Theoretically Practical

Theoretical knowledge is assumed by some to be an alternative to practical experience. These people regard practical experience as the only real understanding of a process. In the sound reinforcement industry, there are practitioners from both backgrounds. However, an understanding of both the theory and practice is not necessarily an either/or situation.

Loudspeaker array design is one example of an area where practical experience should be supported by a deeper theoretical understanding. Many sound contractors have a background in touring sound reinforcement systems. The TST (touring sound tech who focuses entirely on practical experience) will have years of practice putting systems into venues for one-nighters that has yielded a wealth of practical knowledge. However, typically when TST is presented with a project requiring a complex loudspeaker cluster, TST will default to using a touring sound approach. When issues of loudspeaker interaction are brought forward, TST will fall back to the assumption that the "arrayable" full-range loudspeakers will work in any configuration. When discussing the destructive interference that can result, TST will respond with the "it's always like that". TST's practical experience does not extend to the design and installation of a component (horns and boxes) system. These component arrays are often called "junkyard clusters" that require complex rigging to achieve the orientations and component proximity required. This complexity made these arrays disappear from touring inventories over the past 20 years.

TST will have encountered venues with "old horn systems" that lack the modern day processing and performance associated with current sound reinforcement technology. Yet, TST somehow assumes that the placement of the horns into wooden boxes will magically create a "music" loudspeaker. Whether installed as components or in packaged full-range cabinets, given similar signal processing, these loudspeakers have much in common. However, TST will always call for a packaged loudspeaker system. If questioned, the answer is: "Those old horns are only good for speech, it takes a processed, packaged system to do music."

Touring systems are designed to meet the practical needs of one-nighters. The loudspeakers became modular units of full-range and subbass loudspeakers to allow the rig to be installed in a few hours and loaded in a truck within minutes of the end of the concert. These loudspeakers had to be suited to handling by stage crews, repeated trucking, and simple rigging for daily use. The installed system can afford to be more complex to install and adjust as this only happens every 10 or 15 years. The installed systems should ensure that there are no bad seats in the house and cannot excuse the audible results of destructive interference between drivers.

Most clusters of packaged full-range loudspeakers (especially when there is more than one vertical row) will create significant distances between drivers, which then overlap in coverage (especially with direct-radiating woofers) and cause large changes in frequency response across the listening area. The larger touring systems overcome these problems by using complex processing (array shading) and large numbers of drivers that combine to narrow the width of the interference notches in the frequency response. When a small numbers of loudspeakers are used, such as the three to six in a theatre or church cluster, the destructive interference can be profound.

High performance loudspeaker arrays do not always require individually-packaged devices for each frequency band, but careful attention to the resulting polar response of a cluster of loudspeakers will often lead the array design towards using a minimum of devices to achieve uniform coverage. The use of adequately large horns to minimize the overlap of the upper bass and mid-range can also reduce the destructive interference. The devices that share the same passband should be densely packed together. This can be achieved in simple clusters with packaged full-range boxes but is far more difficult to successfully install in complex multi-tiered configurations. When wide coverage angles or near/far throws are required, a "junkyard cluster" must be considered. The advantages of even audience coverage and loudspeaker cost will favour this approach in many installations.

It doesn't matter if you have installed 10 or 10,000 sound systems, unless you have been learning by both listening and studying the theory behind what you hear. Clearly everyone that did touring sound and has now moved into installing sound systems does not take the narrow first-hand experience approach of TST. They know it's never too late to read the works of Olson, Beranek, Davis and Davis, Ahnert and Steffen, Egan and others that have written about the fundamental theories that we must practice every day. No book can provide the necessary listening experience that vital in the evaluation of sound system performance, but the written theory can guide your listening and broaden your design concepts. Whether you toured or not, make your experience more valuable by understanding the principals that influence the audible result.

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