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Speaker Cabs: The Not-So-Silent Partner To Your Tone

Written by
Dave Hunter
Published on
January 21, 2021 at 12:20:22 PM PST January 21, 2021 at 12:20:22 PM PSTst, January 21, 2021 at 12:20:22 PM PST
When you pluck a note on the electric guitar, so much happens virtually simultaneously to create that sound that when you start to consider breaking down “your tone,” it can become an almost mind-boggling undertaking. Any attempt at analyzing the individual contributors to this elusive thing we call “tone,” often begins with the wood the guitar was made from, the pickups it carries, and other general design parameters of the instrument itself, before moving on to the type and size of amp you’re using, the tubes it carries, whether it’s preamp-stage-driven or more output-stage-driven… and eventually on to the speakers you’re using. For many guitarists, though, the last point of consideration is the box that those speakers go into, yet that speaker cabinet is the final shaper of everything that comes before, adding resonance and color to every note you play.
You could write a book on speaker cabinet design and construction, but our purposes here will be best served with a quick overview of how the most common types perform sonically. Remember, though, there are no absolutes in the world of tone; and, given the variables involved, you will occasionally encounter products that perform against type, depending on the mix’n’match factors of speaker, timber, box shape, construction of back panels and baffle, the amp you’re injecting into it, and so on. But these sketches should provide pretty good guidelines on what to expect from different speaker-cabinet types, 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.
This blending of reverse-phase sound waves also lightens up an open-back cab’s low end a little; as a result, these boxes don’t sound as full, chunky, and gut-thumping as closed-back cabs usually do. Along with fuller lows, closed-back cabs have slightly attenuated highs, and a more directional sound projection; beaming 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 or her 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 positions other than directly in front of the cab.

Cabinet Size

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 generally produce less bass, while larger cabs produce 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.
Cabs that are too big can produce a bass response that is boomy and overwhelming, so there’s no hard-and-fast “bigger is better” rule to apply here. Cabs that are too small, on the other hand, will often sound boxy and lightweight, with underdeveloped lows in particular. For most amp and cab designers, finding the sweet spot involves a lot of trial and error, and the hit-and-miss process of working toward a size that’s somewhere in between.

Wood Types

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, although this might be the perfect solution for a more “hi-fi” sounding cabinet, where you want to hear far more of the speaker than of the cabinet in which it’s mounted.

Baffle Construction

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 only  very, very rarely 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 of a 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 which 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 in the so-called “floating baffles” used in many tweed amps of the 1950s—they really get moving when the amp is cranked and roaring. Moving from the thicker, more rigid, and more firmly affixed baffle to the thinner and looser as described here in the tweed “floating” model take us progressively closer to a cabinet which itself acts more as a resonant instrument in partnership with the guitar and amp… which might be highly desirable for some playing styles, but not at all desirable for others.

Cabinet Design Parameters

In addition to the materials it’s made from, other aspects of any speaker cabinet’s design will influence its sound… although as often as not, the results of this might be semi-random at best. Jim Marshall created the most emulated template for the closed-back 4x12 cab when he crammed and unprecedented four Celestion G12 speakers into one chunky box. The resultant Model 1960 speaker cabinet was devised simply as the most logical container for the number of Celestion G12 speakers required to handle the JTM45’s power (these speakers were rated at only 15W originally). Marshall himself said on several occasions that the design was largely random.
As an interesting side note, and further proof of a speaker cab’s contribution to any amp’s overall sound, consider that a late ’50s tweed Fender Bassman and an early ’60s Marshall JTM45 have almost exactly the same circuit, and Marshall even used the same 5881 output tubes at the start. These amps sound quite different in large part because one is an open-back 4x10" combo and the other uses a closed-back 4x12" cab. (Along the same lines, a 2x12" Bluesbreaker combo version of the JTM45 and a 45-watt tweed Fender Twin can sound astoundingly alike.) The majority of closed-back cabs manufactured today still follow Marshall’s example to some extent, although there are many different approaches to the format.
Many makers have also tried to apply a lot more science to their cabinet design. One such calculated approach involves the design for the legendary Thiele ported cabinet, as often used by Mesa/Boogie and others, as well as other ported designs from companies like Port City and 3rd Power. Most of these use carefully calculated internal dimensions, reflective baffles, and ports to channel the speaker’s back-of-cone sound production into the ideal blend with what’s coming from the front of the cone, and many can sound superb as a result of the effort, generally with a well-balanced tone that’s both firm in the lows and articulate in the highs. For all that effort, though, you’ll find just as many players—if not more—who are entirely happy with the sounds emanating from their semi-randomly-designed speaker cabs.

Good, Better, Best?

Okay, most of us know by know that there’s no “best” as far as tone choices are concerned. As such, the type of cab that’s best for you can only be determined by… well, you. As you can see from these descriptions, however, certain types are predisposed toward performing better with certain styles of music: a sturdy, closed-back plywood cab is the classic choice for heavy rock, for example, while an open-back cab made from solid pine might be more appropriate to jazz or classic electric-blues sounds, and so on.
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.