This part of IEC 60268 specifies objective methods for rating the transmission quality of speech with respect to intelligibility. The objective of this standard is to provide a comprehensive manual for all types of users of the STI method in the fields of audio, communications and acoustics. This standard does not provide STI criteria for certification of transmission channels (e.g. criteria for a voice-alarm system). Three methods are presented, which are closely related and are referred to as STI, STIPA, and STITEL. The first two methods are intended for rating speech transmission performance with or without sound systems. The STITEL method has more restricted uses. NOTE None of the methods are suitable for the measurement and assessment of speech privacy and speech masking systems, as STI has not been validated for conditions that represent speech privacy applications [1]1. The following information is included: - measurement techniques; - prediction techniques.