Pro AV and MNEC

Mass Notification Design Considerations 7/18/2011 9:46 AM Eastern

Pro AV and MNEC

Jul 18, 2011 1:46 PM, by Edouard G. Charland

Mass Notification Design Considerations

As anyone knows who has struggled to hear a gate announcement (or talk over one), airports present challenges to intelligibility. Now with the historic revisions to NFPA 72, the National Fire Alarm and Signaling Code, poor intelligibility and sound quality will not just be a violation of customer comfort and convenience, it may be a violation of code requirements. That's because airport display and PA systems will be part of mass notification and emergency communication infrastructure.

In this article we'll look at acoustical considerations for intelligibility in airports—these same principles would also apply to arenas, stadiums, etc. And we will look at the major component parts of mass notification systems and some design considerations for each.

Let's start with a summary of the key acoustical issues—reverberation, reflection, and noise control—and the things that work for and against intelligibility. Airports are full of reflective surfaces—plaster or gypsum board ceilings and walls, glass, and terrazzo or ceramic tile floors that increase ambient noise from footfalls and rolled baggage. There are absorbing materials that can be substituted to decrease reverberation—lay-in, perforated metal or acoustical deck ceilings, acoustic plaster, fiberglass panels wrapped in fabric or perforated metal, carpet, and other sound absorbing materials that can be used as a visible feature and/or hidden above ceiling clouds and bulkheads or behind panels. Similarly, certain architectural features can contribute to or detract from intelligibility—convex, angled, or faceted ceiling and wall surfaces diffuse sound rather than causing discrete reflections and are acoustically preferable to concave surfaces and parallel walls. For example, to prevent flutter echo between walls or floor/ceiling, surfaces should be at least 10 degrees out of parallel. If the architectural situation is not optimal and/or if it is impossible to avoid aiming a line array or other loudspeaker at a reflective surface, sound absorption treatments can reduce undesirable reflections, especially when placed at listening height.

Airports not only have airplane noise, like most facilities they are full of mechanical noise from necessary infrastructure. Again, there is much that can be done to minimize unwanted noise. As much as budgets will allow, using mass (i.e. heavy building materials such as layers of gypsum, laminated, or insulated glass, or double-walled construction as for machine rooms) will do the most to reduce noise; duct lining, silencers, and diffusers also play a role, reducing noise transmitted through the HVAC system.

Once the acoustical environment created by the building design and construction is known (whether it will support adequate levels of speech intelligibility or present obstacles related to a poor acoustical environment), we can then look to maximize the intelligibility of the mass notification system itself. One of the first steps is to choose the right components for input, control, and output—paging stations, control systems, and distribution/playback systems.

Paging stations—where live or prerecorded announcements are initiated—should use microphones with wide, flat frequency response, pop filters, low proximity effect, and low handling noise; noise cancelling microphones may result in non-uniform response. They should be rugged (especially the cord), shock mounted in secure stations, and field replaceable. The stations themselves should be networkable and purpose-built (due to low speech intelligibility and other issues, telephone paging should be limited). These stations come in a range of form factors with various features and ergonomics.

Control systems must be flexible enough to support zone configurability, system fault management, the ability to automatically control level based on ambient noise, and when necessary, the ability to interface smoothly with other systems. Software should be flexible and adaptable, and easy to use. Considerations in choosing a control system include: variety of announcement (live/prerecorded), zone configuration, and considerations for flexibility, maintenance, security, and reliability. Announcement-type considerations include whether the live function is simply for gate announcements, whether the system will control for emergency live announcements, and how many languages the systems must accommodate. When considering zone configuration, remember that while a variety of discreet zones may be needed for some announcements, the ability to group zones to create a larger control area may also be desirable. Control systems should accommodate audio level control through ambient noise sensors and time-of-day programming. They should be able to monitor and test system components such as power amplifiers and loudspeaker circuits and generate system fault and activity logs and messages. Control systems may or may not have to interface with other systems for content or monitoring, such as other paging systems, fire alarm and security systems, background music, TV distribution and noise-masking systems, information display systems, master clock synchronization, and telephone systems (again, only where necessary).

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.

Figure 1: Line array design challenges related to zone separation and cumulative signal delays.

Figure 1: 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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