iAudioInterface2 Mic Calibration
Instructions
Calibrating with a Calibrator
Calibrating with a Reference SPL Meter
Table of Typical Settings
Using Frequency Compensation Files
Calibrating for Very Low Noise Microphones
Import / Export of Calibration Profiles
iAudioInterface2 supports many professional microphones, even those
that require full 48 Volt phantom power.
The Basics
First, you need to know if your mic requires phantom power or not. In
theory, a non-phantom microphone will not be damaged by applying
phantom power, since the phantom voltage is common mode and will be
rejected, but it is possible to damage it depending on how you plug it in.
In this procedure, we will be setting the hardware mic preamp gain in
iAudioInterface2, and setting two trim values in the calibration page
in AudioTools.
Multiple Calibrations Supported
On the Input Source page, you will initially see two entries: iAI Mic,
and iAI Line Input. If you have only one microphone, you will use the
first entry. But if you have more than one mic, or end up getting
another one later, you can always tap the + button and add a new mic
calibration entry.
And, you can delete entries that you have added by swiping left on the entry, and tapping the Delete button that appears.
Understanding Mic Ranges and Gain
Ranges
AudioTools supports the concept of 2 mic ranges, High and Low. This
lets you quickly select an appropriate range, based on your situation.
Think of this as having two stored calibrations for your mic that you
can pick at any time.
The low range will be set up to work best in quiet situations, and the
high range will be setup to work in loud environments.
In order to set these, you will be adjusting the amount of preamp gain that
iAudioInterface2 applies. This is called the "gain range", and is a
hardware setting, like a gain control knob on a typical mic preamp.
Since our gain ranges are fixed, they are repeatable, and perfect for
test and measurment, because you will always get the correct calibrated
level, without worrying about nudging a level control and ruining your calibration.
So in these instructions, we will explain how to set the hardware gain
ranges in iAudioInterface2 to create two mic ranges in AudioTools.
The Calibration Screen

The calibration screens has these fields:
- Range - High or Low
Selects the gain range that the values on the screen apply to.
- dB
Shows the actual dB that is being read from the mic. Can also be an input field.
- Trim
The adjustment being added to calculate the dB value.
- + / -
Nudges the Trim value up or down
- dBFS
Shows how close to clipping ( 0 dBFS) you are.
- Name Field
Name of the current mic
- Description Field
A text description of the mic
- Gain Range
The amount of preamp gain that iAudioInterface2 is adding
- Autorange
Setting to turn autorange on or off (used by some modules)
- 48V Phantom Power
Turn microphone phantom power on or off.
- Cal File
Open the Calibration Files page.
- Output
Turn on to listen to the mic, or off to avoid feedback
Calibrating with a Calibrator
If you have access to a calibrator, follow these instructions. This
assumes a typical setup, with one more sensitive setting (low range)
and one for louder situations (high range).
Before you plug the mic in, select the proper phantom power setting,
Now select Low Range, plug the mic in, and attach and turn on the calibrator.
Try different gain range settings (perhaps start with +20) and see how
much headroom you have, by looking at the dBFS field. You want to get
close to 0, but not too close. -7 to -2 dBFS would be perfect.
Once you have picked a gain range, now you can enter the calibrator
value. If your calibrator is 94 dB SPL, just tap the dB field, clear it
out (tap the tiny x) and enter 94, and press Done.
You should now see 94 in the dB field, and some value (doesn't matter what) in the trim field. The app has set the Trim for you.
This one's done, now let's switch the high range.
Select High Range, and do the same thing, except this time you want
more headroom. How much more? Depends how high you want to measure. If
you allow 20 dB headeroom (reading -20 dBFS), then you could measure 20
dB above the calibrator, or 114.
Want to measure 130 dB SPL? Leave 36 dB headroom above your 94 dB
calibrator setting. You can go as high as you want, as long as your mic
actually works at that level.
That's it, your done. This calibration will be stored, and will be in effect automaticaly next time you start them app.
Calibrating by Comparison to a Calibrated Meter
This procedure is largely the same as calibrating with a calibrator,
but you will be adjusting the Trim value up and down until the dB
reading matches the dB reading of the reference meter.
Here are the steps.
Low Range Calibration
Select Low Range, set phantom power on or off, and connect the microphone.
