well calibrated mic this method can be more accurate if done correctly.


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## Phillips (Aug 12, 2011)

Thank you

Just a question, wouldn't this be level (volume and mic) dependant on the pre/soundcard?

Thanks again


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## jtalden (Mar 12, 2009)

Yes, in my case I set the mic input volume at about 9.5 (out of 10). If I calibrate it that way I end up with SPL headroom of about 103 dB if I want more headroom then I turn down the Input volume a little and redo the calibration. The new headroom will be reported as something higher.

After the cal is completed we cannot change the mic input volume on the audio interface or the SPL calibration is lost.

We just need to experiment a little with our particular equipment to find an appropriate calibration setting.


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## HifiZine (Feb 7, 2013)

Phillips said:


> Just a question, wouldn't this be level (volume and mic) dependant on the pre/soundcard?


The key thing is that you've used the DVM to set a voltage that is related to the mic sensitivity. e.g. if the voltage is 110 mV, then that is 20 dB more than the sensitivity figure of 11 mV @ 94 dB - so you tell REW that it's 114 dB.

What the actual signal level that REW sees is not important, other than for setting the amount of headroom, as jtalden says. If you end up with too much headroom / "signal level too low" then one approach would be to reduce the level in the signal generator by 20 dB (after measuring it at 110 mV) and enter 94 into the SPL calibration.

I didn't get time to work through this last night but will in the next few days.


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## Phillips (Aug 12, 2011)

HifiZine said:


> The key thing is that you've used the DVM to set a voltage that is related to the mic sensitivity. e.g. if the voltage is 110 mV, then that is 20 dB more than the sensitivity figure of 11 mV @ 94 dB - so you tell REW that it's 114 dB.
> 
> What the actual signal level that REW sees is not important, other than for setting the amount of headroom, as jtalden says. If you end up with too much headroom / "signal level too low" then one approach would be to reduce the level in the signal generator by 20 dB (after measuring it at 110 mV) and enter 94 into the SPL calibration.
> 
> I didn't get time to work through this last night but will in the next few days.


Thank you very much for your time

I am very new to this so very interested in the workings.

Thanks again


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## HifiZine (Feb 7, 2013)

Sorry for the delay. I wrote this up a few days ago but hesitated to publish it as I don't have a way to verify my results:

http://johnr.hifizine.com/2013/03/room-eq-wizard-spl-calibration-without-an-slm/

I _think_ it's OK, but with the caveats explained there. If you have a go at it, please do report your results. Thank you


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## jtalden (Mar 12, 2009)

Very nice write-up


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## Phillips (Aug 12, 2011)

HifiZine said:


> Sorry for the delay. I wrote this up a few days ago but hesitated to publish it as I don't have a way to verify my results:
> 
> http://johnr.hifizine.com/2013/03/room-eq-wizard-spl-calibration-without-an-slm/
> 
> I _think_ it's OK, but with the caveats explained there. If you have a go at it, please do report your results. Thank you



Thank you very much i will give it ago once i find the correct connections.

I had a quick read, and as far as the cable, i used 2 x TRS for the (loop back) soundcard calibration, no XLR.

The MIC XLR connections are both male and female.


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## HifiZine (Feb 7, 2013)

Cool, thanks  For the method to work as written up the XLR needs to be used so that you get the same gain as for the mic. The TRS on the combo jack inputs is for line level, the gain could be 20 dB different. If the gain difference was known the method could be adjusted but I can't find anything in the specs for your soundcard about that.


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## HifiZine (Feb 7, 2013)

I updated step 6 - I forgot to say that is where you adjust the input gain of the soundcard.


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## Phillips (Aug 12, 2011)

HifiZine said:


> Cool, thanks  For the method to work as written up the XLR needs to be used so that you get the same gain as for the mic. The TRS on the combo jack inputs is for line level, the gain could be 20 dB different. If the gain difference was known the method could be adjusted but I can't find anything in the specs for your soundcard about that.


Thank you

So is TRS to TRS not suitable for loopback or just SPL calibration?

Is XLR to TRS ok?

I asked the supplier (Steinberg C1) today (before this post) and he said to use the top two bands of the TRS connection to save more cabling as this is the same as using the XLR pins.

Thanks again


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## HifiZine (Feb 7, 2013)

Phillips said:


> So is TRS to TRS not suitable for loopback or just SPL calibration?


TRS-TRS is fine for frequency response calibration. In fact it's possible that it's better, but it's not something I'd worry too much about.



> Is XLR to TRS ok?


I don't understand the question. If you mean for frequency response calibration, yes, it's fine. If you get a TRS-XLR cable you can try it both ways and compare 



> I asked the supplier (Steinberg C1) today (before this post) and he said to use the top two bands of the TRS connection to save more cabling as this is the same as using the XLR pins.


The question to ask is whether the TRS and the XLR connections have the same _gain_. Usually, they don't, as TRS is designated for "line" and the XLR is for "mic". Line level signals are higher in level than mic, and hence the gain is lower. If you knew the exact gain difference, it could be accounted for, but the specs for your soundcard (like most) are shy on detail like that.

Hope this helps


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## Prodba (Oct 12, 2010)

Phillips said:


> Hi how do i calibrate the SPL without a SPL meter?
> 
> I have a Steinberg C1 and Behringer ECM8000 (calibrated).
> 
> Thanks in advance


Pardon me for asking, but what exactly do you mean with calibrating the SPL?
Do you maybe mean; knowing what the Sound Pressure Level is of your monitoring, given a certain position of your volume knob or reading on your meters?

