Several times each week, someone calls, saying, “HELP! I am a (fill in the blank) contractor and I have to install a sound system.”
Whether the system is a telephone, a laptop computer, a television, a home theater, a paging system, a fire alarm voice evacuation system, a corporate conference room, or a concert hall, sound is sound. The real issue is how to integrate it into useful real-world systems.
Successfully installing and integrating a sound system requires three different skill sets. First, you need to understand the flow of signals through an audio system. Second, you need to know if the system is working properly. And third, you need to be able to integrate these systems with other equipment that is in use.
Understanding signal flow
Every audio system project should begin with a flow chart of the signal path. It starts with one or more “sources.” Typical sources include microphones, tape decks, VCR/DVD (which would be an audio and video source), or external sends of pre-recorded material such as background music or messages on a fire evacuation system.
The source then goes to a “pre-amplifier,” which allows the contractor or user to adjust the level (volume) of the system. In a large system, this would be done by a multi-channel mixing console. The signal is then “processed” by a device, such as a compression/limiter or equalizer. The final step is one or more amplifiers, which raises the line level signals used to this point, to the higher speaker levels required to drive a loudspeaker.
Does it work?
The first question a contractor should ask is: “What problem am I seeking to solve?”
All sound systems have the same basic function; they make sounds louder. Accordingly, the next question to ask is, “Will it be loud enough?”
The simplest analysis of a sound system starts with measuring the sound level. (How loud is it?) To decide whether a system is loud enough, many people just turn the system on and listen to it. If there is a sound system performance specification, however, it should look something like “85dB(a) sound pressure level (SPL)” at a distance of 30 feet on axis. Every specification for how loud something is should be specified in decibels “dB” of SPL (as a dB is just a ratio, whereas dB SPL is a description of how loud). It should then state the position for the A/C weighting switch on the SPL meter, the distance at which the measurement is to be made from the loudspeaker, and, whether the measurement is made on axis, meaning straight ahead of it.
A second issue, however, is equally important: “Is it intelligible?” This is the piece of the puzzle that many contractors do not understand, and causes the most sound system complaints. It is possible to have a system that is sufficiently loud but where the sound is distorted, so people cannot understand the message. In most of these cases, turning up the volume further will simply worsen the condition.
To test intelligibility, you must listen to the system under normal operating conditions. If the system is for voice evacuation, ask yourself, “Can you understand speech over the system when the factory is operating, the HVAC is turned on, or when forklift trucks are driving around the factory floor?”
If the answer is no, then you have not satisfied the basic reason for installing a sound system. The standards in this area have changed over the past few years, and future changes are likely. For example, National Fire Protection Association “NFPA-72,” National Fire Alarm Code 1999 provides:
4-3.1.5 Intelligibility Emergency voice/alarm communications systems shall be capable of reproduction of prerecorded, synthesized, or live (for example, microphone, telephone handset, and radio) messages with voice intelligibility.
While the Code only applies to emergency voice/alarm communications, the logic behind the Code makes sense for all systems. If you cannot understand the message, why install a sound system in the first place? There is also equipment to measure and quantify intelligibility, including standards such as RT60, %ALcons, and speech transmission index (STI). The key to all of these tests, however, is to ensure that people not only hear the sound, but also understand the content.
The final step for an audio system is that it must be controllable. Traditionally, this meant little more than a volume control. Today’s customers, however, want the convenience of taking a single action, which then controls multiple functions.
The simplest control systems are the ducking features found in many paging products. A typical use for ducking is to turn off background music. When the user keys the paging microphone, the music is “ducked” off until the mic key is released. This prioritizes the message sent to the sound system.
In modern systems, increasingly, an LCD panel controls the entire room. These are typically connected to a computer, and are programmed with custom icons to represent the task to be accomplished.
In such a system, the first icon might simply say
“Power On.” Pressing this key on the touch pad would send out a series of signals. The first signal would trigger a relay, which would turn on the power at the audio rack. A second relay would turn on the power to the video projector. A third would turn on the lights, and/or a specific set of lights within the room. A fourth could be connected to environmental controls, such as heating or cooling. A fifth might open the door locks for the public entrance. Additional icons would then control other systems within the room or building.
Typically, a second LCD panel would be mounted where the senior personnel could control actions, such as the system volume. An icon will often exist to open curtains or doors to expose a video projection screen, and would simultaneously send a signal to the lighting controls to dim the lights, or to change to small aisle or walkway lights.
In planning a modern audio system, the contractor must ask, “Which systems interact with which other systems?”
Running high-voltage cabling in the same conduit as low-voltage audio cabling can create hum problems. This is especially true where the audio wiring is not balanced cabling. When working with audio, there should be dedicated and isolated neutrals and grounds for each circuit. Sharing of grounds or neutrals between audio gear and other types of equipment is a formula for noise problems. Other common culprits are old-style light dimmers, which emit radio frequency interference (RFI).
No matter how fine a system you install, it is of no value if the user cannot operate it. The true indication of great integration is that the user is completely unaware of the complexity.
MILLER is a sound systems expert at Goldline in Danbury, Conn. He may be reached at (203) 938-2588.