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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.

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.

The array prediction process is embraced by some, shunned by others, and amazingly accurate when done within the limitations of data and prediction methods.

We need arrays because many venues cannot be covered adequately with a single loudspeaker. Yet implementing multiple loudspeakers can introduce a lot of problems. Ask any RF engineer what happens when you radiate radio waves into a space from two different antennas operating on the same frequency. His likely answer: “Drop outs. Don't do it.”

The same principles apply to loudspeaker arrays. When multiple antennas are used, either they are located in very close proximity to create a desired radiation pattern (such as the log periodic dipole often used with wireless mics) or they are spread out with minimum overlap to cover a large area (such as cell phone towers).

Loudspeaker arrays are like the first example. This seems easy until you consider that the ideal loudspeaker array must have the same radiation pattern for over 8 octaves, not at a single frequency. This means that the appropriate spacing for one octave band will be incorrect for another, forcing the array designer to vary loudspeaker placements and orientations as a function of frequency.

The tools for successful array design include array modeling software, accurate loudspeaker data, an understanding of the underlying physics, and an open mind. The last item is important, because array design is not intuitive. In fact, this is one place where following intuition alone can lead to disasters. The biggest mistake made in array design is assuming that the sound goes where the loudspeaker is pointed, and that all one must do to form an array is to cluster some loudspeakers and point them at different seats.

In this column in the May issue of Pro AV, I showed the coordinate system for how loudspeaker data is gathered and explained the far-field dependence of both the data and the predictions. Each loudspeaker is treated like a “point source with directivity” in order to cover modeling and SPL calculations. The larger the loudspeaker, the more difficult it is to get into the far-field for accurate measurements.

Arrays become especially problematic, because an array is really just a big loudspeaker made from smaller ones. In fact, arrays can be so large and the far-field so remote that it is not practical or even possible to measure full spherical data to describe their performance. The next best thing is to predict the performance using array modeling software.

Points to Ponder

There are several key factors in modeling arrays, and failure to consider them will result in inaccurate predictions.

First, each array element must be measured properly as an individual device, giving us our “point source with directivity.”

Second, the array elements must be positioned accurately relative to each other in the array. Modeling programs let each point source be placed at a unique XYZ coordinate. In the upper octave bands, even fractions of an inch can affect the predictions and performance. This hypersensitivity to positional accuracy affects the performance of the modeled array as well as that of the physical array. Your array will perform no better than the tolerances of the loudspeaker rigging and its aiming features. For these reasons, the highest octave band that can be predicted with reasonable accuracy is 8kHz.

Last, array predictions must include relative timing information between array elements. This is because the radiation pattern of the array is determined by the relative phase relationships between the individual elements, just like an antenna. When several “point sources” are placed in three-dimensional space, the result is a non-point source with an entirely different radiation pattern that is both frequency and distance-dependent.

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