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TECHNOLOGY SPOTLIGHT All photos courtesy Cooper Notification Top: Mass Notification systems are particularly useful in school and other campus situations, where a voice message must be quickly distributed among a large number of people. Above: Many poor evacuation systems are the result of trying to compensate for an insufficient amount of speakers with too much volume. Left: When designing an outdoor mass notification system, be sure to account for wind, temperature, humidity and changes resulting from time and season. room. It is the obscuring of voice due to background noise — the higher the ratio, the greater the intelligibility. In Chapter 18, NFPA recommends an average of 15 decibels (dB) over ambient noise. A level higher than 15 dB results in reduced returns in terms of improving intelligibility. Frequency Response: Most fire alarm system speakers produce frequency responses of 400 hertz (Hz) to 4 kilohertz (KHz). Commercial sound systems can usually distribute frequency responses of 125 Hz to 12.5 KHz, and professional sound systems can deliver frequency responses of 20 Hz to 20 KHz. Since humans can hear from approximately 50 Hz up to 22 kHz, the wider the fre46 quency response of a speaker, the better it is at reproducing the frequencies in the original signal; thus, the chance is increased that the message will be understood. Another key consideration is that most of the average energy is in vowels, which lie below 3 kHz; however, the most critical elements of speech are the consonants, which lie above it. The burst of high-frequency sound that distinguishes consonants occurs between 4 kHz and 14 kHz. For example, in an emergency, intelligibility reduces the chances of Stair B being misinterpreted as Stair D, Stair C or Stair E. If the loudspeaker reproducing that speech cannot handle certain frequencies, some of the information will be lost. According to the NFPA, an ADS that differs from another space because of frequency and level of ambient noise might require speakers and system components that have a wider frequency bandwidth. Physical Room Characteristics: Room reverberation depends on the physical characteristics of the space, such as room dimensions, construction materials, occupants and furnishings. NFPA states that the amount of reverberation in a room diminishes when the room includes construction features, people or furnishings that absorb sound. To reduce reverberation, designers should locate loudspeakers away from hard surfaces and point the speakers towards soft, absorbent surfaces. Designing for intelligibility must include collaboration between the system designer, architect and interior designer. The ECS designer should have an understanding of the acoustics of the architectural design. According to the Fire Protection Research Foundation 2008 Report, "Intelligibility of Fire Alarm and Emergency Communication Systems," designers will need to know all room dimensions, use, occupancy, finishes and treatments, as well as speaker polar plots. A design analysis could reveal that an intelligible system is not achievable unless some features of the architecture are changed. Design Trends An intelligibility report developed by the National Electrical Manufacturers Association www.SecurityInfoWatch.com | SD&I; | September 2013