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Speech Intelligibility ("SI")

Speech Intelligibility ("SI")

Speech intelligibility can be defined as how clearly speech is comprehensible to a listener. In an emergency situation a reduced level of speech intelligibility can lead to misunderstanding and confusion potentially placing the occupants of a building at greater risk from fire or other emergency.

There are five primary factors that can adversely affect the intelligibility of speech;

  1. the sound pressure level (loudness);
  2. ambient or background noise;
  3. reverberation;
  4. frequency range of the area or message being transmitted;
  5. distortion or signal-to-noise ratio of the loudspeaker system (i.e. the quality of the audio signal).

Measuring Speech Intelligibility

Speech intelligibility can be measured and is required to be measured for some occupant warning systems and emergency warning systems as part of the commissioning and approval for the system for conformance with the relevant Australian Standards AS1670.1 or AS1670.4.

To measure speech intelligibility a test signal is played through the occupant warning system or emergency warning system loudspeakers. The test signal is made up of seven octave band signals which encompass a combination of vowels and syllables from common speech. There are 14 modulating frequency tones that emulate your speech pattern. This is referred as the modulating transfer function (MTF).

At the location to be tested a specialised electronic device is used to compute the signal to noise ratio based on the impact of background noise, reverberation and distortion. A weighted average of the signal to noise ratio for each modulation frequency is calculated and adjusted for auditory masking and the threshold of hearing. This result is a value between zero and one, representing the estimated intelligibility of the system, where zero is BAD and one is excellent.

Speech Intelligibility STI-CIS Scale where ZERO is BAD and ONE is EXCELLENT


In preparing this definition, we have drawn from various sources including Legislation, Codes, Standards and industry information, research and knowledge.  Like the english language, these definitions may subtly change from time to time. As such these definitions are provided solely on the basis that users will be responsible for making their own assessment of the definition and and are advised to verify all relevant representations, statements and information.