Developed in the early 1970’s, the Speech Transmission Index (STI) is an machine measure of intelligibility whose value varies from 0 (completely unintelligible) to 1 (perfect intelligibility).

In STI testing, speech is modeled by a special test signal with speech-like characteristics. Following on the concept that speech can be described as a fundamental waveform that is modulated by low-frequency signals, 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.