Speech Transmission Index

Speech Transmission Index, short STI is a measure of intelligibility of speech. The understanding of speech, the intelligibility is directly dependent of the background noise level, of the reverberation time, and of the size of the room. For the rating of the understanding of speech, we have different methods, where STI and % ALcons are established.

STI was developed in the beginning of the 1970’s, it is a machine measure of intelligibility whose value varies from 0 = completely unintelligible to 1 = perfect intelligibility. On this scale, an STI of at least .5 is desirable for most applications.

In STI testing, speech is modeled by a special test signal with speech-like characteristics. Speech can be described as a fundamental waveform that is modulated by low-frequency signals. Therefore STI employs a complex amplitude modulation scheme to generate its test signal. At the receiving end of the communication system, the depth of modulation of the received signal is compared with that of the test signal in each of a number of frequency bands. Reductions in the modulation depth are associated with loss of intelligibility.

Another standard defines a method for computing a physical measure that is highly correlated with the intelligibility of speech as evaluated by speech perception tests given a group of talkers and listeners. This measure is called the Speech Intelligibility Index, or SII. The STI is calculated from acoustical measurements of speech and noise.

The Speech Transmission Index STI is a machine measure of intelligibility whose value varies from 0 (completely unintelligible) to 1 (perfect intelligibility). There are also simplified versions of STI developed for use in specific situations. RASTI (Room Acoustics Speech Transmission Index, or Rapid Speech Transmission Index) and STIPA (Speech Transmission Index for Public Address Systems).