RE/P Files: Concert Sound Reinforcement Approaches Maturity

An industry-wide move in this direction (driven by a clear demand in the marketplace) is already well under way, signaling a fundamental shift in attitudes toward loudspeaker design. One of the oldest debates in sound reinforcement, after all, turns on precisely this question: Should the loudspeakers behave as accurate linear reproducers, or should they distort the signal in a way that is subjectively “pleasing?”

Until recently, the latter attitude has held sway; for example, most of us have heard the assertion that such and such loudspeaker is “no good for rock and roll” (or some other particular musical style). Indeed, many audio professionals have been powerfully attracted to the notion of specific system non-linearities, preferring to talk about loudspeakers in the same way that a violinist might discuss a musical instrument.

Romantic though it may be, that attitude has some major pitfalls.

Non-linear systems militate against stylistic pluralism, because they tend to make everything sound the same. Being relatively unresponsive to equalization changes, non-linear loudspeaker systems severely limit the mixing engineer’s creative options. Most importantly, however, they’re simply going out of style because of changing demand on the part of artists and audiences alike.

The overwhelming acceptance in the consumer marketplace of highly linear sound reproduction systems for example, Compact Disc players and high quality headphones is a clear signal to the audio profession. Without a doubt, the future of sound reinforcement lies in linear loudspeaker systems whose performance is objectively verifiable with standard audio test procedures.

A cluster of JBL Concert Series cabinets supplied by Stanal Sound for Neil Diamond at the St. Paul Civic Center, MN.

Testing and Equalization

One could easily argue that the need for testing is greater in live sound reinforcement than in any other professional audio application. Particularly in touring concert sound – where systems are continually disassembled, moved and reassembled – the chances for faulty connections, mis-wiring and component failures are multiplied. Often there is very little time for discovering and correcting problems.

Reinforcement professionals also face a bewildering variety of acoustical environments (most of which are anything but ideal), and have long known that their efforts can be radically affected by hall acoustics.

Even if a loudspeaker system measures ruler-flat in free field, because of physical reflections and resonances its frequency and phase, response will differ in an enclosed space. The environment becomes, in reality, a part of the system itself, and has to be regarded as such.

Little wonder, then, that onsite testing and equalization is a long-standing practice in reinforcement. The most prevalent acoustical testing technique in professional audio is third- octave analysis, favored for its speed and cost effectiveness.

For sound system equalization, the technique is normally used in conjunction with multiple-band, fixed-frequency equalizers. Because third-octave analysis is a low resolution technique, it offers a kind of built-in curve -smoother, making it easy to observe general trends in a system’s frequency response.

Low resolution testing can yield misleading results, however. For example, a third-octave analyzer will not accurately display high-Q peaks or dips in the system’s response; these will simply be averaged into the display of the nearest frequency band(s).

As a result, a system that appears quite flat on a third-octave basis may in fact have significant frequency response aberrations that balance out in the averaging process. When third -octave analyzers are used with graphic-type equalizers for system equalization, the process can actually degrade the system’s performance.

In trying to correct a frequency response peak, for instance, one might unknowingly introduce a separate, adjacent dip that averages out the peak in the analyzer display – making the response rougher even though it appears, at low resolution, to be smoother.

This is not to denigrate third -octave analysis, however; the data gathered must simply be interpreted correctly, with full knowledge of the limitations in resolution. Such knowledge will lead to proper application of the technique.

For example, third-octave analyzers are very useful in checking for consistent polarity among loudspeakers in an array, since cancellations are immediately apparent from the display. They can also be used to check addition between drivers through their respective crossover points (if the loudspeaker /crossover system is phase aligned).

Ultimately, hand held third-octave analyzers will largely replace “phase popper” testers in sound reinforcement because they can perform the same functions more elegantly,. accurately and reliably while providing more information.