NTi Audio Acoustilyzer AL1 User Manual

Page 93

STI - Speech Transmission Index

The STI result is based on the complete set of 98 measurements. Since

this approach requires a rather long test period, it is less frequently applied

in practice. However, STI represents the most detailed method to measure

the speech intelligibility and is mostly used if alternative approaches do not

provide reliable results due to unfavorable environmental conditions.

In practice, the STI result is preferably calculated out of the impulse

response (MLSA) that has been acquired e.g. with a PC-based system.

This approach is much quicker, but requires a lot of experience and

especially a linear behavior of the setup, i.e. there must be no non-linear

processing or conditions, including compressors or limiters, which is a

rather rare situation.

RASTI - Room Acoustics Speech Transmission Index

RASTI acquires only few segments of a complete MTF, which at first glance

obviously represents an extreme simplification of STI. Therefore, tight use

restrictions must be met to acquire reliable speech intelligibility results with

RASTI. Furthermore, the RASTI result does not consider significant and

practical auxiliary parameters such as the frequency response, echoes or

frequency-dependant reverberation times.

For a RASTI measurement, only two simultaneously generated frequency

bands are considered, i.e. the 500 Hz and the 2 kHz band which are then

modulated with four and five frequencies respectively.

The practical application of RASTI is mainly restricted to acquisitions

between two persons. However, RASTI was, for a long time, the only

method to measure the speech intelligibility with a portable instrument