This standard defines the STI model, test signals, measurement and prediction methods. The objective of this standard is to provide a comprehensive manual for all types of users of the STI model 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, but some typical application values are provided in Annex G. Every measurement method has limitations, and the reader is referred to clauses relating to limitations such as speech privacy, echo and systems using digital voice compression (vocoders). This standard does not cover the case of fluctuating noise on the STI, although some general comment on dealing with this complex issue is provided in 7.13 and 8.9.3