Voice Intelligibility

Voice intelligibility requirements for emergency voice alarm/communications systems and mass notification systems is a subject that causes much concern and confusion amongst fire alarm professionals. There is a lot of confusion in the industry of exactly what voice intelligibility is, where and when it is required, and how it is tested.


There is a difference between intelligibility and intelligence of a voice message. Consider the message “Go to Stair B if there is no smoke.” Intelligibility determines if the message is understandable; it is clearly “Stair B”, not “C”, “D”, or “E”. Intelligence determines if the message makes sense: no smoke here or no smoke in the stairs?


Voice intelligibility requirements begin with the acoustically distinguishable space (ADS). According to NFPA 72, chapter 3 (Definitions) an acoustically distinguishable space (ADS) is all or part of an emergency communications system notification zone, enclosed or otherwise physically defined, that might be distinguished from other spaced because of different acoustical, environmental, or use characteristics.


NFPA 72, chapter 3 also define intelligible as the ability to understand a message. While all parts of the building must have ADS’s defined, only some of these spaces may require that voice communications be intelligible. The ADS’s that require intelligibility must be determined during the design phase and may have their intelligibility measured after installation.


For guidance on intelligibility, NFPA 72 provides Annex D (Speech Intelligibility). Annex D provides guidance on design and testing considerations. The AHJ determines which ADS’s require intelligibility during the design phase, with the appropriate ADS’s identified on the design documentation. The AHJ also determines the method of testing after installation.


Intelligibility testing after installation us intended to identify potential issues to remedy. It is not required to be a pass/fail of the system, nor is it intended to require a re-design of the system.


Intelligibility testing may be subjective or objective. Subjective testing uses human subjects to assess intelligibility. Objective uses test equipment to provide a quantitative test of the system and provide an intelligibility score. It is important that the testing method be determined during the design phase of the project.


Subjective testing uses a group of qualified subjects to listen to the message to identify if it is understandable. Testing is done using live and recorded messages. Subjective testing of live messages is also subject the vocal characteristics of the speaker using the microphone.


Objective testing is accomplished using calibrated test instruments. The equipment produces a special, non-voice audio signal that is registered by test equipment to provide an intelligibility score for the ADS. Objective testing is typically done using STIPA (Speech Transmission Index for Public Address) standards. STIPA testing uses a talk box to generate signals through the system microphone or the amplifier directly. The test equipment at the notification location is able to rate intelligibility.



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