In previous work, the authors examined the tendency of changes in the Speech Transmission Index (STI) using several modeled sound fields. The results showed that energy concentration due to strong reflections at any delay time, short or long, increases the STI. This means that the STI evaluates only the degree of energy concentration or dispersion in the time domain regardless of the delay time. As such, the STI fundamentally contradicts the generally accepted concept that early reflection is important in speech intelligibility. However, it has not yet been clarified as to whether such a property of the STI corresponds to intelligibility or merely reveals a defect in the STI. To examine the validity of the STI, speech intelligibility tests were conducted using sound fields with different energy concentration points. The results show that energy concentration at shorter delay times increases intelligibility, thus reconfirming the concept of importance of early energy, and indicates clear disagreement with the STI. The STI cannot be considered to correspond to intelligibility because it does not distinguish useful early energy from non-early energy, which does not contribute to intelligibility.
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.
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