NTi Audio Acoustilyzer AL1 - Manual (Page 93)

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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