A new verification of the speech transmission index for the English language

The speech transmission index (STI) is one of the most widely used and standardized methods for objective prediction of speech intelligibility of transmission channels. The original verification of the relationship between the STI and the intelligibility for the English language was published in 1987. The methodology employed then for the listening tests and the different input spectrum recommended today by the current STI method suggest that the relationship STI vs. speech intelligibility needs to be verified for the English language. This paper presents a new verification of the current STI for the English language with binaural listening and the speech materials presented to the listeners in a real room from a real sound source. Two hundred and ten subjects participated in speech intelligibility tests designed to replicate real-life listening conditions. The speech materials were contaminated with natural reverberation, noise, band pass limiting and echoes, and the listeners were not familiarized with the speech materials before the tests, in order to better replicate everyday situations. Results showed lower intelligibility scores than the earlier verification presented in Annex E of the current STI standard (IEC60268-16:2011) for most of the scenarios that were investigated. The correlation between STI and speech intelligibility was also investigated for the English language with a newly proposed male speech spectrum. The accuracy of STI intelligibility prediction with the new male spectrum was found higher than that attained with the current IEC specified male spectrum. These new findings give new and additional insights into the interpretation of intelligibility using the STI values.