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5-4
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 Iav-,
as specified in B3 of IEC 60849, exceeds this value. Iav is the arithmetical average of the
measured intelligibility values on the CIS and (sigma) is the standard deviation of the results.
Objective means of determining intelligibility are found in Part 16 of IEC 60268, The objective
rating of speech intelligibility by speech transmission index. Subject-based techniques for
measuring intelligibility are defined by ANSI S3.2, Method for Measuring the Intelligibility of
Speech Over Communications Systems. ANSI S3.2 should be considered an acceptable alternative
to ISO TR 4870, where referenced in IEC 60268, Part 16, Second Edition: 1998, The objective
rating of speech intelligibility by speech transmission index.
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.
Continued on next page
Intelligibility
Intelligibility