There are several ways to work with a multi-channel audio interface in AudioTools. For example, you may want to plug a USB audio interface into the USB-C connector on your iPad, iPhone, or Mac to be able to use one more microphones plugged into it. The mic you want to use may not be in channel 1, or you may want to average several mics using our multiplexor, or run our Low SPL module to get lower ambient noise measurements. These use cases are discussed here.

Connecting and Selecting the Interface

On iOS, using an iPhone or iPad, just plug the interface into the USB connector. iOS automatically selects it as the input source.

On Mac, after plugging it in, you need to select the interface in the Mac Settings app, on the Sound panel. Make sure you select it on both the input and output pages.

Channel Mapping

Mapping is used to select the main microphone (L) that is used for all of the single-input modules, including RTA, FFT, SoundScope, SPL Meter, and more. Mapping can also select channel 2 (normally R) that is used for dual-input functions like Transfer Function or Audio Scope. To set this up, go to Settings->Channel Mapper, and you will see the number of available channels on the top of the screen. Use the +/- buttons to select which channel on your audio interface that you want to use for these channels.

Now when you navigate to any module, the selected audio input will be used. Note the display will still show Mic Low Range or High Range.

Microphone Multiplexor

AudioTools includes a microphone multiplexor that cycles through a set of mics so that you can get a spatial average of a measurement. It is implemented in SPL Pro, RTA, FFT, and SoundScope.

While the Mux is active it bypasses Mapping completely.  When exiting any of these modules, or disabling the Mux, the previously set mapping, if any, is reapplied.

To set up the microphone multiplexor, or Mux, first plug in your multichannel interface, and set up the microphones around the room. Then, navigate to the Settings->Mux function. If you have two or more channels available, you will see the Mux setup screen. Here you can select and turn on as many channels as you would like to include in the microphone rotation selection. 

To calibrate microphones for the Mux, it is important to start by calibrating the microphone plugged into channel 1 by using the normal  microphone calibration page. Once this has been done, open the Mux, and for microphones 2 above, tap the mic cell to select it, or use the selector on the bottom of the page. Place a microphone calibrator on the selected mic, and turn it on. Now adjust the hardware gain on the preamp to get as close as possible to the calibrator setting (typically 94dB), and then use the +/- trim to make any final needed adjustments. Note that if you change the calibration for mic 1 you will need to go back and adjust the other mics on the Mux page. 

If you have a microphone calibration file for your microphone, you can tap the file name field to select it. Note that for mic 1, the calibration file and SPL calibration are selected on the main mic settings page.

Next, set the amount of time for the complete multiplexing cycle. For example, if you have 4 microphones selected and a 10s Cycle Time, that will cause each microphone to be active for 2.5 seconds at a time.

Finally, turn on the Enable switch. Now, when you go to a tool like RTA or FFT that support the Mux feature, it will be active.

The microphone multiplexor is not available when using iAudioInterface2.

Selecting a Single Mic from the Mux

While doing FFT or RTA tests, it may be useful to quickly change to a particular mic while using the Mux. To do this, go to the Mux page, and use the selector at the bottom of the page to pick the mic. The mux settings will adjust to select only that mic, and not cycle through the other mics. 

Low SPL Module

The Low SPL test works a little differently, since it only uses two microphones. It bypasses mapping, and does not use the Mux. Simply select the two microphone channels directly in Low SPL setup. The calibration for these microphones however is taken from the main settings (for mic 1) and the Mux settings page (for mics 2 and above).