Convention Centers

by Dennis A. Paoletti

Having enough sound absorption in a large convention center space, where most surfaces tend to be hard and sound reflective, has always been a challenge for acoustical consultants. Traditional formulas indicate that a considerable amount of sound absorbing material (e.g. acoustical ceiling panels and fabric wrapped wall panels) is required for large volume spaces with footprints that extend hundreds of feet and ceilings that are at least 40 to 60 feet or more in height. The same is true for any similar space such as ballrooms, arenas, and multi-purpose spaces of comparable dimension.

The reason for the sound absorption is twofold: 1) to control excessive noise and reverberation from occupants, and 2) to minimize reverberant sound build-up for the sound amplification system to be “intelligible.” When budgets get tight, it is not uncommon for the sound absorption in a convention facility to be removed from the project during a value engineering exercise. Whereas for general exhibits, the raw space without sound absorbing material may be fine, for any loud activities or events that utilize sound amplification, the resulting sound level and intelligibility of voice announcements suffer terribly. Audio systems in convention facilities are typically used for background and foreground music and to make voice announcements, as well as for high-level sound reinforcement. In the past, general voice announcements sometimes supported fire alarms as the main source of evacuation warning in public assembly spaces, with the alarm.

Recently, the National Fire Protection Association (NFPA) established a performance- based code to ensure that voice announcements for fire and emergency evacuation can be clearly heard throughout any and all occupied spaces within a public assembly venue. Some recent projects have had difficulty meeting the new NFPA 72 code. Not only must a facility have a well-designed sound system with adequate loudspeaker coverage, but also, the reverberant sound build up within the space must be controlled to optimize the intelligible sound emanating from the loudspeakers.

“Speech Intelligibility” can be predicted and measured. The Speech Transmission Index (STI) is a quantitative method of measurement of speech intelligibility that has proven extremely accurate. Reference the International Electrotechnical Commission (IEC) Standard 60268-16. The STI accounts for all of the factors that affect intelligibility, such as: source-to-listener distance, background noise levels, and sound reflective / sound absorptive room characteristics. In general, speech is not intelligible, just because it is audible. Being able to understand at least a certain percentage of words and sentences is how speech intelligibility is measured. The current documentation recommends a minimum STI of .50, which is subjectively defined as “fair” intelligibility. An STI of .60 to .75 is “good;” above .75 is “excellent.”

A new ISO Standard #9921 “Assessment of Speech Communication” is being developed and will discuss the topic of speech intelligibility. STI can be measured rather quickly and simply to evaluate conformance with a written specification. If the intelligibility criterion is not met, it could be fairly costly to have to make the necessary modifications to comply with the standards. Simply adding additional loudspeakers may not help, if the sound coverage or system design is not optimized. However, the use of strategically located loudspeakers with highly directive coverage patterns may help; but to have to go back into a space that may have just been constructed, to add additional sound-absorbing material may be impractical, and it could certainly get prohibitively expensive.

Owners and architects need to realize that large public assembly spaces that depend on audio systems for voice evacuation announcements must allow for an adequate amount of sound absorbing material, as an essential component of the facility architecture. The audio system needs to be well designed technically, using well-established scientific principles of physics. This combination will result in a comfortable aural environment for the attendees, and a safe environment in which speech will be highly intelligible.

Dennis A. Paoletti is a principal with Shen Milsom & Wilke, Inc., a consultant company in acoustics, audiovisual, telecommunications and security systems based in San Francisco, California. He may be contacted at dpaoletti@sf.smwinc.com.