Setup a speaker playing pink noise, at a moderately loud level, say 75
- 80 dB SPL. Set your reference meter to unweighted or C weighting.
Orient the test microphone and the reference meter microphone close together, pointing in the same direction.
Now select a gain range on the calibration screen that will give you
about 20 to 25 dB of headroom, so the dBFS reading will be -20 to -25.
Ajust the Trim field, either by entering a number, or by using the +
and - minus buttons, until the dB field reads the same value as the
reference SPL meter.
High Range Calibration
Follow the same steps as above, but first select High range, and then
select a preamp hardware gain setting that gives you 40 to 45 dB of
headroom, so the dBFS reading will be -40 to -45.
Now adjust the Trim field until the dB reading on the screen matches the reference meter.
Being Absolutely Sure You Are Not Clipping
Since we have mulitple tools in AudioTools, it's easy to check
that you haven't set the gain too high. Just look at the waveform from
the calibrator in the Audio Scope. Here's how to do that:
The only tricky part is setting the scope to read the mic. Open the
Audio Scope on the Line Input menu, and go to the Setup page, and set
it like this:
- Line Input Mode - Mono
- Input Source - Mic
Now go back to the Scope screen, and turn on the calibrator. Make sure
the correct mic range shows on the Scope screen, if not, change it in
one of the Acoustic modules, like SPL Meter. Just look at the waveform,
on 1X Gain and 1 ms / Div, and see how close to clipping you are.
Anywhere below full screen is fine.
This shows a mic set to about -2 dBFS:

Calibrating Another iOS Device
If you have another iOS device, you can simply copy the Trim settings
and enter them directly. Since the analog parts and conveters are in
iAudioInterface2, the same Trim values will work in another iOS device. Or, use our Import / Export feature described below.
Calibrating for Extremely Low-Noise Microphones
Some microphones can measure very low noise levels, and if the noise
floor of the mic is below 5 or 10 dB SPL, you may not be able to take
advantage of its full capabilities, if you calibrate with a 94 dB SPL
calibrator.
This is because the dynamic range of a 16-bit system such as this one
is at most 96 dB, so it is possible that the noise floor of our system
will be reached before the noise floor of the microphone, if the preamp
gain range is set such that the upper clip point is near 94 dB.
To get around this, there are two methods. First would be to use and
SPL attenuator, or a lower calibrator level, such as 74 dB SPL. If this
is not possible, then the calibration can be translated down to a more
sensitive range.
These are the steps needed for this process:
1) First, do the normal low-range calibrattion, as described above.
Write down the Trim value and preamp gain range setting for this range,
in case you want to go back to it.
2) Next, determine how much additional sensitivity is required. You may
want another 10 ro 20 dB of sensitivity, to take full advantage of your
quite mic. Let's call this value "x".
3) Now, you will need a calibrated generator, set to 1000Hz. Plug the
generator into the mic input, and reduce the generaotr level until you
are reading "x" dB, below the calibrator level. For example, if you
want an additional 20 dB of sensitivity, adjust the level so that the
dB field reads 74.
4) Now set the preamp gain range higher by "x" dB (or slightly more).
Adjust the Trim field value until you once again read the same 74 dB
SPL.
This new Trim number is your "super-sensitive" Trim value, at the new
preamp gain range. Remember, with these settings your mic will clip "x"
dB sooner, so use the appropriate mic range.
Table of Typical Calibration Values
This table lists typical calibration Trim values for some
microphones that we have tested. Given analog component tolerances, and
microphone variations, you should be with a few dB if you use these
values.
AudioControl Type 1 Mic
Low Range, +25dB gain, -12.3 Trim
High Range, +10dB gain, -16.7 Trim
AudioControl CM-20
Low Range, +45dB gain, -10.5 Trim
High Range, +25dB gain, -12.0 Trim
Josephson C550H
Low Range, +45dB gain, -11.6 Trim
High Range, +25dB gain, -13.6 Trim
Using Microphone Frequency Compensation Files
Beginning with AudioTools v4.1, you can import a file containing
frequency response data for a microphone, and that response curve will
be applied to the microphone in many AudioTools modules.
The microphone response correction data will be applied in all modules
that display frequency dependent information: RTA, FFT, SoundScope, and Transfer Function. Note that it will NOT be applied
to basic SPL measurements.
