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INTELLIGIBILITY – The capability of being understood or comprehended.
In simple terms, intelligibility is an evaluation of changes that occur to speech that impact
comprehension. More specifically, intelligibility is concerned with evaluating reductions of the
modulations of speech that cause undesired reductions in speech comprehension. These
modulation reductions can also be thought of as a degradation of signal (speech) to noise ratio.
Over the last few years, the drive towards intelligible Emergency Voice/Alarm Communications
Systems has been gaining momentum throughout the fire alarm industry. NFPA 72
®
, the National
Fire Alarm Code
®
now requires that emergency voice/alarm communications systems be
intelligible and discusses methods for verifying intelligibility.
In the past, the fire alarm industry primarily focused concern on audibility requirements, assuming
that if the sound was loud enough it would be sufficiently intelligible. Furthermore, many designs
did not take into account ongoing changes in the construction of the building, the construction
materials used in a building, or its furnishings. It is possible that many emergency voice/alarm
communications systems designed under those conditions do not provide sufficiently intelligible
communications. While those systems may provide highly audible alert and evacuation tones,
speech information may not be properly delivered.
This guide provides general information on the concepts of intelligibility and on the design of
emergency voice/alarm communications systems. It provides you with a better understanding of
the factors affecting the intelligibility of these systems in public spaces and is intended to help
design a system that meets the requirements for speech audibility and intelligibility in a
cost-effective manner.
This guide is separated into the following chapters:
• Chapter 1. Speech Intelligibility Overview: Provides an overview of audio intelligibility and
an introduction to the topics covered in this publication.
• Chapter 2. Background Information: Provides several sections of background material that
are essential to designing an intelligible system. Topics such as room acoustics, speaker design
layouts, and audio math are discussed.
• Chapter 3. Speech Intelligibility: Details the influences and measurements of intelligibility.
• Chapter 4. Emergency Voice/Alarm Communication Systems: Details emergency
voice/alarm communications systems and describes the advantages of an emergency system
compared to a typical non-emergency paging system.
• Chapter 5. Regulatory Issues: Discusses National Fire Protection Agency (NFPA) Codes.
Several excerpts of the 2002 Code are included.
• Chapter 6. Fire Alarm Audio Speaker System Design Method: Provides examples of
speaker designs created by using the Tyco Safety Products iTool.
• Chapter 7. Glossary of Terms: This chapter includes definitions of the important terms
used in this publication.
Refer to the page number listed in this table for information on a specific topic.
Topic See Page #
Importance of Audible and Intelligible Emergency Communications 1-2
Chapter 1
Speech Intelligibility Overview
Introduction
Chapters of this
Publication
In this Chapter