Speech transmission index
measure of speech transmission quality
Speech transmission index (STI) is a measure of speech transmission quality. The absolute measurement of speech intelligibility is a complex science. The STI measures some physical characteristics of a transmission channel (a room, electro-acoustic equipment, telephone line, etc.), and expresses the ability of the channel to carry across the characteristics of a speech signal. STI is a well-established objective measurement predictor of how the characteristics of the transmission channel affect speech intelligibility.
The influence that a transmission channel has on speech intelligibility is dependent on:
the speech level
frequency response of the channel
non-linear distortions
background noise level
quality of the sound reproduction equipment
echos (reflections with delay > 100ms)
the reverberation time
psychoacoustic effects (masking effects)
History
The STI was introduced by Tammo Houtgast and Herman Steeneken in 1971, and was accepted by Acoustical Society of America in 1980. Steeneken and Houtgast decided to develop the Speech Transmission Index because they were tasked to carry out a very lengthy series of tedious speech intelligibility measurements for the Netherlands Armed Forces. Instead, they spent the time developing a much quicker objective method (which was actually the predecessor to the STI).
Houtgast and Steeneken developed the Speech Transmission Index while working at The Netherlands Organisation of Applied Scientific Research TNO. Their team at TNO kept supporting and developing the STI, improving the model and developing hardware and software for measuring the STI, until 2010. In that year, the TNO research group responsible for the STI spun out of TNO and continued its work as a privately owned company named Embedded Acoustics. Embedded Acoustics now continues to support development of the STI, with Herman Steeneken (now formally retired from TNO) still acting as a senior consultant.
“Speech transmission index” enters the record as measure of speech transmission quality. Crown Archives preserves that source wording while asking what Speech, transmission and index can confirm, complicate or overturn.
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