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5. Record Impulse Response

Create New IRIS Project


IRIS software has a concept of projects containing a collection of measurements. A project typically represents a single space or measurement session.

  1. Open IRIS software

    The initial window looks like this:

    IRIS Start Screen

  2. Click NEW PROJECT to create a new project. Please fill in the necessary details. At least a project name should be provided.

    IRIS New Project

  3. Click OK to create the project.


    IRIS creates a new folder within the Projects Folder directory with the name of the project. All project data is stored in this folder and measurements exist as subfolders.

Before adding a measurement, you must set the correct hardware configuration in the IRIS software.

Configure Audio Settings

  1. Open AUDIO SETTINGS in the toolbar. This window is used to configure the audio hardware and the corresponding calibration documents.

    IRIS Audio Settings


    On first load IRIS will attempt to enter the correct details automatically, but you should always check before recording any measurements.

  2. Set the ASIO Driver to the correct audio interface:

    • Zoom F4 driver is ZOOM Audio IF with Rec ASIO
    • MOTU 4pre driver is MOTU Audio ASIO
  3. If using a Zoom F4, select ASIO Control Panel and set the buffer size to the largest value.

    Zoom Control Panel

  4. Set the Receiver Type. For Revision 1 or 2 kits we'll be using Core Sound TetraMic.

  5. Set the correct calibration data. Audio Device should contain the serial number of your specific audio interface.


    Audio Device calibration is for matching the input channel gains, and also sets the input and output channels on the right side of this window.

    This calibration is optional but highly recommended. It is not necessary if the inputs on your audio device are inherently matched to within 0.1 dB.

  6. Specify a System calibration document if you have previously calibrated your entire IRIS kit and source in lab for measuring sound strength (or open plan office parameters).

  7. Select your TetraMic serial number from the TetraMic calibration list.


    The TetraMic serial number is printed on the back of the mic.

  8. If you did not set an Audio Device calibration document, you must set the correct OUTPUT, INPUT and LOOPBACK channels.

  9. Set the desired sample rate (44100 Hz or 48000 Hz).


    If using a Zoom F4, you must select 48000 Hz as the AIF with Rec driver we are using is fixed to this sample rate.

  10. Click OK to save changes.

Configure Stimulus Settings

IRIS currently uses only one type of excitation signal - a single-shot exponentially swept sine wave.


    IRIS Stimulus Settings

  2. Set the Sweep Time (s) to an appropriate value.


    • For a smaller space such as a rehearsal room, classroom or office, 10 seconds is probably adequate.

    • Use 30 seconds for a medium to large room, such as a concert hall.

    A longer sweep time improves the resulting signal-to-noise ratio (SNR) of the recorded impulse response.

  3. Set the Impulse Response Time (s) to be sufficiently longer than the estimated decay time in the room. This is the length of the impulse response recording.


    The Impulse Response Time cannot be shorter than 2 seconds to accommodate the very low modulation frequencies required by STI calculations.

  4. Advanced Sweep Settings allow you to control the start and end frequencies of the sweep signal.

    IRIS Stimulus Settings Advanced

    These should be left set to 20 Hz and 20 kHz, respectively, otherwise you risk having incomplete information in some bands. Only adjust these frequencies if there is concern of damaging your source loudspeaker.

  5. Click OK to save changes.

Add Measurement

  1. Click on ADD MEASUREMENT button in the toolbar. This adds an empty (not yet recorded) measurement to the list on the left.

    IRIS Add Measurement

  2. Enter in details of the measurement such as the source and Receiver position.


    The Description field will auto-complete based on the Source and Receiver text unless you manually type into the Description field.

Record Measurement

With all settings above set correctly, the RECORD button should now be enabled.

  1. Click on RECORD to open the Record Measurement window.

    IRIS Record Measurement


    You should see some activity on the input level meters. Clap your hands to check.

  2. Set the sound source to an appropriate level:

    1. First, set the source power amplifier to an appropriate level.


      If you are using a calibrated system (e.g. for sound strength measurements with an omni-source), please ensure the amplifier is set to precisely the same setting as when your system was calibrated.

    2. Set the Output Gain slider in IRIS to the minimum value.

    3. Click TEST TONE and then slowly increase the output gain until the sweep signal is an appropriate volume.

    4. Click TEST TONE again to stop the test tone. The test sweep also stops automatically after 30 seconds.


    Please do not adjust the power amplifier once it is set as IRIS will not know a change has been made.

    If you need to change the level, use only the output gain slider in IRIS. IRIS internally compensates for this value, so you may change the level without affecting the level reference feature or system calibration (e.g. sound strength measurements).

  3. Click RECORD to record the measurement. IRIS will play a sweep followed by a period of silence. Please ensure the room is quiet as possible for duration of the recording. If a noisy event happens during the recording, please cancel and restart the measurement.

    The IRIS window will glow red while recording is in process. You may make noise again as soon as the red background disappears.

  4. If the measurement completes successfully, the Record Measurement window will disappear and IRIS will display the results.

    If the recording is excessively noisy, IRIS will give an error message as shown below.

    IRIS Bad Measurement

    A bad measurement is probably a result of a loose connection, or a too quiet/short excitation signal.

Main IRIS Window

After the measurement has completed successfully, IRIS will look something like this:

IRIS Completed Measurement

The window is divided into four main areas:

  1. List of measurements

  2. Measurement details and a summary of the results

  3. Detailed results

    • IRIS 3D PLOT shows the IRIS PLOT (only available with a 3D receiver).

    • PARAMETERS lists the various room acoustic parameters from ISO 3382-1 and 2.

    • SPEECH contains speech specific parameters including STI.

  4. Waveform view - the time domain impulse response signal in a linear or log form.


    For 3D measurements, the waveform view displays the sound pressure channel (the W channel).

Inspect Measurement Quality

Before analysing the measurement in detail, you must check the quality of the impulse response recording. It may be necessary to repeat the recording. There are two things to inspect - the decay range, and the onset time.

Decay Range

The decay range is effectively the signal-to-noise ratio of the impulse response.

  1. Click on PARAMETERS tab

  2. Inspect the Decay Range (first row) in each band of interest, for example:

    IRIS Decay Range

    You should aim to have at least 45 dB in each band. In the above example, this requirement was easily met in each band except 63 Hz. This is expected as the loudspeaker used (dodecahedron) is not very efficient at 63 Hz, and results at this band should be ignored.


    To increase the decay range, you can either increase the sweep time or increase the volume of the excitation signal. Please only make level adjustments using the slider in the Record Measurement window and not on the actual hardware.

Onset Time

The onset is the start of the impulse response signal - the moment when the direct sound arrives. All calculations depend on this time so it is very important it is correct.

IRIS automatically detects the onset, but it may select an earlier time if there was excessive crosstalk in the measurement hardware.

To check for the presence of crosstalk:

  1. Zoom in to the start of the waveform where time = 0 (use buttons or mouse wheel to zoom).

  2. Crosstalk will appear as a peak before the actual start of the impulse response, for example:

    IRIS Crosstalk

  3. Manually adjust the onset time by click on Edit Onset Time below the waveform. Use the cursor to manually set the actual onset, for example:

    IRIS Onset Time Adjusted


    Crosstalk is usually caused by the cable after the power amplifier being too close to the input cable. The high power output is electromagnetically induced into the input cable, causing an impulse before the acoustic direct sound.

    This can be avoided by keeping the cable after the power amplifier away from any other cables. Alternatively, use an active loudspeaker, where the power amplifier is contained within the loudspeaker unit.