REW Pro - relative SPL calibration for 4 mics

[jhaider]

What's the most efficient workflow to calibrate four microphones to be equal in SPL for a given input for multi-mic measurements in REW? Is there a decent way to do so without using an external SPL meter?

I recently upgraded to REW Pro with an eye to using it for two things. First, taking multiple in-room measurements for spatial averaging in one go, and second, polar response measurements of speakers with less movement of the DUT - basically 4 mikes in a plus sign with the speaker in the middle, and rotate the speaker a quarter-turn for a full 360 degree measurement. To that end I also picked up a Focusrite Clarett 4Pre and 4 iSEMcon measurement mics. One thing that I did not consider until I first experimented with the full rig tonight is how to most efficiently calibrate the 4 mics to be equivalent level without introducing extra equipment.

I've never previously calibrated for SPL because absolute SPL has never been important to me. I generally only care about relative response differences for the measurements I take. But with 4 mics rather than one, they need to be calibrated to read the same SPL for a given input when they're in the same place. So absolute SPL still irrelevant, but relative SPL between the mics is quite important.

Is there a way to do a set-it-once (assuming no mic drift over time at least) and forget? Like cranking the mic gains all the way up, doing one SPL calibration (maybe periodically repeated, like every other year) and hacking the cal files' sensitivity line to make the SPL cal "stick?" The iSEMcon files do have a sensitivity line, but it's not in dB (unit is mV/Pa) and I'm not sure REW reads that line. At least I didn't see any information about that in the REW documentation.

 

[John Mulcahy]

You can add a line of the form

Sensitivity -12.34 dBFS

at the start of each mic's Cal file where the -12.34 figure is replaced by whatever the dBFS input is for each mic on a 94 dB calibrator. If the SPL is different, 75 dB say, the figure just gets adjusted accordingly so if your dBFS input is at 75 dB add 19 dB to that figure to get the value to put in the file. You could also use 20*log10(mV/Pa) to see the relative sensitivities of the mics and offset all those figures to arrive at a 94 dB value.

I will come up with a better way of calibrating the mics for multi-input, it's awkward at the moment.

 

[jhaider]

That would be great, thanks!