As we say in the South, “There are many ways to skin an opossum.” Such is the loudspeaker design process: each designer has his own way. Some processes are very simple and some are extremely scientific. The following article outlines some of the most fundamental elements all loudspeaker design processes must take into consideration. We can forgo discussing the types of loudspeakers, (two-way, three-way, subwoofers, etc.), and focus on the fundamental purpose of loudspeaker systems, define basic design criteria, and outline design configurations.
WHAT ARE THE FUNDAMENTAL DESIGN CRITERIA OF ANY LOUDSPEAKER SYSTEM?
When you consider the most basic, fundamental purpose of a loudspeaker system, you will discover it is simply to increase the volume at the listener's ear. However, there are numerous other purposes of loudspeaker systems that extend well beyond the fundamental. Loudspeaker systems provide the tools audio designers and engineers use to create artistic audio images, specific sound signatures, and audio perceptions.
The loudspeaker design process begins with remembering the basic fundamental purpose of the loudspeaker system and building on that base with the many other purposes. As the process matures, the system design takes shape.
GETTING STARTED
Loudspeaker systems must provide even sound coverage to designated seating areas. Depending on the shape of the venue, multiple loudspeaker systems are often required to ensure equal coverage from seat-to-seat. The size and location of the seating planes is the first criteria that will begin to determine the quantity of loudspeakers required. History proves it is hard to provide even coverage to all seats from a single reference point (center cluster). However, digitally controlled column loudspeakers and line arrays are challenging the old notions. Both newer technologies are proving useful when minimal point sources are called for.
The loudspeaker system must provide adequate volume to the coverage areas. The source material to be reinforced (speech, music, effects) plays a role in defining the sound pressure level (SPL) required for each seating area. Coupled with the coverage criteria, the target SPL determines the type, size and quantity of loudspeaker devices that will be required to generate not only the proper coverage, but also the required volume (SPL) conducive to the program material.
Providing even coverage to numerous seating areas and delivering the required sound pressure level typically requires several loudspeaker systems per area. Creating a specific audio image is subject to the location of the loudspeakers and their coverage characteristics. We know the brain localizes on what it hears first, not necessarily what it hears the loudest. As such, a listener is going to localize on the closest loudspeaker system. By way of digital signal processing and numerous processing tricks, creating audio images is much easier today than it was in days gone by. The art of creating an audio image is experienced not only in motion picture theaters (surround), but in famous entertainment complexes, as well as performing arts venues. The ability to direct one's attention in a specific direction or create an impact with excessive level is yet another tool the loudspeaker system provides to deliver an audible experience.
SOME LOGISTICS
There are several types of loudspeaker system configurations, as well as hybrids. The four most common configurations are central, left/right, left/center/right and distributed. Each configuration has pros and cons, as well as specific purposes. It is important to remember that one's attention is drawn to the sound (source) that reaches the ear first. With that said, let's discuss the four most common configurations.
Central loudspeaker systems (center clusters) were extremely popular in the 1980s. They were known for their accurate point source characteristic: hearing the sound from a single point of geometric origin with exceptional vocal intelligibility. In essence, everyone hears from the same point in space. Imagine a rock dropped into a pool of water; notice the concentric ripples emitting from the single point of entry. This illustrates the sound pressure waves generated by a central cluster.
However, the audio image from a central point source was found to be very small, and this typically did not match the visual width of the stage. This posed a challenge for the listener, seeing a wide stage but hearing the sound from a narrow, specific point. Additionally, most central arrays were less than musical, producing a very “mono” sound. Stereo imaging was not easily accomplished.
Today, central clusters are still used, depending on the venue's primary purpose, but typically include a left and right array for stereo music reinforcement. This hybrid configuration became popular in the early 1990s—an attempt to provide the best of both worlds. It became known as L/C/R (left, center, right).
Left/right loudspeaker system configurations were present well before central arrays. Many readers will recall the two column loudspeakers in churches well back to the 1960s. The left/right configuration is sometimes called ‘stereo' configuration but does not necessarily produce a stereo image. Stereo imaging is a product of audio processing that requires two loudspeakers.
Left/right loudspeaker configurations are quite popular today when line array loudspeakers are used. Line arrays provide fairly wide coverage with unique vertical coverage characteristics. Left/right configurations are easily accomplished using line array or multiple loudspeaker cabinets. If required to deliver stereo audio, each loudspeaker array must cover the majority of the venue's width. This is fairly easy in square or rectangular spaces. Fan shaped seating venues are a bit more complex. If the left and right arrays do not provide adequate coverage to the entire seating area, a stereo image will only be truly experienced by listeners sitting in the middle section(s).
As mentioned above, the left/center/right configuration emerged when a left/right array was added to a central array in an attempt to provide the best of both worlds. The configuration requires numerous loudspeakers and moderate processing. It's safe to say digital technology has had a major impact on the ability to do things now that we were unable to do in the past by way of digital signal processing. Thus, the old L/C/R provided both stereo and a central point source—a pseudo three-channel “flat-surround.” With today's DSP, the L/C/R configuration can provide a host of creative images and effects.
Distributed loudspeaker systems provide very even coverage. The most noted is the eight-inch ceiling loudspeaker system common in retail outlets and restaurants. The configuration found its way into larger venues in the mid 1990s when DSP became available. The digital delay unit provided the ability to delay the signal to the various loudspeakers in order to steer the listener's attention to the stage. Remember, the brain localizes on what it hears first, not necessarily the loudest. Distributing smaller loudspeakers throughout the venue met the coverage need and desired SPL easily. Churches no longer had a large enclosure (inverted house) suspended above the pulpit. The configuration required more loudspeakers and wiring infrastructure but became a popular design approach. Creating an audio image or stereo listening field was not easily accomplished. As such, most distributed systems were mono.
The digital signal processor has changed the playing field. Today, we can use pretty much any of the typical loudspeaker configurations and create numerous creative images and effects. Not only has the DSP brought new possibilities, but the incorporation of amplification and DSP onboard the loudspeaker has simplified the loudspeaker system overall. By using self-powered loudspeakers, with digital audio network connections, a power receptacle and a CAT5e cable at the loudspeaker location, the audio designer and engineer are allowed to control and monitor the loudspeaker system from a laptop computer (and/or a mobile device).
So how is one to determine which loudspeaker configuration is best for a given venue? It can be as simple as 1, 2, 3: (1) do not lose sight of the most fundamental purpose of the loudspeaker system (increase volume at the listener's ear), (2) define the listening planes to determine the type and number of loudspeakers required to cover the plane evenly and deliver the required SPL, and (3) formulate the DSP requirements needed to provide a specific artistic audio image, effect or impact.
Too bad skinning an opossum isn't as easy….
