To begin calibration, select Low Range (darker). Select gain range +10 as a starting point (see
picture below). Turn on Phantom Power, if required by your microphone. Now attach and turn on
your calibrator. Leave the calibrator running for the remainder of this procedure.
Before setting the calibration value, you need to select an appropriate microphone gain range. This
gain is added before the A/D converter, so for the low range you typically want the highest dBFS
value possible, without clipping. Note that for a sine wave 0.0 dBFS is the highest value that can be
reached without clipping, so you want a number that is lower than 0.0 dBFS by several dB.
Note: AudioTools uses the AES standard for dBFS, which is that for a sine wave just reaching
clipping, the reading will be 0.00. A square would by this formula show +3.00 dBFS.
Slowly increase the gain range setting until the dBFS value nears, but does not exceed, 0.0. This is
your input gain range setting for the low range. Now enter the calibrator output setting (typically
94) in the dB field by tapping on the field, clearing the value in the field, and entering your
calibrator’s fixed value. Tap Done. The trim value will be set automatically by this process, so do
not alter it.
The low range setting is now complete.
Now select High Range (darker). Select a gain range that is lower by as many dB as you wish to
have as headroom above the calibrator value. For example, if your calibrator value is 94 dB, and
your low gain range was +35, and you wish to be able to measure 120 dB SPL, set the gain range
value to 0 dB, which would actually allow 94 + 35 = 130 dB SPL. After selecting the desired gain
range setting, once again enter the calibrator value in the dB field.
You can if you wish rename the microphone, and change the description to something that describes
your microphone. This name will appear on the screen of any function that is using the microphone.
Since these settings will be lost if you delete the app or move to another iOS device, we recommend
writing them down somewhere that you can recall them. Also, the settings are transferrable to
another device. Simply open the Microphone Calibration screen and enter the values.
After setting the ranges, it’s a good idea to double check your headroom using Audio Scope. To do
this, exit this screen, and go to the Line Input menu, Audio Scope screen. Go to settings (tap the
wrench icon), and select Line Input Mode Mono, Input Source Mic. Now go back to the scope
screen and check that the sine wave is not clipped. It should look something like this picture:
iAudioInterface2 User Guide
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