SPL has nothing to do with voltmeters, which measure the voltage of the signal inside or between audio equipment, and can be translated to dBu's, which are a different thing than dB SPL.
SPL is a measurement of actual soundwaves (what you actually hear).
You can have your equipment producing whatever voltage, and even your loudspeakers at any volume and still get different SPL readings, depending on where you stand in the room.

Also XLR to TRS is no problem at all; you can have balanced I/O on both XLR as well as TRS.
And even if you have to go from balanced to unbalanced or vice versa, it's no big deal if the wiring is done correctly.
A different thing is the level difference between your input and output; +4dBu versus -10dBu or -20dBu.

Regards,


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## HifiZine (Feb 7, 2013)

Prodba said:


> Pardon me for asking, but what exactly do you mean with calibrating the SPL?


So that the decibel scale of a measurement made in REW is correct.



> SPL has nothing to do with voltmeters


Sure it does  The microphone converts from SPL to a voltage signal. If you know that conversion ratio, and the gain of the subsequent preamp, then you can figure out what the SPL is by measuring the voltage. That is, in essence, what an SPL meter does.

The question in this case is, knowing the conversion ratio between SPL and voltage, how can you get REW to display the right SPL values? What the voltmeter is used for is to generate the same voltage as the microphone does at 94 dB SPL. Then you tell REW that signal represents 94 dB, and it all works


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## Prodba (Oct 12, 2010)

HifiZine said:


> So that the decibel scale of a measurement made in REW is correct.
> 
> 
> 
> ...




Thanks for the clarification; I get the theory and I also understand the question now.
Not understanding the question also made me read over all the replies that, in retrospect, were very clear (how my psychology puts me on the wrong foot, once again).

Anyway, I'm going to re-read those posts as, even though I do own an SPL meter, they are very interesting to try out; you never know when you may find yourself having to get all Mc. Guyver and stuff, hi, hi, hi.

Regards,


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## Phillips (Aug 12, 2011)

Thank you

I will purchase a XLR to TRS cable and try the SPL calibration.

I will report back with the results.


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## Phillips (Aug 12, 2011)

HifiZine said:


> Cool, thanks  For the method to work as written up the XLR needs to be used so that you get the same gain as for the mic. The TRS on the combo jack inputs is for line level, the gain could be 20 dB different. If the gain difference was known the method could be adjusted but I can't find anything in the specs for your soundcard about that.



Hi i haven't had a chance to do the calibration.

With a TRS to XLR cable are they always balanced?

Thanks in advance


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## HifiZine (Feb 7, 2013)

Phillips said:


> With a TRS to XLR cable are they always balanced?


I would assume that they are, I can't think of why they wouldn't be.


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## Phillips (Aug 12, 2011)

HifiZine said:


> I would assume that they are, I can't think of why they wouldn't be.



Thank you very much

Once i get the correct cable i will post with some results.


Matter of interest have you had any dealings with the Antimode Dual Core 2.0?

I am trialing one currently, if you are intersted check out my thread "Check my meaurements".

Thanks again


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## HifiZine (Feb 7, 2013)

Phillips said:


> Matter of interest have you had any dealings with the Antimode Dual Core 2.0?


No, never seen or used one, although I've read a little about it. My own preference is for DSP units that are complete active crossovers, but the AM dual core looks like a well thought out package. I would be interested to know how you go with it (send me a PM if you like  )


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## Phillips (Aug 12, 2011)

HifiZine said:


> No, never seen or used one, although I've read a little about it. My own preference is for DSP units that are complete active crossovers, but the AM dual core looks like a well thought out package. I would be interested to know how you go with it (send me a PM if you like  )



Absolutely will PM you.

Attached is some REW graphs which is the Auto EQ.

It has auto eq, as well as a Manual PEQ with 18 filters per channel with large plus and minus gains and BW.

It has alot of different settings

What is your view on EQing from say 500hz up, and how? I believe from this frequency range you are EQing the speaker, not the room.


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## HifiZine (Feb 7, 2013)

Phillips said:


> What is your view on EQing from say 500hz up, and how? I believe from this frequency range you are EQing the speaker, not the room.


Well, it's a bit of a complex topic. My preference is for active systems, in which case you do EQ to correct the response of individual drivers and the box/baffle using "quasi anechoic" measurements (or as near as you can get). With speakers in a room, you can then choose to apply EQ or not. The problem then is that if looking only at the amplitude response in the frequency domain, the measurement program/equalizer is not able to distinguish time or direction of arrival, never mind change in directivity with distance (an issue for some speakers like array and panel speakers), so can easily be led off on a false scent, as it were.

So I don't have a specific conclusion, I don't think it's either good or bad. You just have to be careful about what the measurement actually _means_. I have no hesitation in applying EQ (above 500 Hz, well more like 200 in my case) over and above what is done for the drivers and box/baffle, if I think it makes the system sound better, but in the case of automatic algorithms I think it's going to come down to how smart the algorithm is and to some extent, how smart the operator is in giving the algorithm the parameters to work with (the position of the microphone would be one such parameter...).

I think this is an area where we are likely to see rapid development in the next few years. DSP capability seems to be coming down in price and units like the Dual Core are helping to break down prejudice against DSP in general.

How's that


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