On the disagreement between speech transmission index (STI) and speech intelligibility
Hiroshi Onaga, Yoshihiro Furue, Tetsuo Ikeda · The Journal of the Acoustical Society of America · 2000
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.