The disagreement between speech transmission index (STI) and speech intelligibility

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.