Achieving Critical Mass Notification

High profile incidents are fueling greater demand than ever before for mass notification systems (MNS). Find out how new technologies, standards, designs, installation techniques and practices are helping fire/life-safety solution providers grow their MNS business and help clients better deal with emergencies.

4 Factors That Affect Messaging

An intelligibility report developed by the National Electrical Manufacturers Association (NEMA), states that designers and engineers have the greatest effect on speech intelligibility by their choice of equipment, the number, distribution and placement of loudspeakers, and the power at which they are driven. However, designing an ADS for intelligible voice can be complicated by many variables.

Signal-to-Noise Ratio (SNR) — Compares the sound level output from the speaker to ambient noise in the room. It is the obscuring of voice due to background noise; the higher the ratio, the greater the intelligibility. Humans can tolerate a significant amount of background noise. However, once intelligibility begins to diminish, it does so rapidly. In Chapter 18, NFPA recommends an average of 15dB over ambient noise. Higher than 15dB results in reduced returns in terms of improving intelligibility.

The rule of thumb for better intelligibility is increasing the number of speakers and utilizing the lower tap settings instead of increasing the sound output on the speakers. Increasing the wattage of speaker can often distort the message.

Harmonic Distortion — A well-spoken intelligent message can be misunderstood if it is distorted by the delivery system or by the acoustic environment. Distortion is the noise that masks the original speech signal, resulting from one of the electrical or electro-acoustical components in the transmission.

Frequency Response — Most fire alarm and mass notification systems generally produce frequency responses of 400Hz to 4KHz. However, commercial sound systems usually distribute frequency responses of 125Hz to 12.5KHz, and professional sound systems deliver frequency responses of 20Hz to 20KHz. Since humans can hear from around 50Hz up to 22kHz, the wider the frequency response of a speaker, the better it is at reproducing the frequencies in the original signal. Thus the better the message is understood.

Most of the average energy is in vowels, which lie below 3kHz. However, the most critical elements of speech are the consonants, which lie above it. The burst of high-frequency sound that distinguishes constants occurs between 4kHz and 14kHz. For example, in an emergency, intelligible voice communications reduces the chances of stair B being misinterpreted as stair D, stair C or stair E. If the loudspeaker reproducing that speech can’t handle certain frequencies in that speech, some of the information will be lost in what was originally said. According to 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, whether or not there are occupants in the space 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.

From space to space or room to room, the amount of reverberation varies. It relies on the absorption characteristics of the materials in a particular space. Soft surfaces absorb sound, and hard surfaces reflect sound rather than absorb it. To reduce reverberation, designers should locate loudspeakers away from hard surfaces, and point the speakers towards soft, absorbent surfaces.

Designing for Best Results

For future occupancies, designing for intelligibility must include collaboration between the system designer, the architect and the interior designer. The ECS designer should have an understanding of the acoustic characteristics 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, room use, room occupancy, room finishes and treatments, and speaker polar plots. Conducting a design analysis could reveal that an intelligible system is not achievable unless some features of the architectural design are changed.

An important point to remember is that effective coverage in a room depends more upon speaker placement, the type of speaker and spacing than upon volume of the speaker themselves. To achieve an intelligible voice message in certain situations, according to NFPA, requires a distributed sound level with minimal sound intensity variations, which differs from past fire alarm design.

For example, a garden hose has a certain spray pattern. Increasing the water pressure will not give the person greater coverage for the garden but adding another nozzle certainly will. In a similar fashion, multiple speakers “spray” sound evenly to all areas and create a uniform sound field. Achieving sufficient intelligibility is not simply a matter of turning up the volume or increasing the wattage of the speaker. Many poor evacuation systems are the result of trying to
compensate for an insufficient amount of speakers with too much volume.

The general rule of thumb is that a lot more speakers at lower wattage taps are required for designing for intelligibility. It can be as much as eight times as many speakers used to achieve audibility alone, and increasing the wattage can often distort the message. The ideal point source (speaker to listener) is 20 feet or less. Ceiling height is speaker spacing. For example, if the ceiling is 12 feet high, space the speakers 12 feet apart for intelligibility versus 24 feet apart for audibility.

The same principles also pertain to designing intelligible systems for outdoor areas. However, there are additional parameters to consider such as wind, temperature, humidity, changing of the environment due to seasons, age and time. For example, vegetation growth affects sound. Designing a wide-area MNS in the middle of winter when trees are bare without considering trees in full bloom can greatly affect the intelligibility of the system.

Referring to NFPA 72 2013, consulting a sound professional and utilizing commercially available software tools are recommended when designing spaces and rooms for voice intelligibility. EASE Evac is an example of a sound modeling program that simplifies audio design with tools to define the space finishes so a true design can be developed for intelligibility.

Ted Milburn is Vice President Marketing for Cooper Notification. He can be reached at (941) 487-2301 or [email protected].

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