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Speech Intelligibility

In general terms, the speech intelligibility of a speech transmission system depends on the characteristics of the system (e.g. a room, or sound system in a room), and the absolute sound levels of speech and background noise at the receiver.

IRIS calculates two speech intelligibility parameters:

  • Speech Transmission Index (STI)
  • Useful-to-Detrimental ratios (U)

Speech Transmission Index

IRIS calculates the full STI following the indirect method as described in IEC 60268-16 Ed. 4 (2011)1. Three STI metrics are presented: STI Male, STI Female and STIPA (IR). The implementation was validated with the technique by Cabrera et al.2.

Useful-to-Detrimental Ratio

Useful-to-detrimental ratios, proposed by Bradley3, can be determined from the early-to-total sound energy ratio and the speech-to-noise ratio. IRIS calculates U50 and U80 ratios in octave bands using:

\mathrm{U}_{t} = 10\log\left[\frac{\mathrm{D}_{t}}{1 - \mathrm{D}_{t} + n/s}\right]

where D is the early-to-total sound energy ratio with integration time t (50 or 80 ms), and n/s is the noise-to-speech ratio. Without the n/s term, U is the same as C (Clarity).


  1. International Standard IEC 60268-16 Ed. 4.0, Sound system equipment - Part 16: Objective rating of speech Intelligibility by speech transmission index (2011). 

  2. D. Cabrera, D. Lee, G. Leembruggen, D. Jimenez, Increasing robustness in the calculation of the speech transmission index from impulse responses, J. Build. Acoust. 21 (3) (2014) 181-198. 

  3. J. Bradley, Predictors of speech intelligibility in rooms, J. Acoust. Soc. Am. 80 (3) (1986) 837-845.