A revised speech spectrum for STI calculations

The ability of the Speech Transmission Index (STI) to predict speech intelligibility under noisy conditions is highly dependent on the assumed spectrum of the speech signal. Examination of the literature showed that the long-term average speech spectrum of male talkers differs substantially from the speech spectrum recommended for STI calculations (IEC 60268-16). To explore these issues, the long-term average speech spectrum of forty male British English people was first measured, compared with the available literature and proposed for STI calculations. Then, using several voice alarm systems, the influence of the measured spectrum on STI calculations was assessed and comparisons made with the standard speech spectrum. The results showed significant STI differences under noisy conditions and considerable reductions in the required electrical power with the use of the new proposed male spectrum. This indicated that the current STI method could benefit from a revised speech spectrum.