Pro AV and MNEC
Jul 18, 2011 1:46 PM, by Edouard G. Charland
Mass Notification Design Considerations
With the right components to generate clear content and to control the system in place, intelligibility is obviously reliant on output—the distribution system, which requires quality signal processing and power amplification, as well as the right loudspeakers and video displays (for visual paging). Signal processing should provide compression on inputs (microphone paging stations), equalization for each zone to account for loudspeaker types and/or acoustical conditions, and, if needed, equalization for inputs to accommodate different microphone station types. This won't improve poor quality microphones, but it will improve uniformity among differing types of microphones. The large number of zones in an airport, and many other installations, dictate high-density power amplifiers be used to provide space savings; modular and/or card-file-type systems can provide for easy maintenance and minimize downtime. Backup systems should be used judiciously, and only where they do not compromise the integrity of primary systems. Loudspeaker selection will be based on acoustical and architectural requirements of the facility; zoning is dictated by functional considerations or physical characteristics of the space, as is the choice of visual paging and display devices. There are considerations in choosing ceiling and surface-mounted speakers: They must be spaced and zoned properly, and be integrated with architecture and acoustic treatments as necessary. Line arrays present the most challenges, in part because they are often necessary in situations with limited mounting options. Line arrays do allow sound to be directed toward listeners in highly reverberant spaces and fewer devices may be needed to achieve coverage. However, line arrays present challenges to zone separation, and they may add to the reflections in the space; signal delays may be needed for overlapping zones to prevent echoes. Active devices also require local AC power and control circuits adding to the complexity of the design. Figure 1 illustrates line array design challenges related to zone separation and cumulative signal delays.
Acoustic modeling and simulation can be used to evaluate the design for acoustics, loudspeaker distribution, and equalization to help predict and optimize intelligibility. Finished models can include actual proposed finishes, measured performance data for loudspeaker polar response, sensitivity, etc., and approximate recommended equalization settings. They can offer a way to test various design ideas and quickly perform sound pressure and intelligibility calculations, as well as create "auralizations" that allow one to "hear" what the designed MNS may sound like in a particular space. These models may also serve to prove that design will meet international intelligibility standards and to demonstrate justification for acoustical treatments or specific loudspeaker distribution schemes. Acoustic modeling and simulation should be considered only as a tool, not a substitute for knowledge and experience.
In addition to proper design, MNS intelligibility also depends on proper equalization of the system; many installed systems are never properly equalized. In fact, post-installation commissioning in which the installation of systems components are verified, the system properly equalized, system settings configured, and operations staff trained, can make or break a project—especially when mass notification systems are involved.
Edouard G. Charland is VP at the acoustics/AV consulting firm Coffeen Fricke & Associates. He is an engineer with more than 20 years of experience in design, testing, and commissioning of AV system and sound reinforcement systems most recently for the Detroit, Las Vegas, Philadelphia, and Washington Dulles airports.
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