1997 Edition, June 6, 1997

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American National Standard Methods for Calculation of the Speech Intelligibility Index

Includes all amendments and changes through Reaffirmation Notice , June 18, 2012

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Product Details:

  • Revision: 1997 Edition, June 6, 1997
  • Published Date: June 18, 2012
  • Status: Active, Most Current
  • Document Language: English
  • Published By: Acoustical Society of America (ASA)
  • Page Count: 28
  • ANSI Approved: Yes
  • DoD Adopted: Yes

Description / Abstract:

The predictions of this Standard apply to listening conditions where the input variables of the Speech Intelligibility Index (SII) model can be accurately estimated. The input variables include the equivalent speech spectrum level, the equivalent noise spectrum level, and the equivalent hearing threshold level. This includes the conditions where either speech or noise may not exist as directly measurable physically quantities (e.g., conditions where speech correlated noise is present, such as reverberated speech) but where equivalent speech spectrum level, equivalent noise spectrum level, and equivalent hearing threshold level can, nevertheless, be calculated. The predictions made by use of this Standard are correct only on the average, that is, across a group of talkers and a group of listeners of both genders. The scope of the Standard is limited to natural speech, otologically normal listeners, and communication conditions which do not include multiple, sharply filtered bands of speech or sharply filtered noise. In addition, the listeners should have no linguistic or cognitive deficiencies with respect to the language used.


This Standard defines methods for computing a measure, called the Speech Intelligibility Index (SII), that is highly correlated with the intelligibility of speech under a variety of adverse listening conditions, such as noise masking, filtering, and reverberation. The SII is computed from acoustical measurements or estimates of speech spectrum level, from noise spectrum level, and from psychoacoustical measurements or estimates of hearing threshold level. Various frequencies contribute different amounts to speech intelligibility, and, within a certain range, a higher speech-to-noise ratio contributes to intelligibility. By measuring the speech-to-noise ratio in each contributing frequency band and adding the results, the intelligibility of a speech communication system can be predicted.

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