Tone Tips: Speaker Cabinets
When you pluck a note on the electric guitar, much happens virtually simultaneously to create the sound that you consider “your tone” that breaking it down can be an almost mind-boggling undertaking.
[originally Dave Hunter | 03.15.2008, Gibson.com] - Throughout this Tone Tips series we have looked at many different components of the guitar itself, considered parts of the amplifier, and even discussed how different speakers can alter your sound. Now it’s time to take it a step further and examine speaker cabinets themselves, these simple-seeming boxes that many guitarists consider just a “carrying case” of sorts for the drivers, but which add resonance and color to every note you play.
In combination with the speakers themselves, the cabinet into which they are mounted contributes enormously to an amplifier’s overall tone. The size and construction of the cabinet influences resonance, low-end content, projection, and even frequency content; closed-back cabinets sound very different from open-back; multi-speaker cabs sound different from single-speaker cabs; solid pine sounds different from plywood, and chipboard sounds different from both, and so on. And while perhaps not enough guitarists give due consideration to how the box itself is helping to shape their tone, you can bet any great amplifier manufacturer worth their salt is putting a lot of thought into matching the right cabinet to their preciously conceived amp design.
You could write a book on speaker cabinet design, but our purposes here will be best served with a quick overview of how the most common types perform sonically. Remember, however, that there are few absolutes in the world of tone, and given the variables involved you will occasionally encounter products that perform against type, depending on the mix-and-match factors of driver type, timber, box shape, construction of back panels and baffle, and so on. But these sketches provide a pretty good guideline in most instances.
Open vs. Closed Back
The seemingly simple decision to build a cab with its back entirely closed off by a sheet of plywood or to leave it partially open is one of the single greatest sound-influencing factors in speaker cabinet construction. Open-backed cabinets accentuate the higher frequencies and present a wider, more “surround-sound” style of sound dispersion. They tend to offer a broad, round, and fairly realistic frequency response, partly because the sound waves escaping from the back of the cab are blending with the sound waves escaping from the front—but in reverse-phase, being produced from the rear of a speaker cone pumping backwards, rather than the front of a cone pumping forwards—and as such are helping to tame any low-end boominess or woofiness the cab might produce otherwise.
Of course, this blending of reverse-phase sound waves also lightens up an open-back cab’s low end a little, so these boxes don’t sound as full, chunky, and gut-thumping as a closed-back cab. Along with fuller lows, closed-back cabs have slightly attenuated highs, and a more direct sound projection, shooting the sound waves out from the front, while sounding pretty subdued from behind. This in itself can be desirable in some situations (if, for example, your drummer doesn’t want to hear too much direct sound from a cab placed in front of him on-stage); likewise, the open-back cab can be a boon in situations where you want to be able to monitor the amp sound from a position other than directly in front of the cab.
Cabs of different sizes will sound very different, even connected to the same amp and with the exact same speakers bolted into them. The most obvious factor of size is that smaller cabs are heard to produce less bass, and larger cabs more. That said, any cabinet needs to provide enough internal airspace to give sound waves produced by the speaker(s) in it—depending upon their size—enough room to develop and, therefore, to present a realistic sonic picture. Too big a cab, however, can produce a bass response that is boomy and overwhelming.
Broadly speaking, plywood and chipboard offer less cabinet resonance than do solid woods, while pine and cedar (the most common solid woods used in guitar cab construction) contribute more of their own resonance to the brew. This resonance is usually described as contributing to “warmth” or “texture,” but it also produces a slight blurring of notes. Where there’s resonance there’s also absorption of sound, so while a solid pine cab might sound full and round, it also usually won’t project quite as much as a well-built cab made from quality plywood, nor will it sound quite as punchy and loud. The top choice for high-end plywood cabs is 11-ply Baltic birch, which offers a tight, muscular performance while still sounding fairly musical, though less resonant than solid wood. What resonance does occur in a non-solid-wood cab made of lesser materials such as chipboard or MDF—and there’s always some resonance—will sometimes sound dead or atonal.
The board at the front of the cab to which the speaker(s) is affixed is called the “baffle.” As with every other factor so far, baffle type and construction can vary widely. A firmly affixed baffle made of relatively thick plywood (baffles are never made of solid wood), say 3/4-inch 11-ply Baltic birch (or sometimes two sheets back to back), makes for extended punch and projection, and like the quality plywood cab, gives you more of the speaker sound and less of the cab itself. A thinner baffle (and they’re found right down to sizes of 3/8-inch on some vintage amps) naturally vibrates more, and therefore produces its own soundwaves that blend in with those of the speaker cone. When such a baffle is also less firmly affixed to the front of the cab, such as with just one bolt or screw in each corner, as with the so-called “floating baffles” used in many tweed amps of the 1950s, they really get moving when the amp is cranked and roaring.
Good, Better, Best?
As with virtually ever other link in the sound chain, the type of cab that’s best for you can only be determined by… well, you. As you can see from these descriptions, certain types are biased toward performing better with certain styles of music—a sturdy, closed-back plywood cab is the classic choice for heavy rock, an open-back cab made from solid pine might be more appropriate to jazz or classic electric blues sounds, and so on—but you need to weigh up your own requirements, likes and dislikes, try as many cabs as you can plug into, and figure out what’s best for you. If anything, this knowledge might also make it a little easier for you to determine why a sound is not working for you, and that’s always a good place to start.
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