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Making It Flat: Analyzing Loudspeakers & DSP
How to view and understand the on-axis and off-axis response of a typical single 2-way loudspeaker...
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As sound reproduction has transitioned to the digital realm, audio professionals have a wealth of capabilities that were unthinkable in the analog days. Today, loudspeakers with companion digital processors are increasingly a tool of choice, going well beyond just the higher end of the application spectrum.

Virtually all types and brands of digital loudspeaker processors offer a rich suite of features, often overwhelmingly so. Some have capabilities that are unique, while others seek to capture the most important functions at the lowest possible price point. Nearly all provide a comprehensive set of functions that, if compared in a vacuum to the analog crossovers of yesteryear, would actually seem to be science fiction rather than reality.

The advanced capabilities—now often taken for granted—have unquestionably raised the performance potential of sound systems by at least an order of magnitude. But they also open a Pandora’s Box.

For example, how do we discern the choice of a Bessel curve over that of a Linkwitz-Riley? Why select a 12 dB/octave crossover slope instead of a 48 dB/octave slope? What’s actually going on with asymmetrical crossovers, extensive EQ capabilities, the use of digitally adjustable all-pass filters, incremental driver delay, phase filter controls, and the many other features that have become commonplace in the modern DSP loudspeaker controller?

Product user manuals do a great job of telling us how to access the various features of their products. But do we actually know why we might want to utilize these features? The answers are not simple and come from a deeper understanding of electro-acoustics, loudspeakers, and processing.

This is the first in a series of articles, to be continued regularly here on PSW in a step-by-step manner, to provide those answers. Here, we’ll start by looking at how to view and understand the on-axis and off-axis response of a typical single 2-way loudspeaker.

The Bruel & Kjaer 4007 Series microphone positioned to measure the Apogee AE-5 2-way loudspeaker. (click to enlarge)

Importance Of Instrumentation
While many aspects of sound system calibration can be set by ear, it’s physiologically impossible to set up incremental time delays, all-pass filters, and phase filters relying solely on listening. Yes, you can select recommended crossover points and crossover rates, and even woofer delays from a loudspeaker manufacturer’s list of recommendations, and that might get you pretty far down the road.

But there is no substitution for taking the measurements yourself and making the optimal adjustments accordingly. Manufacturers are as overworked as the rest of us, and while most of them do their best to provide optimal guidelines, they don’t have access to all possible loudspeaker controllers, or in many cases, to optimal measurement instrumentation. For 17 years I was one of these folks. (By the very nature of the business, their suggestions are generic at best.)


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