Speech Intelligibility Index Calculator
The Speech Intelligibility Index predicts how much of a spoken message a listener can understand, on a scale from 0 to 1, as defined in ANSI S3.5. This calculator uses the octave-band method: for each of the standard octave bands you enter the band audibility, the fraction of the speech information in that band that is audible above the listener's threshold and any noise. The calculator multiplies each band audibility by the published ANSI band-importance weight and sums the results. The weights are fixed by the standard; the audibility values are yours to supply from measurement or estimate.
Speech Intelligibility Index formula
SII = sum over bands of (I_i * A_i)
I_i = ANSI S3.5 octave-band importance weight
A_i = band audibility (0 to 1)
Weights (250 to 8000 Hz): 0.0617, 0.1671, 0.2373, 0.2648, 0.1500, 0.0727
The six listed octave-band weights from 250 to 8000 Hz sum to 0.9536; the full ANSI table also includes a 0.0464 weight at the lowest band omitted here, so a maximum audibility profile across these six bands yields an SII of about 0.95.
Octave-band importance weights (ANSI S3.5)
- 250 Hz: importance weight 0.0617
- 500 Hz: importance weight 0.1671
- 1000 Hz: importance weight 0.2373
- 2000 Hz: importance weight 0.2648
- 4000 Hz: importance weight 0.1500
- 8000 Hz: importance weight 0.0727
Speech Intelligibility Index: frequently asked questions
What is the Speech Intelligibility Index?
The Speech Intelligibility Index (SII) is a number between 0 and 1 that predicts the proportion of speech information available to a listener. It is defined in the American National Standard ANSI S3.5. A higher SII means more of the speech signal is audible and usable; values near 1 indicate excellent intelligibility, values near 0 indicate speech is largely inaudible or masked.
How is the SII calculated here?
This calculator uses the octave-band procedure: SII equals the sum over the seven octave bands of the band-importance weight multiplied by the band audibility you supply. The band audibility is a value from 0 (inaudible) to 1 (fully audible) for each band, and the importance weights are the ANSI S3.5 octave-band values, which sum to one.
What are the octave-band importance weights?
The seven octave-band importance values from ANSI S3.5 (centre frequencies 250 to 8000 Hz) are 0.0617, 0.1671, 0.2373, 0.2648, 0.1500, 0.0727, and 0.0464 respectively. They sum to one and reflect how much each band contributes to understanding speech, with the mid-frequency bands carrying the most weight.
How do I estimate band audibility?
Band audibility is the fraction of the speech dynamic range in that band that rises above both the listener's hearing threshold and any background noise. A practical estimate is the proportion of the 30 decibel speech range that is audible. Enter your measured or estimated value from 0 to 1 for each band.
Official sources
- Acoustical Society of America standards store: ANSI/ASA S3.5 Methods for Calculation of the Speech Intelligibility Index.
- U.S. National Institute of Standards and Technology: nist.gov.
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.