On the disagreement between speech transmission index (STI) and speech intelligibility

Abstract

The change tendency of speech transmission index (STI) was examined by calculation for several modeled sound fields. As a result, it has been revealed that STI contradicts the generally accepted concept of the importance of early energy for speech intelligibility. Namely, for single reflection fields, (1) STI takes the same value for positive and negative levels of reflection relative to direct sound, and for positive and negative delay times. (2) STI takes the lowest value when the reflection level is 0 dB (equal to direct sound), and the increase and decrease of reflection level from 0 dB causes a rise in STI. For multiple reflection fields, (3) STI evaluates only the degree of energy concentration or dispersion in the time domain. Also, energy concentration due to strong reflections with any delay time increases the value of STI. However, the already published experimental results of intelligibility on single reflection fields differ from the above-mentioned properties of STI, (1) and (2). Moreover, the authors’ intelligibility experiment with multiple reflections indicates the importance of early energy for intelligibility and shows the disagreement with the properties of STI (3). From the above findings, STI cannot be considered as corresponding to intelligibility.

A note on meaningful acoustical parameters for open-air theatres

“…However, there are problems with the STI, especially when applied to a situation with low reflection density. Onaga et al [19] have shown that STI responds to single reflections in the same way whether the time delay is positive or negative. Thus, a delayed reflection that causes a disturbing echo is not treated unfavourable in the STI.…”