Dave’s Corner: Don’t Ignore The Speaker Cab

Most tone-conscious guitarists are well aware that virtually everything that touches their rig plays a part in the sonic equation. Even among that cadre, however, relatively few put a lot of thought into the significance of the speaker cabinet. And I’m not even talking about the speakers that go into the box — but just the box itself. The size, shape and design of that box, and the materials it is made from, play an enormous part in shaping your sound, so it’s an issue worthy of some consideration.

Plugging the same guitar and amp into two significantly different designs loaded with even the same speaker(s) can be like night and day. Some cab designs might utterly destroy your tone, while others might make a particular amp sound better than you ever imagined it could. Beyond the better-or-worse dichotomy, though, there’s a huge range of very acceptable cab topologies that might be considered very different, but all good, depending on what you want from your sound.

A Boutique Amp-Maker’s View

Before diving into some of the earmark differences between different cab designs, however, consider these thoughts from one of the country’s leading high-end amp makers, Mark Bartel of Tone King. Bartel is so convinced of the role that a cabinet plays in any amplifier’s overall tone that he newly designs the combo or extension cab for every new amp model he releases, usually going through dozens of iterations before landing at a satisfactory result. “I’ve built hundreds and hundreds of test cabinets over the years,” he tells us. “I’m like the Thomas Edison of test cabinets.

“I’ve discovered an enormous amount about how to make a cabinet sound a certain way,” he adds. “The two important things are, one, you have to know what you want before you start. You can’t just start screwing around and say, ‘that sounds pretty good, let’s try this…’ Because that’ll lead you down a wormhole and you’ll never come to a design. And number two, your ears are going to play tricks on you. The only way to make progress with speakers and cabinets is to compare to a known reference. And to compare in a way that you’re switching back and forth like that [clicks fingers]. Ten seconds’ delay, and you’re going to lose a lot of information.”

Cab Comparisons

So let’s look at a handful of different types of speaker cabs by way of considering several of the fundamental variables in design and construction. Even when loaded with the exact same speaker(s), each of these different types of cabs will result in a distinct variation in your sonic signature.

Birch Ply vs Solid Pine

Birch plywood is more rigid than solid softwoods like pine, the most common such choice, or cedar, which is used occasionally. As a result, cabs made with this material tend to have a firm, punchy response with good clarity and good projection. Note that cabs are also made from other types of engineered wood: chipboard, MDF and inferior ply types. Birch Ply, however, and particularly void-free Baltic birch ply, is the elite of ply-built cab woods. In general, this tends to be a more “modern” sound, but is also part of the classic “big British” sound, since the most popular Marshall cabs have long been made with ply.

Properly seasoned pine, on the other hand, is a soft, relatively light wood that will absorb more of a speaker’s acoustic energy than will a rigid-ply cabinet, translating that into a different form of vibrational energy within the cab itself. As a result the speaker and cabinet tend to work more “in harmony” in a solid-pine cab, which might be harmonious when it works well -- with a reedy, woody resonance that enhances the sound of the amp and speaker -- or inharmonious when it doesn’t, with a jarring dissonance and cab resonance at a pitch that clashes with the amp and speaker’s tone or frequency range. Broadly speaking, solid-pine cabs offer a more “vintage” sound, since Fender amps of the ’50s and ’60s used this construction. The tweed amps of the ’50s also had a minimally supported “floating” baffle (the front panel to which the speaker is mounted) that contributed further to their “singing” quality.

Open or Closed Back

The differences between open and closed-back cabinets are not entirely black and white, because there’s an infinite variety of configurations in between. In general, though, what we call “an open-back speaker cab” has panels covering about half to two-thirds of its back opening, thereby allowing some of the sound from the back of the speaker, and that resonating around inside the cab, to escape. A closed-back cab, well, doesn’t.

To evaluate the difference between the two types — say, in identically constructed cabs, one with the back entirely closed off, the other with panels across the top and bottom only and the middle third open — you need to consider the fact that a moving speaker cone produces sound from its back as well as its front, but that the sound waves emanating from the back side are reverse-phase of those emanating from the front. When that back-of-speaker/inside-of-cab sound bounces out front to be blended with the front-of-speaker/cab sound it typically adds some sparkle and dimension to the overall tone. Also, though, because reverse-phase sound waves are hitting the listener’s ear at different times in partial combination with in-phase sound waves, the contributions tends to cancel out some frequencies and enhance others.

The result is usually heard as a warmer, more low-end-heavy tone from the closed-back cab and a more broad-spectrum tone from its open-back sibling. Closed-back cabs are also often known for an aggressive midrange punch. In addition, closed-back cabs are usually more directional, while open-back cabs give the impression of a more “surround-sound” presentation.

Ported Cabs

Many players find a “best of both worlds” solution in some of the many ported speaker cabs available. “Porting” involves creating a sound hole, or port, in the front (or occasionally sides) of a cab to project some of the rear-firing sound out front to the listener. The objective here is to carefully engineer the port design to blend a little of everything the speaker produces in a careful and calculated way — rather than the semi-random blending of most standard open-back cabs — for a result that positively enhances your overall tone.

Most carefully designed and well-constructed ported cabs thus blend the characteristics of good open- and closed-back cabs. They tend to offer full lows, good warmth and chunky mids, and a healthy dose of open-back-style shimmer and dimensionality.

Size, Depth and Shape

If you mix and match the factors already discussed above, clearly there are already many variables involved in cab design, and therefore in your choice of the best cab for you — and we haven’t even touched on the effects of multiple speakers vs. single speaker, mixed types in multi-speaker cabs vs. multiples of the same type, and so on.

Do be aware, though, that in addition to all of the above, the size of any speaker cabinet also plays a big part in its sound. As a rule of thumb, the larger the cab, the fuller the low end. Go too big, though, and “full” can become “boomy,” at which point lows will overwhelm mids and highs, making a cab sound lacking in those elements. Small cabs, conversely, can be lacking in lows, but might also sound “boxy,” that is, a little dull and constipated. They can sound as if they don’t give the speaker quite enough room to breath. Many makers using compact cab designs help to make up for this by adding extra depth to smaller cabs, which does generally help to bolster the low end and allow small cabs to sound fuller in general.

And, if you think all this means that size has no impact on closed-back cabs — given that you don’t hear any sound waves coming from the rear of the speaker in such designs, or very little, at least, think again. The air inside the space of a closed-back cab provides a damping factor, a cushion of sorts, which influences the movement of the speaker cone, so the cab’s size and, more to the point, its internal dimensions still play a part in its tone, whatever its configuration.

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