Speech Transmission Index for the English Language Verified Under Reverberant Conditions with Two Binaural Listening Methods: Real-Life and Headphones

The speech transmission index (STI) is one of the most widely used standardized methods for objective prediction of speech intelligibility. The original verification of the relationship between STI and intelligibility for the English language employed an STI model that differs from the current STI model included in the IEC60268-16:2011 standard. This paper presents a verification of the current STI for the English language under reverberant conditions employing two binaural listening methods: real-life and headphones. One hundred subjects participated in speech intelligibility tests designed to replicate real-life listening conditions. The speech material was contaminated with natural reverberation and the listeners were not familiarized with the speech material before the tests, in order to replicate everyday situations. This method produced lower intelligibility scores than the verification presented in Annex E of the current STI standard (IEC60268-16:2011) for most of the scenarios that were investigated. An additional experiment was carried out in which 80 subjects listened to the contaminated speech material through headphones. The headphone method produced lower mean intelligibility scores than those obtained with the real-life method and also a higher standard deviation from the mean.