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.
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. According to NFPA, in order to achieve an intelligible voice message in certain situations, a distributed sound level with minimal sound intensity variations is required, which differs from past fire alarm design. It is not simply a matter of turning up the volume or increasing the speaker wattage. Many poor evacuation systems are the result of trying to compensate for an insufficient amount of speakers with too much volume.
The rule of thumb for enhanced intelligibility is increasing the number of speakers and using the lower tap settings instead of increasing the sound output — as increasing the wattage of a speaker can often distort the message. Optimizing intelligibility can equate to as much as eight times as many speakers used just to achieve audibility.
The deal point-source (speaker to listener) is 20 feet or less. Ceiling height should be speaker spacing. For example, if the ceiling is 12 feet high, space the speakers 12 feet apart for intelligibility, as opposed to 24 feet apart for audibility.
Designing for Outdoor Areas
The same principles also pertain to designing intelligible systems for outdoor areas; however, there are additional parameters to consider such as wind, temperature and humidity, as well as the 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. It is also important to note to include all stakeholders as part of the design process, such as the architect, interior designer and the AHJ.