Splitting the Difference

Mar 14, 2008 1:58 PM, By Mike Sokol

           

A SOUND KNOWLEDGE OF CROSSOVERS IS KEY TO UNDERSTANDING YOUR P.A.

Three, Four, or More

A biamped system splits the audio spectrum somewhere in the middle, usually around 1,600 Hz, for two-way speakers. However, you can add as many crossover points as necessary for really large speaker systems. A system that uses three-way active crossovers is triamped; a system that has four-way active crossovers is quad-amped. Within the heady world of professional sound systems, triamping is extremely common because it can split the speakers into three spectral ranges: 20 to 120 Hz, 120 to 1,600 Hz, and 1,600 to 20,000 Hz. Of course, these are all approximate crossover points; each speaker manufacturer has specific frequency values for each particular speaker type and enclosure.

One of the larger systems I regularly use is a three-way active system with a four-way passive split. The woofers, midrange drivers, and horns are each powered by their own amplifier set, but the high range is passively split at approximately 8 kHz or so between a dome tweeter and a horn driver. If necessary, I can add in some big 18-inch subwoofer cabinets, using the lowpass 100 Hz filters common on many professional amplifiers (from companies such as QSC, Crown, and Mackie). These filters transform an amplifier into a subamp that outputs only the frequencies below 100 Hz. In this case, the system becomes a quad-amped, five-way affair with a passive split.

However, more is not necessarily better, because each crossover point tends to introduce phase errors and makes the system more prone to feedback. That's the reason that sound engineers are invariably interested in the crossover points of an unfamiliar sound system. Any big feedback frequencies will probably appear somewhere around the crossover points.

Over and Out

I hope this simple introduction to the complex and wonderful world of crossover technology has improved your basic understanding of sound-reinforcement concepts. Of course, variations on the technology show up in everything from the simplest two-way home-stereo speakers to multicomponent studio-monitoring systems costing tens of thousands of dollars.

dbx DriveRack

Delay Tactics

Crossover units sometimes include certain features that enable them to become an entire speaker-management system. The more sophisticated products don't simply regulate the crossover frequencies and slope; they also provide limiters, parametric equalizers, and even digital delays, or they allow for the insertion of those devices into the signal path. The need for limiters and EQ should be obvious; however, why it might be desirable to delay one or more drivers may not be as readily apparent.

Basically, in order for a large array of speakers to work together cohesively, some of the drivers must be delayed to allow the sound from a far driver to catch up with the sound of the drivers closest to the audience. Sound travels at approximately 1,100 feet per second, or 1 foot per millisecond, so if sets of speakers are located hundreds or thousands of feet apart, the time delay can be substantial. There is even a slight delay when horn drivers are placed only a foot or so behind the woofer diaphragms, as is often the case. Delaying the woofer by the appropriate amount of time — 1 millisecond for a distance of 1 foot — aligns the entire speaker cabinet so that the sound emerges from it in a single pulse rather than stretching out.

Excellent examples of this variety of high-end crossover include the BSS Omnidrive (an industry standard) and the new dbx DriveRack. Both of those units include all the goodies you could want in addition to storing all the crossover information for dozens of speaker types by name. All you have to do is select the speaker type and press the Recall button, and within a heartbeat you have all of that speaker's crossover points, delays, slopes, equalization, and limiters as specified in the manufacturer's recommendations.

Onstage Hotlinks

www.linkwitzlab.com/crossovers.htm
A short primer on crossovers by Siegfried Linkwitz on the Linkwitz Lab site.

www.scrounge.org/speak/xvrprim.htm
Another short crossover primer by Wayne Larmon on the scrounge.org site.

Mike Sokol is a live-sound and recording engineer, a musician, and a computer integrator with 30 years of experience on both sides of the microphone. He lives in western Maryland with his wife, four boys, and three cats.