AudioTools supports the pending AES standard for frequency response
data (.frd) files, as well as most typical mic cal file formats.
Specifically, files that conform to these rules may be read in:
- File names must have either the .txt or .frd extension.
- Files must contain ASCII characters (not binary data).
- Lines must have a frequency value followed by a dB value,
separated by a tab, comma, or blanks. Values may be in IEEE format,
integer format, or floating point. Extra values are ignored.
- Lines must be in order of ascending frequency.
- Any number of data points may be present.
- Lines beginning with '*' or ';', or blank lines, are skipped.
Note that a frequency entry takes effect at or after the frequency in
the file, and will remain in effect until the next frequency entry is
reached. The final entry is applied for the remainder of the frequency
spectrum.
To import a file into AudioTools, you have two options, to use Dropbox, or a web browser.
Using Dropbox to Upload Mic Compensation Files
First, link AudioTools to Dropbox. Start at the Settings->General page, and tap the Dropbox button.
When you have linked to Dropbox, just copy your mic compensation files
to the public/calibration folder, under the app name. The full path for
this folder is Dropbox/Apps/AudioTools/public/calibration.
Each time you open the Calibration Files screen, all of the files in
that folder are copied to your iOS device, and can be selected. If you
need to change a file, just drop the modified file into the Dropbox
folder and re-open the Calibration Files screen. The new file will
overwrite the file on the iOS device.
Using a Browser to Upload Mic Compensation Files
Second, to use a browser on a PC or Mac, follow these steps.
First, create the file and copy it to your iOS device:
- Create your calibration file according to the rules above.
- In AudioTools, go to the Settings->Microphone screen, and tap the Cal File button (Calibration File on iPad).
- Tap the Files button to bring up the Files utility screen.
- On your computer, open a browser and enter the URL from the AudioTools Files screen. Make sure your iOS device and computer are on the same WiFi network.
- On your computer browser, select the "calibration" link to access the calibration folder.
- Now select the "Choose" button on the browser, and navigate to the mic cal file that you wish to upload.
- Click "Send" to transmit this file to AudioTools.
Then, apply the calibration file to the microphone calibration entry:
- On AudioTools, If the Files window is open, tap "Done" to go back to the Calibration Files screen.
- Tap the Reload button (rounded arrow) to refresh the screen to make sure all of the files are showing.
- Tap the mic cal file name, which should now be visible.
- Tap "Apply". If the import is successful, you will see a message showing the number of data points imported.
- The import is complete. Tap done to go back to the Calibration screen.
At any time, you can revert to the default configuration by tapping the "Default" button on the Calibration Files screen.
The name of the current calibration file (or the word "default") will appear on the Calibration screen.
Export / Import of Microphone Calibration Profiles
Once
you have defined your microphone calibration profiles, you can export
them in a text file, and then import them on another iOS device. It's a
good idea to keep a copy of your microphone profiles, in case you
delete the app, which also deletes the profiles stored on the device.
Export
To
export the profiles, tap the "Export" button, and then go to your
browser as described above, navigate to the calibration folder, and
download the profiles, which are stored in a file named
"calibrationprofiles.txt". Or, if you have Dropbox file synchronization
enabled, the file will automatically appear in your Dropbox.
Import
If
you already have a calibrationprofile.txt file stored on your device,
you must first delete it. To do this, go to the Utility-Files page, tap
the Documents button, select the calibration folder, tap Edit, and
delete the file.
Now use your browser, as described above, to navigate to the
calibration folder, which should now be empty. Select the file to
upload from your computer, using the Select button on your browser, and
then tap send.
Now go to the Settings-Microphone Setup page in AudioTools, and tap Import. After confirming, the profiles will be imported.
File Format
Calibration profiles are stored in plain text files, named calibrationprofiles.txt. The format of the file is a follows:
First line -- header
Subsequent lines -- calibration profiles for one microphone
The data is comma separated:
name -- text
description -- text
phantom -- 0 for off, 1 for on
polarity reverse -- 0 for normal, 1 for reversed
compensation filename -- text, name of compensation file, if any
low trim -- float, trim value
low gain range -- integer, gain selection index, 0 = max gain
high trim -- float, trim value
high gain range -- integer, gain selection index, 0 = max gain