A comparison of speech coherence index and measured speech intelligibility

Perceptual tests were taken in real and simulated spaces to determine whether Speech Coherence Index (SCI) predicts human performance better than Speech Transmission Index (STI). SCI is a proposed method of estimating speech intelligibility during event conditions, that is, in real time with program material. The complex valued coherence function is used to estimate the signal-to-noise ratio on a per frequency basis, and is calculated with short time windows at high frequencies and longer time windows at low frequencies to mimic the multi-resolution nature of human hearing. SCI has been shown to track STI very closely under simplified conditions. Under more realistic conditions, SCI is more sensitive to reverberant energy and is always less than STI. Under certain conditions, such as long source to listener distance or low SNR, SCI predicts significantly lower intelligibility values than STI.