The concept of intelligibility expressed in Clause 3.2.4.22.(1)(b) is intended to mean that a person with average hearing and cognitive abilities is able to understand the messages that are transmitted into the space occupied by the person. There is no absolute measure to predetermine the effect of loudspeakers and it maybe necessary, once the building has been furnished and occupied, to increase to the number of loudspeakers to improve the quality of the messages.
The intelligibility of the message depends on the speech level, the background level, and the reverberation time of the space.
ISO 7731, “Ergonomics - Danger Signals for Public and Work Areas - Auditory Danger Signals”, addresses audibility. The standard suggests that an A-weighted sound level at least 15 dBA above the ambient is required for audibility, but allows for more precise calculations using octave or ⅓ octave band frequencies to tailor the alarm signal for particular ambient noise conditions. Design of the alarm system is limited to ensuring that all areas receive an adequately loud alarm signal.
If a public address system is to be used to convey instructions during an emergency, then the requirements of the system are less straightforward. In general, however, a larger number of speakers operating at lower sound levels would be required.
Additional guidance on how to design and evaluate the intelligibility of a communication system can be found in the following documents
- IEC 60268-16, Sound System Equipment — Part 16: Objective Rating of Speech Intelligibility by Speech Transmission Index
- ISO 7240-19, Fire Detection and Alarm Systems — Part 19: Design, Installation, “Commissioning and Service of Sound Systems for Emergency Purposes”
- NEMA SB 50, “Emergency Communications Audio Intelligibility Applications Guide”
- Annex D of NFPA 72, “National Fire Alarm and Signaling Code”.