In Part II of our two-part segment on loudspeakers and loudspeaker system design, we tackle the subject of multichannel sound systems and more on loudspeaker placement in the room. But first we discuss more about loudspeakers as systems.
Directivity is simply the ability for a loudspeaker to control where sound goes. We do it all of the time with light. A light bulb dangling on a wire in free space has no directivity. We say that it is radiating into “whole” space. If you put it against a ceiling the light radiation is confined to “one-half” space, and the light intensity goes up because the light that strikes the ceiling is reflected back into the room. If you put it where the wall and ceiling come together, the radiation is now “one-quarter” space. If you put it into a corner then it’s “one-eighth” space. The more we limit the radiation angles, the brighter the light becomes. We can do a similar thing with sound. Room surfaces can be used to confine the sound’s radiation angles. If those surfaces are not in the right place (most often the case), then we build “little rooms” called horns to mount in front of the loudspeakers.
Horn loading is virtually a prerequisite for radiating sound into a large auditorium. The horn serves two purposes. First, it confines the sound to a smaller area, making it more intense. Second, it keeps the sound from going to unoccupied areas, room surfaces, and microphone locations, which is just as important. It’s like stage lighting, whose job it is to illuminate the stage, not the audience. And the only way to accomplish this is through the use of directivity.
For sound, the required physical size for a horn is related to the frequency and wavelength of the sound being controlled. The horn’s size must increase as frequency decreases (bigger horns control lower frequencies), which can yield fairly large horns for full-range systems. When you place an arbitrary limit on the size of the loudspeaker, you are also placing a limit on its sound radiation control. It is better to consider the size required to get the job done, and then consider the aesthetic issues involved in incorporating such a device or devices into the room. Any competent system designer can quickly determine the required horn size based on room size, listener distance, required bandwidth, and expected use of the system. It’s the architect’s job to make it look right.
Multi-channel Sound Systems
We use face-to-face communication to describe what is expected from a speech
reinforcement system. Music is a bit different, and a better analogy is to
consider a live ensemble or a choral group. When we sit out front and listen
we are getting different sounds from different places. The ideal music reproduction
system would emulate this, and many approaches have been used, ranging from
“stereo” to “surround sound.” The idea is to create
a spacious sound image rather than have all of the sound emanate from one location.
Will a multi-channel sound system work in my venue? Maybe. Maybe not. True
multichannel formats use program material that is unique for each playback
channel, meaning that every listener must be able to hear every channel. And
the more channels that are used, the harder this is to accomplish. Two-channel
systems can be the most difficult, because each audience member must be able
to hear both channels at relatively equal levels. If monaural sound (single
channel) is mixed to both left and right channels, then we have violated one
of the main criteria for a good speech system - a single source for each listener.
Most of the audience will be plagued by “hot spots,” “cold
spots,” and unnatural localization of sound (all are common auditorium
system complaints).
So why does “stereo” work so well at home? There are several reasons, all of which are violated in large spaces. First, at home the listener can sit at the same distance from the left and right loudspeakers. The next time you check out a big screen TV or listen to a pair of loudspeakers at a hi-fi store, take a look at the position to which you naturally gravitate. It is the one that is directly in front of and equidistant from the left and right sources. The same is true for our seating preference in a movie theater or live music show. For this position (and this position only), sounds that are mixed equally to the left and right loudspeakers will appear to come from a position exactly between the two loudspeakers. We call this a “phantom” center channel, and it works great in front of your personal computer or five feet from a television. If you move off-axis, the phantom image moves from center or eventually disappears altogether, leaving only the left or right loudspeaker.
In an auditorium we simply can’t get the audience into the “sweet spot” where stereo works. A better solution is to not rely on the human auditory system to create the center channel. Instead, we can use a physical center channel between the left and right loudspeakers. Sounds that are mixed to the physical center channel (rather than the left or right) will localize properly for a much larger segment of the audience. For that reason, it is my opinion that house-ofworship sound systems should be either single channel (like the speech system) or utilize three channels (often called Left- Center-Right or LCR). Stereo is out unless you have a very small congregation or are willing to compromise the sound reproduction for the majority of the audience (remember the 80/20 rule?).
A Video Comparison
Let’s apply all of this to something a bit more evident than the sense
of sound - the sense of sight. Modern houses-of-worship use video projection
technology to bring the information sources closer to the listeners. The idea
is to let everyone see what the camera sees. It’s great if this can be
accomplished with a single projector and screen. More often than not, it requires
multiple projectors and screens for the entire congregation to see what a single
camera is seeing. If we wish to project two cameras simultaneously, then the
equipment list gets doubled unless we mix both cameras to the same projector
(back to the single-source system). Most video designers wouldn’t consider
trying to cover the entire audience with three discrete channels of video projection.
It wouldn’t be impossible, just impractical. While multi-channel video
may work fine in CNN’s control room, it’s much more difficult to
accomplish in an auditorium. The sound system is very similar. The most important
thing is to get the main mix to everyone, usually as a single channel mixed
down from the various sound sources. Stereo or LCR systems require a doubling
or tripling of equipment - not impossible, but often impractical. And remember
that the multichannel system is only multi-channel if the mix and the mixer
make it so. If the sound operator mixes all of the mics and instruments to
all of the loudspeakers, the whole effect is lost. A LCR mixing console (as
opposed to the traditional “stereo” board) and a well-trained operator
are mandatory for these types of systems.
Auxiliary Loudspeakers
No matter how good the design of the main system, there are often some audience
areas that fall outside of its coverage. This can include balconies, under
balconies, overflow areas, etc. These areas must often be covered with auxiliary
loudspeakers, usually in the form of small format (lower directivity) devices
located physically near the area to be covered. The same criteria apply for
these as for the main system - make sure that everyone can hear one of them,
ideally from the direction of the stage. Since these are often located at remote
distances from the main array, separate amplifiers with delay are required
to drive them.
Conclusion
As I stated at the beginning of this article, it is never possible to deliver
optimum sound reinforcement to every listener in an auditorium. A methodical
hierarchy of compromise is required to deliver the most important aspects of
the reinforced sound to as many of the audience members as possible. Good system
designers carefully evaluate the client’s needs, and then follow the
80/20 rule in designing the system.
