# Calibrating SPL with mic & preamp question



## morik (Mar 15, 2011)

So I have a mic & preamp (ecm8000 + m-audio mobile pre), and am wondering about the calibrate SPL step. I have a radioshack analog SPL meter.

The guide says to uncheck 'c-weighted spl meter' in the preferences... does this mean I should switch the radioshack meter to use a-weighting? Or do I leave it on c-weighting?

Thanks!


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## EarlK (Jan 1, 2010)

morik said:


> So I have a mic & preamp (ecm8000 + m-audio mobile pre), and am wondering about the calibrate SPL step. I have a radioshack analog SPL meter.
> 
> The guide says to uncheck 'c-weighted spl meter' in the preferences... does this mean I should switch the radioshack meter to use a-weighting? Or do I leave it on c-weighting?
> 
> Thanks!


To Review ; The guide directs one to turn off the "C Weighting" in the preferences panel when ( & only when ) one is using a test mic instead of using the RadioShack SPL meter ( as a test mic for sweeps ) .

When running sweeps ( tests ) it is advised to first/always calibrate the system to a known spl level ( taken at the same location as the test mic ) before running the test ( REW has a pop-up advising the user of this ) . 
- After the sp level is noted and entered into the SPLs calibration info box / run a sweep ( without changing any level settings. 
- If level settings are changed ( due to demands made by REW ), it is again necessary to calibrate the SPL meter section . 
- During this mode keep the "RS SPL" meter on C weighting . Really, the "A Weighting" curve has little utility for this program ( when testing speakers ) .


<> cheers


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## raptor18 (Mar 11, 2011)

Hi

I don't have a SPL meter, only a Behringer ECM8000 microphone.

I have been trying to find some information on what to do on the SPL calibration page when it opens up with the mic only. 

I have calibrated the mic (with a downloaded file) and the soundcard. But what about the SPL? I can't do it, or? If i can't do it, what values should i set? What will i loose if i don't calibrate the SPL?

Thanks :bigsmile:


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## EarlK (Jan 1, 2010)

All you can do is *guess* how loud your test setup is ( for that particular session, at that particular mic location ) . 
- Guessing at the test sessions' db level, simply means that REWs' results ( in the db scale ) will only be relative .

If you _*guess*_ it's 75 db ( at the test mics position, say at 1 metre from the speaker ) then enter into the SPL calibration window , 75 db .

If you are taking very close-up measurements ( say 2" off the woofers cone, etc. ) enter in 85 - 90 db ( assuming the playback level seems the same as your previous session ) .

While anyone can make guesses, (& I've got more 3 decades of experience under my belt doing live sound ), I still use Radio Shack meters for level setting . :blink:

<> EarlK


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## morik (Mar 15, 2011)

Do you have a receiver with a version of Audyssey which has you adjust your subwoofer volume knob?
If so, you can use that...

(E.g., my receiver plays a tone through each of my subwoofers, and displays the SPL of the tone detected by the audyssey microphone. I placed my ECM 8000 so that the mic part was very close to the audyssey mic, and entered the DB level my audyssey mic was showing)

I am not positive, but I think the only thing you miss out on with calibrating the SPL in REW would be that the offset of your graphs may be off... I don't believe it would affect the accuracy other than that. (E.g., calibrated or not, if you have an 8 db dip somewhere, its an 8 db dip. Calibrated may say its from 80 db to 72, uncalibrated may say 76 to 68, but either way it would be accurately telling you how big the dip is)

My reason for guessing this is that you only enter one value into the SPL calibration.


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## raptor18 (Mar 11, 2011)

So, each time i want to do a measurement, i will need to raise the speaker volume so they play the noise at 75 dB and to find out what volume (level) is playing (at the listening spot), i will need the SPL instrument.

Excellent! Thanks, now i know! I feel then i don't need a SPL meter because i don't need really accurate values. Only a rough plot of how my rooms sounds like


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## Max Dread (Oct 20, 2010)

morik said:


> Do you have a receiver with a version of Audyssey which has you adjust your subwoofer volume knob?
> If so, you can use that...
> 
> (E.g., my receiver plays a tone through each of my subwoofers, and displays the SPL of the tone detected by the audyssey microphone. I placed my ECM 8000 so that the mic part was very close to the audyssey mic, and entered the DB level my audyssey mic was showing)
> ...


I'm quite new to this and still learning, but I'd have to agree with your intuitions here. Would be good to hear clarification or confirmation from someone who knows more though. 

One thing sprung to mind...... and please correct me if I'm wrong - or tell me I'm right(!).... the lack of calibration will only tell you where you have problems (peaks and nulls) but it will not tell you where the correct level is. And because the resulting graphs are "wavy", there's no reference point which says "this is the right point". So a dip to 60dB - if done correctly with levels set to 75dB - will tell you there is a loss of 15dB at whatever frequency. But if you are guessing the SPL to be 75dB when in actual fact it is more like 85dB, you are undersetimating the null by 10dB (the true null is actually 25dB). And of course all other measurements are similarly out. 

Excuse me for thinking all this aloud! But is my thinking correct?

Cheers

Max


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## EarlK (Jan 1, 2010)

Morik said:


> I am not positive, but I think the only thing you miss out on with calibrating the SPL in REW would be that the offset of your graphs may be off... I don't believe it would affect the accuracy other than that. (E.g., calibrated or not, if you have an 8 db dip somewhere, its an 8 db dip. Calibrated may say its from 80 db to 72, uncalibrated may say 76 to 68, but either way it would be accurately telling you how big the dip is)
> 
> 
> Max Dread said:
> ...


Your instincts are correct in that if REW is used uncalibrated the following happens ; 

- One still achieves relative accuracy within the resulting trace ( ie; an 8 db dip is still an 8 db dip / while a 3 db peak is still a 3 db peak ) .
- One only loses the absolute SPL level that the trace was made at / which is generally not a show-stopper .


*OTOH ;*
- My recommendation is that when people first start using REW, they employ the KISS method .
- That translates into *using the computers on-board sound-card with a Radio Shack SPL meter ( or SilverLine ) as the test mic .*
- For 75% ( or more ) of the casual users out there / that's all they'll ever need ( for measuring subs ) .
- Therefore , with these recommendations, one can start with the SPL actually calibrated ( all cheap & cheerful ).

<> EarlK


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