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IRIS Plot Details

This section outlines the process of generating an IRIS plot from a set of first-order Ambisonic signals.

A-format and B-format

The four A-format signals from a first-order Ambisonic microphone array aren’t used directly. After a recording they are converted into first order B-format, which is also made up of four channels:

  • Sound pressure (omnidirectional pattern)
  • Sound particle velocity in X, Y and Z directions (figure-of-8 pattern)

Onset and Noise Detection

The relevant part of the impulse response to evaluate is identified: from the onset of the direct sound to where the impulse response decays into noise.

Note

The position in time where the impulse response decays into noise will also depend on the selected Filter.

Filter

The four B-format signals are filtered according to the specified Filter:

IRIS Plot Filter

Tip

Typical octave band filters are provided, as well as a 1-8 kHz filter which is useful for assessing early reflections in studio control rooms (see EBU3276).

Time Windowing

The filtered B-format impulse response is then divided into a series of non-overlapping rectangular windows. The length of these windows is determined by the Resolution in IRIS.

IRIS Plot Resolution

The recommended resolution for most situations is 2 ms. For very small rooms, like a control room, we recommend 1 ms.

These time windows are annotated on the waveform if you zoom in - note the marks along the top, which indicate the start of each window:

IRIS Plot Waveform

Note

Short time windows introduce a low frequency cut-off. For example, the lowest frequency a 2 ms can contain is 500 Hz. Bands below this frequency cannot be selected in the Filter selection.

This also means a Broadband setting with a 2 ms resolution is effectively showing 500 Hz and above.

Levels

The RMS sound pressure level for each window is calculated. There are two factors which control sound levels on an IRIS plot:

  • The Level Reference determines what 0 dB means. It is either the broadband level of the first window (the direct sound) of This measurement or another measurement (Global).

    IRIS Plot Reference

    Please refer to Level Reference for more details.

  • The Range control determines the dynamic range of the IRIS plot. The default is 40 dB and this may be adjusted in 10 dB increments.

    IRIS Plot Range

    Note

    A range of 40 dB means rays of levels less than -40 dB are not shown. Sound levels greater than 0 dB will still be displayed, however, which might happen when there is a reflection louder than the direct sound. This means the actual dynamic range of an IRIS plot may be larger than what is stated in the Range control.

Directional Analysis

The average direction of sound in each window is determined using a sound intensity technique.

Sound intensity, I, is defined as the product of instantaneous sound pressure p(t) and the instantaneous 3D particle velocity vector u(t):

\boldsymbol{\mathrm{I}} = \overline{p(t)\boldsymbol{\mathrm{u}}(t)}

where the average is made over a specific time window.

Visualisation

Each window corresponds to one ray on the IRIS plot. The level (in dB) translates to ray length, the ray points in the direction of the incoming sound energy, and the ray is coloured according to its time-of-arrival.

Colour

There are three options for colouring the rays according to their time-of-arrival: music, speech and studio. These can be adjusted using Time Intervals control:

IRIS Time Intervals

Music

The music setting emphasises the integration time for listening to music: 80 ms, which is useful in concert hall and large room applications. Reflections in green correspond to early useful energy (20 to 80 ms). Any strong very early reflections in orange (2 to 20 ms) may result in image shifting issues, and strong reflections after 80 ms (both shades of blue) may be perceived as late echoes.

IRIS Music Interval

Speech

The speech setting emphasises a 50 ms integration time and is intended for use in lecture and drama theatres, offices and classrooms.

IRIS Speech Interval

Studio

The studio setting uses time limits useful in control room applications. For example, EBU3276 defines early reflections as reflections which arrive within 15 ms of the direct sound.

IRIS Studio Interval

Tip

The waveform is also shaded according to the IRIS plot colours.

Note

In the time key, the number in brackets following Inf is the actual position in time IRIS stops analysing the impulse response - the point at which the signal decays into noise.

Align Direct Sound

You may rotate an IRIS plot so that the horizontal component of the direct sound (first ray) is aligned to the X-axis. This can be useful for comparing multiple IRIS plots.