AudioTool Calibration Method by Rex Beckett

I've created an Excel tool to help with calibrating AudioTool platforms. Here's how it works:

Plug a source of pink noise into the device's mic input. This should be a good, flat source. A test CD on a player with a line output should work or you could use a PC program like Audacity if you have a USB sound adaptor. I would not trust the headphone output of any phone or PC. I used an old NTI MR1 which is pretty good. You will need an appropriate adaptor or cable to connect to the mic terminals on the 3.5mm jack. I've seen them on eBay. I made mine by sacrificing an old headset and soldering the cable to an XLR Female. Set the level to give a similar reading as you get from your measurement mic. Set Weighting Flat, Averaging On and Use Single Calibration. Once it has settled, Store the spectrum. This should be the frequency response of the mic input of your device. Copy the at file to a PC.

Open up the AudioTool Calibrate Excel and click the Load Pink AT File button. Select the file you just stored and wait a few seconds. The tool will analyse and plot the 1/3rd octave response of the mic input. The response figures are relative to that at 1kHz.

Now you can add frequency response factors for your measurement mic for each band under Mic dB. This is often provided as a plot or table with the mic or from the spec-sheet. If the plot shows the mic response as -3dB at 100Hz, for example, enter -3.0 in the Mic dB field on the 100Hz row. You can also make adjustments if you want to tailor the overall response by setting offsets in Trim dB. The tool shows plots of the initial pink response, the mic response, the calculated calibration curve and the theoretical final result.

Click the Make CAL File button and chose an appropriate name and path and the tool will create the cal file. Copy this to your device and load it into AudioTool with the Load Cal button. You will probably now need to recalibrate the single-point adjustment for SPL using a mic calibrator or known accurate reference meter. In my case, all the correction was at the low end and it had almost no effect on the SPL calibration. 

Just to be clear: This is not a substitute for proper calibration with reference sources and instruments. It is a technique that may help to improve the accuracy of an otherwise uncalibrated setup. It should also be noted that, left to its own devices, the tool will attempt to level bands that have little or no response at all. The resulting cal file could give strange results in AudioTool. I recommend using the Trim dB factors to curtail crazy corrections and generally smooth the calibration curve to a sensible shape. As with sound systems, it is often better to accept a few ripples and some of the natural roll-off at the top and bottom ends.

Anyway, here it is. I hope it helps somebody.

PS Here is the .at file to REW converter.