IEC 60268-16 Ed. 3.0 en:2003, Sound system equipment - Part 16: Objective rating of speech intelligibility by speech transmission index

defines objective methods for rating the transmission quality of speech with respect to intelligibility. The four methods, which are closely related, are referred to as the "STI," the "STITEL", the "STIPA" and the "RASTI" methods (see Clause 3).