Tyco 579-769 specifications Speech Intelligibility Importance Designing for

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Importance of Audible and Intelligible Emergency Communications

Speech Intelligibility Importance

Designing for

Intelligibility

Emergency voice/alarm communications systems are used in applications where it is necessary to communicate more detailed information to occupants of a building than the simple evacuation signal provided by horns or bells. For example, in a high-rise building, evacuation of all of the occupants at one time could create an unsafe situation in which the routes to evacuation could be blocked by the sheer number of people trying to exit at once.

An emergency voice/alarm communications system can provide a means to ensure a more orderly and safe evacuation. However, if the emergency voice/alarm communications system is not audible (loud enough), or if it is not intelligible (understandable), then emergency information is not properly communicated. Therefore, a safe response to a fire cannot be reliably achieved.

In some ways an inaudible or unintelligible system is worse than not having a system. This is due to a possible false sense of security. Also personnel responding to an incident may operate under the premise that building occupants are getting proper instructions, when in reality they are not.

Historically, the emphasis in emergency voice/alarm communications system design has been on audibility. These systems have been required to have a sound level that is at least 15 dB above the average ambient sound level, or 5 dB above the maximum sound level having a duration of at least 60 seconds, whichever is greater. Starting with the 1999 version of the National Fire Alarm Code (NFPA 72) the fire alarm industry recognized the importance of requiring both audibility and intelligibility.

Although a specific measure of intelligibility is not currently required by NFPA 72, the Code’s Annex recommends the use of International Electrotechnical Commission (IEC) 60849 and a Common Intelligibility Scale (CIS) measurement of 0.70 as a guideline. It is expected that future versions of NFPA 72 will quantify the measurements required to demonstrate intelligibility.

Properly designing emergency voice/alarm communications systems for intelligibility requires knowledge of the acoustical factors that influence intelligibility; awareness of the tools available to predict acoustical performance; and the ability to measure the intelligibility of the completed installation. It is also necessary to identify complicated areas where experienced sound professionals using sophisticated audio design tools may be required to achieve the desired intelligibility.

This document is presented as an introductory guide to understanding intelligibility and its importance in achieving successful emergency voice/alarm communications systems. Please refer to the cited references for more information concerning audio systems design.

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Contents Fire Alarm Audio Applications Guide Page Copyrights and Trademarks Page Table of Contents Emergency Voice/Alarm Communications Systems Glossary of Terms Related Publications Chapter Speech Intelligibility Overview Speech Intelligibility Importance Designing for Chapter Background Information Topic See Page #Equation 2-1. The Decibel Equation 2-3. Power RelationshipsBasic Audio Math Equation 2-2. Ohm’s LawEquation 2-5. dB and Sound Pressure Levels Equation 2-6. Adding DecibelsSound and Hearing Robinson and Dadson Equal Loudness CurvesNature of Speech Speech Pattern that Illustrates ModulationsRoom Acoustics Sabine Equation, used when α Eyring Equation, used when αAreas with high ceilings, specify a more directional speaker Speaker Basics Equation 2-8. The Inverse Square LawSPL = Sensitivity + 20 log Equation 2-9. On-Axis SPL Calculation87dB 6dB/divisionEquation 2-10. Directivity Factor Q for a Conical Source 51º 104 Critical Polar Angle CalculationsEquation 2-11. Coverage Area Calculations Listener Height = 1.5 Meters Ceiling Coverage Diameter2x Edge-to-Edge Layout Pattern Selection GuideSPL Variation by Layout Pattern Minimum OverlapDistributed Wall Mounted Systems Opposite Speaker WidthWall Mounted Speakers In Meters Room Coverage Width Edge-edge Minimum-Overlap Full-Overlap Chapter Speech Intelligibility Influences on Intelligibility Frequency of Speech Contribution to IntelligibilityDegradation of CIS vs. Signal-to-Noise Ratio No Noise With Added NoiseBackground Noise Reverberation Distortion Correlation of CIS and with STI and %ALcons Measures of IntelligibilityALcons STI method with faster measurement times Practical Measurement of Intelligibility STI-CIS Analyzer TalkboxTools for Predicting Intelligibility Page Chapter Emergency Voice/Alarm Communications Systems Typical Emergency Voice/Alarm Communications System AdvantagesParts of an Emergency Voice/Alarm Communications System Class a and B Speaker Circuit Wiring Chapter Regulatory Issues Audibility From Nfpa 72, 2002 EditionHigh Background Noise Large Areas Intelligibility Intelligibility Certification Page Chapter Speaker System Design Method Speaker Design Method Determine the speaker-to-listener distance D2Recommendations for Maximizing System Intelligibility Applying the Methods ITool Office Space ExampleOffice Space Speaker Location Guide Corridor Design Example Corridor Speaker Location Guide Corridor SPL DistributionITool Gymnasium Example 10. Gymnasium Speaker Location Guide 13. Lobby Example 15. Lobby Layout Applying the Methods Conclusion Page Chapter Glossary of Terms Glossary Glossary Page Index IN-2 Page 579-769 Rev. C
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