Calibration Information

Built-in iOS Mic Calibration

iTestMic & iPrecisionMic Calibration

USB Audio Devices

Using Microphone Compensation Files

AudioTools uses default calibration values for all iOS devices, which in most cases will be within a few dB. For more accurate results, you can calibrate AudioTools to your iPhone / iPad / iPod touch.

AudioTools stores separate calibration values for all sources and applies those automatically when you attach any audio input device.

These instructions and more information about microphones are available on our website.

Low Range and High Range
For many iOS devices, we are able to adjust the hardware gain for the microphone. For these devices, you will see a Low Range and High Range. Low Range will provide the most sensitivity, and be able to read the lowest SPL levels. High Range will be able to read louder SPL levels, in most cases up to at least 120 dB SPL.

If your hardware does not support direct control of its input gain from our app, you will not see the Low and High range options.

Calibration Instructions
These instructions are for calibrating the individual settings for the microphone.

If you are using the built-in or Apple headset mic our defaults should work well for you. However, if you have access to a calibrated SPL meter, or you are using a third-party microphone, you can adjust the calibration for more accurate readings.

Note that if two ranges are shown, (High and Low) then there are two separate trim constants stored, so that you can calibrate each range correctly. Not all iOS devices support multiple gain ranges.

Built-in Microphone Calibration
To calibrate the mic input, you will need either a microphone SPL calibrator, or a reasonably constant acoustic noise source, such as pink or white noise, and a calibrated reference SPL meter set to A weighting and Slow response. Do not use a single frequency tone. If you are using acoustic noise, turn down the low frequency. This is important, since often there will be discrepancies in low frequency response of both the iPhone mic, and the reference SPL meter. The idea is to calibrate the center of the measurement frequency window, especially since A-weighting rolls off low frequencies.

Now turn up the noise source until you read about 75-85dB SPL on the reference meter, or attach the calibrator to the
SPL Meter and turn it on.

Go to the Microphone Setup page, and if there is more than one input device showing, select the one that you wish to calibrate. The Calibration screen will now appear:

Low and High Range
Select Low Range or High Range. Now, tap the dBA field and enter the observed value from the reference SPL meter or the calibration value from the SPL calibrator. This will set the Trim field, which is added to the internal dB values. You can also tap the + or - keys to adjust the level, or enter a Trim value manually.

dBA Slow and dBFS
These values show the current measured SPL level, as dBA weighted Slow, and as dBFS. dBFS indicates how much headroom you have before the input starts to clip. Note that many A/D converters will show some headroom even when they are overloaded.

For normal calibration using a calibrator, just tap the dBA Slow field and it will be filled with the default calibrator level. Tap done and the trim will be adjusted so that the dBA reading equals that value.

The default calibrator level may be customize on the Settings->General page, under Global Audio Settings.

Trim

This is the value that will be added to the internal calculations to adjust the readings. Increasing the number here will increase dB levels, and decreasing it will decrease SPL readings.

Gain

This field, if present, will set the iOS device analog input electronics gain setting. 1.00 is maximum gain, which will provide the lowest possible SPL readings, and 0.001 is the lowest gain setting, which will allow reading the highest SPL levels.

We suggest leaving these set to the default values, which are optimized per each device. If you change these, you will need to also recalibrate by using the Trim value.

If the field is not present, it means that the iOS device does not support setting the input gain.

Mic Name and Description
You can rename the mic, or change its description.

Log Cal
Tapping this button will cause a microphone calibration log file to be created. This file will be stored in the public/calibration folder, and will include the date and time as well as the calibration details.

Cal File
Tapping this button brings up the microphone frequency compensation file screen, see below. Note that the name of the current compensation file, if any, will appear beneath the button.

Reverse Polarity
For some microphones, a Reverse Polarity switch will be shown. If you determine that your polarity readings are backward, you can invert the polarity readings by turning this switch on.

AutoRange
If you want to enable Autorange, turn that on. Some modules will change the mic gain to match conditions.

Output
If you wish to monitor the microphone in headphones, turn on Output, and connect headphones that have a 3-conductor plug (not 4-conductor) to the headset jack on your iOS device.

Exit the page, you are done.

iTestMic and iPrecisionMic

When these microphones are plugged in, the app retrieves the trim values from the microphone. These values will be shown in the trim fields. If you change these values, the calibration for the current session will be changed, but the internal stored trim values will not be changed.  If you wish to also update the values in the microphone, use the "Update" button on the bottom of the screen. This will store the new values in the microphone.

USB Audio Devices
AudioTools supports all USB and dock audio sources that are recognized by IOS. Some USB audio interfaces may be used by powering them with an external hub, and using the Apple Camera Connection Kit. Newer iPhones and all iPads will work the the CCK.

When you plug in your USB audio devices, 2 entries will be made in the calibration list. The first entry is for the right channel, and the second entry is for the left channel. Most AudioTools acoustic analysis and SPL modules expect the microphone to be present on the Left channel. The Right input channel is normally used for line input modules, such as THD+n.

Microphone Frequency Compensation Files

AudioTools uses microphone frequency compensation tables that we have created by carefully measuring the frequency response of the internal microphones in all iOS products. This frequency compensation should work well for most devices, but if you would like to override our mic compensation,  you can create your own microphone calibration file and use it instead.

The microphone response correction data will be applied in all modules that display frequency dependent information: RTA, FFT, SoundScope , Transfer Function, and LARSA. Note that it will NOT be applied to basic SPL measurements.

To do this, you will first need to create a mic compensation file.

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:

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 in the file is applied for the remainder of the frequency spectrum.

Importing Microphone Compensation Files

Calibration files can be imported from any location on your iOS device, or in iCloud. The easiest thing to do is to copy the calibration file to a location on your iCloud, which can then be accessed from the app. In addition to iCloud, there are many other ways to get a file onto your iOS device, including copying and pasting the file from a different location using the Files app on your device.

Once the file is on your device or iCloud, importing is easy to do.

From the Microphone Setup page, tap the Cal File button. Now press the Import button on the bottom of the Calibration Files screen. The iOS file picker will pop up. From here, navigate to the location of the calibration file, either on your device or in iCloud, and select it. The file will be imported and will appear on the list of available files.

Select the file in the list, and tap Apply.

Line Input Calibration

If you are using a dock-connector device that has a right-channel line input available, you can calibrate AudioTools to this device. Please note that many dock connector audio input devices use heavy compression as well as low-frequency filtering, so your results may vary. Check with the device manufacturer for more information.

To calibrate this device, connect a known level source to the device right input, and go to the Line Input Setup page on the Utility menu. Tap the dBU field, and enter the known level which is being applied to the right channel line input. Exit the page, you are done.