The captain of Road Prison 36 in “Cool Hand Luke,” played by Strother Martin, said the line that appears as the title of this article. Here, it leads to a discussion on the art and science of designing an effective communications system.
We sometimes feel we haven’t conveyed or received information very well when we install a system and it does not meet the owner’s requirements. The owner described the system desired, but after the installation, you found out that you and he had not communicated.
How would you approach the design and installation of a communications system? Unless you have experience, you should thoroughly research what it takes to layout and install such a system. You need to ask yourself, “What minimum points of information will help me meet my customer’s needs?” No codes exist to govern these systems unless the owner’s request moves from a simple public address (PA)/background music system to a mass notification system (MNS). NFPA 72 2010, the National Fire Alarm and Signaling Code, covers MNSs.
I suggest you start small, especially if the owner’s architect does not provide design drawings or engineering specifications to give you guidance. I cannot overemphasize the importance of knowing the fundamentals of sound and communications. You need to keep in mind that distance and losses through building elements such as walls, floors and doors will reduce sound levels, sometimes significantly.
For example, every time the distance from the sound source doubles, the sound level decreases by about 6 decibels (dB). As an example of the importance of this point, when UL lists a speaker, it measures the manufacturer-provided sound pressure level at a specific power output at 10 feet. So, if you measure, at 20 feet, the output of a -watt speaker listed for 90 dBA at 10 feet, the sound pressure level will reduce to 84 dBA, a 6 dBA loss.
If the owner’s goal stated that his staff members must hear the communications in their offices with the doors closed, then you have to account for the loss of signal through the wall or door when you create your design. Since such losses may range from 15 to 34 dBA, you may need to install a speaker in each office.
Additionally, you must address the effects of background or “ambient noise levels.” For the system to meet any reasonable intelligibility goals—and by “intelligibility,” I mean the ability of a listener to clearly understand the spoken message delivered by the communications system—the system must have an adequate signal-to-noise ratio. To minimize intelligibility loss from ambient noise levels, the system must provide a speech signal at least 15 dB higher than the ambient noise level.
The overall quality of the equipment you buy will also affect the system’s sound quality. “You get what you pay for” applies here. The amplifier must provide a low distortion signal and have enough power to operate all the speakers in your design without overheating.
NFPA 72 2010 Chapter 24’s annex contains instructions for the number of speakers to use.
For example, in a standard building configuration with a ceiling height of 8–12 feet and a ceiling constructed of drop-in acoustical ceiling tiles, “standard wall configurations, and finishes and carpeted floors, ceiling-mounted speakers should be installed in all normally occupiable spaces and in corridors spaced at a maximum of twice the ceiling height.”
One goal that affects speaker placement requires that the design provide the shortest practical distance from the source (speaker) to the recipient (person hearing the signal). Applications may require a combination of wall- and ceiling-mounted speakers. The tap (power)/setting at each speaker can affect the audibility and intelligibility of the communications system. Connecting to a high setting to increase audibility in an area could distort the signal’s intelligibility.
These approximate guidelines apply to areas typically considered “nonacoustically challenging,” including traditional office environments, hotel guest rooms, dwelling units and spaces with carpeting and furnishings. The code recommends “special attention must be given to acoustically challenging areas such as spaces that might incorporate appreciable hard surfaces (e.g., glass, marble, tile, metal, etc.) or appreciably high ceilings (e.g., atria, multiple ceiling heights).”
As you might guess, I cannot possibly cover everything you need to know about communications systems in this column. But my goal is to get you thinking about the kind of knowledge and information you should seek out regarding these systems and, once armed, to lessen your concerns about designing and installing them.
The National Electrical Manufacturers Association Standards Publication SB 50 2008, Emergency Communications Audio Intelligibility Applications Guide—available for free from the Automatic Fire Alarm Association at www.afaa.org—provides valuable design guidance. I recommend you start there.
MOORE, a licensed fire protection engineer, frequent speaker and an expert in the life safety field, is a past chair of the NFPA 72 Technical Correlating Committee. Moore is a principal with Hughes Associates Inc. at the Warwick, R.I., office. He can be reached at email@example.com.