speech transmission index (acoustics) (STI)
In rooms with sound reinforcement, speech intelligibility depends on various factors, such as the reverberation, the size of the room and its structure, the directional characteristics of the loudspeakers, background noise and reflections in the room. There are several methods for evaluating speech intelligibility in rooms with sound. Speech TransmissionIndex ( STI) is one of these evaluation methods, Speech Intelligibility Index( SII), Rapid Speech Transmission Index( RASTI) and ALcons others.
STI measurement is a machine-based method and is based on speech modeling with a test signal whose characteristics correspond to speech signals. Since speech can be considered as a modulation technique, the STI test signal is formed from a fundamental wave modulated at low frequencies.
The entire STI measurement consists of many modulations that cover the entire speech range and are varied in modulation depth. The modulation depth of the received audio signal is measured and compared with the radiated audio signal. From the quotient, the dimensionless STI index is formed, which can assume values between "0" and "1". The STI value "0" stands for insufficient speech intelligibility, the STI value of "1" for excellent speech intelligibility.