Controlling Loudspeaker Coverage
Understanding the physics behind sound radiation is the key to putting sound where you want - and keeping it away from where you don't.
Of course, real loudspeakers are large compared to at least the highest frequencies, and their wavefronts never have exactly the correct shape. Because arrays are usually far from perfect in the technical sense, effective arraying is somewhat of an art form. Most experienced practitioners know what deviations from ideal will produce acceptable results and what will be perceived poorly.
Traditionally, the most common objective of arrays has been a horizontal arc. A specific coverage angle can be approximated by arranging a number of practice, the loudspeakers are usually relatively large and rarely exhibit wavefronts with the correct included angle. However, the results can still be very usable. In the few cases where the wavefronts are nearly correct, the results can be excellent.
More recently, the arraying problem has been applied in the vertical direction. Nearly every manufacturer of professional loudspeakers now offers one or more so-called “line array” modules, which are designed to provide a relatively flat vertical wavefront from an individual loudspeaker with which to construct much larger curved wavefronts by using multiple modules. The wavefronts that can be developed with these modules aren't limited to simple arcs. In fact, the most common configuration is a curve that's relatively flat at the top of the array and more tightly curved at the bottom. The upper part of the array addresses more distant listeners, and the beam it produces is correspondingly louder. This can partially, or in some cases, completely, compensate for the longer distance to the listeners.
A form of array that deserves special treatment is the true “line array,” as shown in the photo. A tall array of sources arranged in a straight vertical line looks very much like our prototypical planar source in the vertical plane, but in the horizontal plane it looks like a point source. The result is a radiation pattern that's unidirectional, vertically, and omnidirectional horizontally — a form referred to as “cylindrical radiation.”
In the most common application of a true line array, a relatively short array is aimed at the farthest listener, and the naturally attenuating underside of the radiation pattern covers most of the listeners. While this technique has been used historically to provide intelligible speech in many difficult environments, the evenness of the response outside the main beam of a line array falls short of the quality standard required for most of today's sound reinforcement applications, especially where music reproduction is involved. A true line array can, however, provide an interesting solution when it's possible to deploy an array that's taller than the audience. In that case, all of the listeners fall within the main beam of the array, and the relatively slow decay of SPL that occurs in a cylindrical wave may provide acceptably even SPL distribution.
Beam steered arrays. The technique commonly referred to as “beam steering” (a name that's more appropriate in the fields of radar and sonar) has been well known by acousticians for decades, but it has rarely been employed in audio until the last few years. The advent of inexpensive digital signal processing equipment and the power of the PC have suddenly made beam-steered systems not only practical, but also inexpensive to implement.
Now that you understand a bit of the connection between wavefront and directionality, the way these devices work should hopefully be much less mysterious. To create a particular radiation pattern, we must simply choose signal processing settings that create the wavefront shape known to produce that pattern.
The most powerful and basic technique for varying the radiation pattern is to apply a different delay to each transducer's signal. To steer the beam downward, the device at the top of the array is given the smallest delay possible, and each successive device is given a bit more delay than the device above it. Then the wavefront emanating from this array issues from the top element first; and each element in succession then follows with its contribution. The resulting wavefront tilts slightly downward, so the sound propagates slightly downward, rather than straight out. To create a diverging beam, the elements at the center of the array are assigned the shortest delays, with the longest delays going to the top and bottom elements. The following illustration shows how the sequenced wavefronts from a number of different sources can combine to produce a tilted or curved wavefront.
While delay alone is capable of producing useful beam shapes, techniques such as gain shading, EQ shading, and other more sophisticated signal processing techniques can improve those shapes, and of course, produce more sophisticated directional effects as well.Wrap-up at a glance
We've covered a wide variety of loudspeaker types and behaviors. But hopefully, focusing on the wavefront sizes and shapes they each produce has created an intuitive connection to the way they perform. In fact, with some practice and experience, this technique can enable a practitioner to accurately judge the directional characteristics of an unfamiliar loudspeaker or array just by looking at it. Try it, and amaze your friends!
David Gunness is director of research and development with Eastern Acoustic Works, Whitinsville, MA. He can be reached at David.Gunness@eaw.com.