The EM Poll

Splitting the Difference

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

           

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

Illustration courtesy www.lolabek.com

Back in the '60s, when I first began playing in bands, speaker crossover systems were almost unknown outside of higher-level professional applications. The first bands I played in used nothing more than a pair of Atlas Sound Reflex Horns connected directly to a little powered P.A. mixer. It was fortunate that the mixer had only enough channels to handle the vocals, because instruments pumped through the Atlas Horns would have sounded really bad.

We took a major step up when our guitarist bought a new Kustom PA 200 with columns that had cone-driver speakers. The Kustom looked very cool with its tuck-and-roll red-sparkle covering; still, something was missing from the sound. Our little P.A. system didn't sound anywhere near as good as my home hi-fi system, but we just accepted the apparent fact that live P.A. systems sounded muffled while home stereos were capable of full-range sonic reproduction.

A few years later, I began experimenting with improving the sound of my P.A. system. I understood enough to know that large drivers are better at handling low frequencies than small drivers are and vice versa, so I built a full-range speaker cabinet with an 18-inch woofer, a 10-inch midrange driver, and a horn for the highest frequencies. Of course, without some way to direct the appropriate frequencies to each driver, it wouldn't work. While attempting to solve the problem, I was introduced to the wonderful world of speaker crossovers.

These days there are many great-sounding speaker systems ranging in size from huge stacks to ultracompact units you can easily carry with one hand. Virtually all of these systems use multiple drivers for different frequency ranges, which means they require some sort of

So whether you have your own P.A. or just use the house systems at the clubs where you perform, a working knowledge of how crossovers function is helpful for understanding what's going on soundwise at your gigs. The logical place to begin this discussion is the place from which sound emanates — the driver (in this article I'll use the word driver to refer to the individual components, such as a woofer or horn, in a speaker system).

Designated Drivers

The very first audio drivers were full-range devices; that is, they reproduced the full bandwidth of whatever you pumped into them. However, it was quickly discovered that you could design a driver with a big diaphragm that would reproduce the bass notes well or with a small diaphragm that would reproduce the high notes well, but it was difficult to make a single driver that could do both well. To better understand the problem, let's first take a close look at driver construction.

Generally speaking, drivers with larger diaphragms handle low frequencies more efficiently, and drivers with smaller diaphragms handle high frequencies more efficiently. Why is that the case? Basically, the lower the frequency, the longer its wavelength, and the more air that must be moved to transfer energy from the driver to the air.

Very high notes have wavelengths only a few inches long, while bass notes can have wavelengths of 30 feet or longer. If all a driver has to do is reproduce a single frequency, then you can tune it to efficiently reproduce just that frequency with no regard to any other. A good visual reference for this concept is a large pipe organ. The length of most pipes in such an organ is half the wavelength of the fundamental frequency they produce, so the longest pipe in a large organ is 28 feet long (half the wavelength of 20 Hz), while the shortest one is an inch or smaller.

Drivers designed to handle low frequencies have diaphragms (in this case, usually cones) that are larger in diameter, thicker, and heavier than those of high-frequency drivers. Those drivers, commonly called woofers for obvious reasons, have to move a lot of air — they work like a big piston pushing in and out with as much as an inch of travel, up to a few hundred times a second.

For a driver to reproduce, say, the metallic sound of a drumstick hitting a cymbal, it must move back and forth approximately 10,000 times per second. A big, heavy driver diaphragm is unable to do that, but a small, lightweight diaphragm can, which is why drivers designed to handle high frequencies are usually only a few inches across and weigh less than an ounce. A high-frequency diaphragm has to move only a tiny fraction of an inch to transform the electrical energy into lots of sound. High-frequency drivers — tweeters in hi-fi systems — are called horns in P.A. systems because they typically have a bell-like flare on the front.

If you run high frequencies into a large woofer, the diaphragm will attempt to move back and forth thousands of times per second, wasting lots of energy that is converted into heat rather than sound. Conversely, putting bass notes into a tweeter will force it to move in and out farther than it's designed to, turning it into so much confetti in milliseconds. If you want to power both types of drivers with a single amplifier channel, you need some sort of routing system that will send the bass frequencies to the woofer and the high frequencies to the horn. Necessity, in this case, gave birth to crossovers.