Intelligibility
Intelligibility
Intelligibility has historically been a difficult parameter to measure. Unlike SPL that can easily be measured with a relatively common dBA meter, intelligibility measurements have previously required trained acoustical engineers or sophisticated/high end evaluations. NFPA 72 requires that voice messages to areas of buildings be intelligible without defining a preset limit within the main body (enforceable part) of NFPA 72.
From NFPA 72, 2002 Edition:
7.4.1.4* Where required, emergency voice/alarm communications systems shall be capable of the reproduction of prerecorded, synthesized, or live (e.g., microphone, telephone handset, and radio) messages with voice intelligibility.
Working without a preset limit gives the AHJ latitude to determine adequate intelligibility. NFPA 72 provides guidance for determining adequate intelligibility with the explanatory information in the Annex.
From NFPA 72, 2002 Edition:
A.7.4.1.4 Voice intelligibility should be measured in accordance with the guidelines in Annex A of IEC 60849, Second Edition: 1998, Sound systems for emergency purposes. When tested in accordance with Annex B, Clause B1, of IEC 60849, the system should exceed the equivalent of a common intelligibility scale (CIS) score of 0.70. Intelligibility is achieved when the quantity
As technology for intelligibility analysis equipment advances, many expect the main body of NFPA 72 will be changed to require a minimum fixed intelligibility score, that is verifiable with the equipment designed for this purpose. The SimplexGrinnell STICIS meter and TALKBOX signal generator is the first such system developed to provide a measurement system that is nearly as easy to use as a dBA meter, provides readout in CIS units, and is portable.
Note: See the “Measures of Intelligibility” section in Chapter 3 of this guide for more information. The system uses a subset of the STI method outlined in IEC60628.
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