# How to calibrate mic to use as the spl meter. Ideas...



## insanegenius (Oct 7, 2010)

Ok, I am about to calibrate my room using REW. I want to use my existing mic to get the room volume levels so I can set my speeker levels accordingly. I don't have a radio shack handy. So, I have an idea.onder:

Now when you first use REW you hook up your line out to your line in. The output is set to 100% and you adjust the line in levels so that they match the line out (speaker out). This I believe calibrates the levels so that you are getting the same db reading coming in as you have going out. (correct me if I am wrong)

If this is correct the my idea is to next calibrate my preamp. So, with the settings above I now hook the line out to my preamp and my preamp to the line in. (Basically puts the preamp in the circuit now) Then since the line in and out are calibrated I now adjust my preamp level to match the line out reading in REW.

This should calibrate the db levels for the whole system right? Next, since my mic has a fairly flat frequency responce I should just be able to hook this up to the preamp now and get fairly accurate db levels in the room right?

Correct me if I am wrong, or tell me if this is right. 
Thanks. :bigsmile:


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## eugovector (Sep 4, 2006)

No, that's right, for the most part. It's not just the overall level that you want to measure, but any abnormalities in the output. In other words, your soundcard may drop everything below 20hz by 2db. By assuming the cable used to connect the output to the input isn't attenuating any frequencies, you create a calibration file for your output/input from the inverse of the measured response. Inserting a preamp, line-level matching box, or filter device will all be measured and calibrated for.

But until you have a calibration file for your mic, you will still get errors there. They may not be great, but they will be present.


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## insanegenius (Oct 7, 2010)

Yeah, I have a calibration file I made for my mic. I have a frequency response chart for my mic and basically plugged those values in place of the calibration files you can download for the berhenger mic.
I believe my preamp is fairly high quality and should have fairly little effect on frequency response. 
I may be 1-2 db out here and there over all using this setup (if it is working the way I think it should).
I figure though that my hearing could be out by that much, So, it should be good enough. 

Any suggestions welcome.


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## eugovector (Sep 4, 2006)

Sounds like you've got it all set.


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## insanegenius (Oct 7, 2010)

Thanks very much for verifying this.

I think I got it working more or less.


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