The Speech Transmission Index (STI)

The Speech Transmission Index is an objective, physical measure of speech transmission quality. The STI is a 0 to 1 index, indicating the degree to which a transmission channel degrades speech intelligibility. This means that perfectly intelligible speech, when transferred through a channel with an associated STI of 1, will remain perfectly intelligible. The closer the STI value approaches zero, the more information is lost. There are standardized ratings linking certain ranges of the STI to subjectively experienced intelligibility. Relations between the STI and various subjective intelligibility tests (such as CVC wordscore and phonetically balanced word lists) are also well established. These relations are shown in the table below.

Part of the appeal of the STI is it wide range of application areas. The STI is extensively used in room acoustics, for instance to assess intelligibility in auditoria, churches and conference rooms. But the STI is also applied to telecommunication channels, such (mobile) telephone lines and radio transmissions.

The scientific principle on which the STI is based, is that information in speech is represented acoustically in the form of modulations. A speech utterance is essentially nothing more than a sequence of modulated tonal and noisy sounds. Loss of these modulations translates into loss of intelligibility. The Modulation Transfer Function, which can be computed or measured, expresses loss and preservation of modulations. The STI is calculated directly from the Modulation Transfer Function.

The STI was first invented in the 1970's by Herman Steeneken and Tammo Houtgast, both then working at TNO in Soesterberg, the Netherlands. The STI has developed gradually since then, into the mature and widely recognized and applied measuring tool it is today. An estimated population of over 10,000 engineers, scientists and consultants have the STI in their professional toolbox, in some form or another.

An overview of the STI and its possibilities is given in the book "Past, present and future of the STI." Registered users can download a PDF version of this book through the downloads section of this website, free of charge.

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