IRIS 1.2 introduced a source-receiver distance field, which can be automatically determined or manually specified (or not set). A value must be set for calculating open plan office parameters.

## Automatic¶

The source-receiver distance can be automatically determined if:

• There is line-of-sight between the source and receiver, and

• Either a loopback connection is installed and configured in the audio interface, or you are using a Dante network. See here.

Information

The loopback connection is used to measure and remove the round-trip delay time caused by various buffers in the audio stream, meaning the start of the impulse response is the precise moment the sound left the source loudspeaker. This is not required if you are using a Dante network, as this system has deterministic delay (and IRIS automatically removes the reported delay from the start of the impulse response recording).

The distance is calculated from the onset time of the direct sound and the speed of sound. The speed of sound in air is calculated by:

c_{air} = 331.3\sqrt{1 + \theta/273.15}

where $\theta$ is the air temperature in degrees Celsius (°C). The temperature in IRIS is set at a project level, when a new project is created, and can be edited in Project Details in the Project menu.

Tip

The default temperature for new projects can be configured in Settings in the Edit menu.

### Calibrating Automatic Distance Measurements¶

Warning

Please check the automatically calculated source-receiver distance against the actual distance (measured to the acoustic centres of the source and receiver).

Modern power amplifiers and active loudspeakers often have internal digital signal processing which will delay playback by a constant amount, and make the loudspeaker appear further away than it really is.

To correct for this, measure the difference in metres and create a system calibration document with the necessary distance adjustment. Apply the resulting calibration document to all affected measurements.

Note

The sound strength part of the system calibration document must also be completed. If you're not interested in sound strength, calculate nonsense strength data but enter the distance adjustment correctly. Take care to ignore G and related parameters.

## Manual¶

The source-receiver distance can be measured manually and entered.

## Not Set¶

IRIS projects before version 1.2 or for new measurements where a loopback/Dante setup was not used, the source-receiver distance will default to Not Set.