The indirect method of calculating STI (i.e. from impulse responses) used by IRIS determines STI in two steps:
- Measure an impulse response (a normal IRIS measurement).
- Configure speech and background noise levels manually on the SPEECH page. Multiple speech and noise scenarios can be evaluated for a single measurement.
Sound Source Requirements¶
For STI measurements of a system with an acoustic input, the directivity of the sound source should match the directivity of a human talker (e.g. unamplified talker, talker speaking into a microphone).
The directivity of an artificial mouth is specified in ITU-T P.511.
For measurements of an amplification system with no specific type of input or an electrical input, the sine sweep excitation should be electrically injected into the amplification system.
Open plan office measurements (ISO 3382-3) require an omnidirectional source.
When making measurements of STI, set the level of the sine sweep excitation to achieve a high quality noiseless impulse response. It has no influence on the speech level, which is configured manually in a second step.
The impulse response method can only be used to measure linear and time-invariant systems. You should disable any non-linear processing that might be present in the system before conducting any STI tests, such as:
- Dynamic range compression
For example, dynamic range compression should be disabled and the system gain set to a suitable fixed level.
IRIS measures impulse responses using a sine sweep technique, meaning non-linear harmonic distortion components in the system are effectively ignored. This is ideal for almost every use case of IRIS except when measuring speech intelligibility of systems where non-linear distortion is significant. For example, a public address system which uses low quality horn drivers in an acoustically challenging environment like a train station. With the horns operating at their maximum level, the harmonic distortion present will likely reduce speech intelligibility and this should be included in the measurement. In this situation, a direct method of measuring STI such as with a STIPA meter may be more suitable. Refer to Section 6.3 of the STI standard2 for details.
For further reading on STI measurements, see Section 6 (Indirect method of measuring STI using the impulse response) and Section 7 (Measurement procedures, post-processing of data and applications) of the STI standard2.