Sound System Design Essentials ? Part 3
Most audio pros know that loudspeaker data can be used to predict important aspects of a sound system's performance at the drawing board stage of a project. But it's also possible to extend the concept to array design.
Array modeling software can be used to predict the shape of the new pattern as a function of frequency, as well as account for the distance-dependence of the response. The numerous variables and their interdependence make array design impossible without the right software tools.
All arrays will exhibit lobing because of phase interaction between the elements. The perfect array that behaves like a single loudspeaker simply doesn't exist. But some array designs are vastly superior to others and can produce adequately smooth coverage to large audience areas.Pick Your Type
In most line arrays, the loudspeakers are relatively small and designed to be tightly packed and aimed in the same direction, allowing their interaction to produce the desired coverage pattern. Line arrays have constraints on minimum and maximum length that let them do their thing. Many line arrays have succumbed to value engineering that resulted in an array that was too short for its intended application.
With the point array, the idea is just the opposite: the loudspeakers are physically large, and the resulting aggressive pattern control is used to minimize interaction between adjacent devices, even when spaced very close together. Each array element is aimed at a different seating area, and overlap is minimized.
Exploded arrays break up the audience into zones and attempt to place a loudspeaker on each one. They are often positioned along an arc with several box dimensions between each element. Unlike line arrays and point arrays, exploded arrays are less dependent on the characteristics of the individual array elements to work. This makes them an attractive approach when conventional point-source loudspeakers must be used to cover a large room.
So, step one is to determine which array type you need. It is also possible to use a hybrid approach, such as using the line array for low frequencies and the point array for high frequencies.
Low-frequency array design is, perhaps, the trickiest, because at low frequency, most of the array elements are quite non-directional. This means that the array's radiation pattern is not determined by where you point the boxes. Instead, it is determined by the complex phase interactions of the individual elements, and it easily can be the opposite of what is expected and desired.
This presents a very fundamental problem for many full-range, trapezoidal loudspeakers when used in overhead arrays. If they are positioned for proper HF coverage, for instance, side by side, they are in the wrong orientation for proper low-frequency coverage. The “bass alley” heard in many concert systems is an example of what can result from this interaction, as does the low-frequency build-up often experienced on stage from an overhead array.
The ramifications of a bad array design are significant. They include uneven coverage, poor gain before feedback, reduced speech intelligibility, improper imaging and seat-dependent frequency response — in short, most of the problems that people complain about concerning sound systems.
Bad arrays are often overequalized in an attempt to correct them, which can make things worse. Some hope that the answer to these common system problems is more signal processing, but in most cases, DSP is just a Band-Aid thrown on a bigger, underlying problem.
The loudspeaker system is a filter through which all sound heard by the audience must pass. The array design may prove to be the most significant single factor in determining the quality of the sound experienced in the space, other than the room's acoustics. Designing good arrays is a monumental engineering task that requires powerful software, good data, and a good understanding of electro-acoustics.
Pat Brown is president of Synergetic Audio Concepts (Syn-Aud-Con) Inc. and Electro-Acoustic Testing Company (ETC) Inc. Syn-Aud-Con conducts training seminars worldwide in audio and acoustics for those who operate, design and install sound reinforcement systems. ETC performs precision loudspeaker tests for the audio industry. Pat can be reached at email@example.com.