Audio Visual Design Guidelines

Speech intelligibility

12 views November 19, 2019 aetm 0

The audience is identified as the most important stakeholder. So it follows that unless a typical listener can clearly discern the transmitted audio and assimilate the communicated information then the acoustics may not be successful. 

To be able to define the design target and to subsequently measure compliance we require a clear technical definition of intelligibility and a means of evaluating it. Intelligibility is defined in IEC60268:16 Objective rating of speech intelligibility by speech transmission

According to the Standard, 

“the Speech Transmission Index (STI) is an objective measure to predict the intelligibility of speech transmitted from talker to listener by a transmission channel.”

The standard describes intelligibility on a scale of 0 (unintelligible) to 1.0 (perfect), and plain-language labels are applied to describe performance to clients. As a general rule,

  • AETM recommends an STI design target in the ‘excellent’ band (0.75+) for all installed audio systems
  • an ‘excellent’ STI (0.75+) may not always be achievable in a large, tiered venue and so a minimum requirement of ‘good’ STI (0.6+) can be deemed acceptable in this category of building.


When briefing a design, technical managers should take care to ensure appropriate language is used to describe the required performance such that it is appropriate for the use cases and remains affordable by the organisation. 

Testing ‘proper’ STI can be quite onerous, and the most commonly adopted implementation of IEC60268:16 is STIPA (Speech Transmission Index for Public Address Systems) which requires a far simpler test sequence and shorter testing time. The inexpensive test equipment typically found in contractor toolkits is often capable of STIPA testing.

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