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The Long And Short Of It: Acoustic Wavelengths
Understanding the nature of wavelengths can aid in optimally setting up and operating sound systems...
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Noise Abatement
You may sometimes be called upon to control noise “pollution” with respect to neighboring structures or to adjacent events. These might be the “air walls” for adjoining meeting rooms in a hotel environment. Or perhaps more specifically, the occupants of the staff residences located behind the rear of the outdoor Greek Theatre at the University of California at Berkeley, who aren’t thrilled with high-volume, late-night concerts. In order to help control such problems you’ll benefit by knowing about wavelengths and their effect on radiated directivity.

This is not just conjecture; in 1982 I was asked by Bill Graham to help the Greek Theatre stay in operation. There was no easy answer at that time – prior to the advent of modern line arrays – but of course we did the best we could with the tools that were then available. The outcome was essentially to reduce LF output across the board and more drastically, reduce overall operating levels.

This issue gave birth to the SPL police in the San Francisco Bay Area, and at the end of the day, nobody was pleased. Fortunately, today there are better ways of dealing with that type of problem. Line arrays and cardioid subwoofers can greatly aid in keeping the sound where it is needed, and minimizing it where it is not. 

Array Directivity
When planning the number of modules – and therefore the size of a line array or a conventional array – it will be easier to determine the required size of the array when you think about the lengths of the wavefront. An array must be quite large if the low-frequency energy is to be controlled and directed in the lower segment of the audible sound spectrum. A small array of four or five elements may control upper mids and highs adequately, but if it’s only several feet tall, it’s certainly not going to provide effective LF pattern control.

Various publications expound about line array length versus the frequencies that a line of drivers can control. A wide range of opinions are stated, sometimes varying substantially. Even the nature of the line array wavefront, whether it is cylindrical in nature – or not – is the subject of debate among loudspeaker manufacturers as well as non-partisan authors. It’s very difficult to determine which authoritatively stated opinion, or collection of opinions, should govern your system design choices.

Low-frequency wavefronts make it feasible to fly subs behind line arrays.

In support of practical applications, a good rule of thumb is that array size must equal at least a half-wavelength of the lowest frequency in which you’re seeking pattern control, but that’s just scratching the surface. A half wavelength will just begin to develop some semblance of control. If you’re looking to keep LF energy from bouncing off the rear walls, it’s wise to increase the array length to at least a full wavelength at the absolute minimum, and preferably several times greater.

Soon, however, this becomes impractical in the real world. A 100-foot-tall line array, which would be five multiples of 60 Hz, will presumably provide very effective vertical control, but is unlikely to be achievable in all but the most esoteric conditions.

DSP Support
There are things that level control of line array modules, time delay, and complex DSP frequency shading can do to potentially improve large-scale array performance beyond the obvious aspect of simply flattening the composite response.

Beam steering is one method that may be the answer to keeping array size manageable, while creating directional control that seems to defy the laws of physics. Beam steering is based on delaying some modules in relation to others, thereby increasing or altering the cancellation effect that is the very essence of how the line array principal works to control directivity. Complex DSP control is a field that’s developing rapidly, and is likely to offer continued improvements in performance for the foreseeable future.

It’s not an easy proposition to assemble and measure large-scale line arrays, let alone to attempt the thousands of variations, in an inert acoustical environment, that are needed to determine precisely what the effect of complex DSP intervention can – or cannot – achieve in performance advantages. Fortunately, computer modeling makes it much easier and less expensive to explore differing scenarios, and that is exactly what drives the majority of much present day research and development. 

Getting Results
Arrays of various types have been with us for decades. Some have proven to be very effective, providing cohesive and consistent sound quality to large numbers of people, while others have been a poor attempt at assembling loudspeakers that have little business being used together in any sort of deployment, not even in a disjointed cluster. But progress goes on.

By understanding the fundamentals of sonic energy, which in large part is being able to grasp the nature of wavelengths, you can authentically evaluate marketing claims, make informed decisions when planning and deploying loudspeaker systems, and deliver optimal results to your audience.

This short introduction to acoustic wavelengths is just that: an introduction. To fully understand how the nature of sonic energy affects the wide range of situations that the practicing sound engineer might encounter, I encourage a serious commitment to learning and understanding acoustical principals, and how they relate to real-world applications.

Senior technical editor Ken DeLoria has mixed innumerable shows and tuned hundreds of sound systems with an emphasis on taming difficult acoustical environments, and he’s also the founder and former owner of Apogee Sound, which developed the TEC Award-winning AE-9 loudspeaker.

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