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Various factors can influence the intelligibility of spoken messages. Here are a few of them:

  • • Signal-to-noise ratio: The louder the background noise, the smaller the speech signal will emerge above it and the more difficult it will be for the listener.
  • • Reverberation time: The longer the reverberation time, the more difficult it will be for the listener to catch the consonants and understand the spoken words.
  • • Distance between the speaker and the listener: The longer the distance, the more the acoustic attenuation, and thus the lower the received speech signal.
  • • Visual cues: The better the vision of the speaker by the listener, the better his understanding of the speech signal.

Assessing Intelligibility

The assessment of speech intelligibility can be performed through a list of words or sentences being read by a normal speaker to a panel of normal-hearing listeners. Unfortunately, this method calls for a significant number of participants, which makes it costly and unpractical. More to the point, it is quite time-consuming.

Depending on the native language of the speaker and the listeners, scores can vary by as much as 20%.

Other methods call for an artificial test signal being diffused at the speaker location and measured at various listeners’ positions. The measurement results can be expressed using some intelligibility descriptors such as the speech transmission index (STI) (cf. Section 6.5). The resulting score is not language dependent.

Which Intelligibility for Which Use?

Depending on the nature of the speaker-listener relation, various intelligibility scores can be aimed at. For example, in a theatre or a conference room, one will attempt to have as high an STI value as possible (at least 0.70, preferably 0.90), while in an open-plan office one will try to have an STI value well under the 0.50 mark between noncollaborating workstations (cf. Chapter 7). The corresponding results will usually be achieved through a mixture of background noise level adjustments, and acoustic propagation enhancement or prevention.

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