Welcome to a description of the Speech Transmission Index (STI).
The basic principles are described in: Basics-STI_2014-V1 , also in “On_measuring_and_predicting_speech_intelligibility” (1992), ISBN: 90-6743-209-1, and in “Past, present and future of the Speech Transmission Index”, ISBN: 90-76702-02-0.
Around 1990 the STI assessment method was reviewed with respect to specific types of distortions. Specifically the effect on intelligibility by irregular frequency transfer functions was studied. This resulted in a revised version of the STI where, in particular, correlations between adjacent frequency bands were involved. Also the effect of gender (difference between female and male speech) and frequency importance functions of different type of consonant groups were studied. The results and validation are given in several papers (i.e. in Elsevier’s Journal of Speech Communication) and in a condensed form in document:
An overview of selected publications of STI-related papers and conference proceedings is given by: STI-references-2014. In order to get familiar with the basic STI test-signals a demonstration of simple test-signals, before and after transmission through a reverberating enclosure, is included in a power-point presentation. It includes three test-signals with a simple sinusoidal envelope modulation of 10Hz, 3Hz and 1Hz respectively. Both the original signal and a version transmitted through a reverberation enclosure are presented. Notice the strong audible reduction of higher modulations frequencies as predicted by the Modulation Transfer Function (MTF) for reverberation enclosures.
You can also replay the original signal in a reverberating environment and compare the original signal (close to the loudspeaker) with the sound in the reverberating environment. (Good luck!)