RASTI

RASTI (Rapid Speech Transmission Index) was a simplified two-band speech intelligibility measurement method using octave bands at 500 Hz and 2000 Hz with a total of 9 modulation frequencies. Defined in the now-withdrawn IEC 60268-16:2003 (superseded by the 2011 and 2020 editions), RASTI was widely used from the 1980s through the 2000s for quick field assessments. However, RASTI was found to be unreliable in rooms with strong frequency-dependent effects (e.g., rooms with only low-frequency noise or only high-frequency absorption) because its two-band sampling missed significant information. IEC 60268-16:2011 officially deprecated RASTI in favor of STIPA, which provides better accuracy with similar measurement speed. While RASTI instruments are still encountered in the field, new measurements should use STIPA. Historical RASTI data should be interpreted with caution, particularly for rooms with non-flat noise spectra or unusual absorption patterns.