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All About Speaker Cabinets

August 21, 2007
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I’m here to tell you, however, that this is an enormous variable in your painstakingly conceived tone.

Try this if you ever get the chance: Keep everything in your rig exactly the same up to the point of your amp’s speaker output, and plug that setup first into an open-back 2x12 cab made from finger-jointed solid pine with a floating baffle. Then, plug into a closed-back 2x12 made from 11-ply Baltic birch with a fixed baffle. Ideally, both cabs should be loaded with exactly the same speakers. Sound different? You bet your sweet fuzzbox they do!

The speaker cabinet is an enormous part of the very last link in the sound chain. It’s the link that ultimately reproduces your tone as sound waves in the air—which are what reaches your listener’s ear, or a studio microphone. Every subtle difference in the way two cabs are made will make them sound a little different. So let’s dig in deep and find out how and why.

Aside, of course, from the speakers you mount in the cabinet, major variables include the size and depth of the cabinet, the type of wood used to build it, the way that wood is fixed together, the type and thickness of the baffle material (the sheet of wood to which the speaker is mounted), and the way this baffle is mounted to the cab. One of the single biggest variables in the sound of any cab, however, involves whether you close off the entire back of the cab, or leave it open. And as with most of these variables, it isn’t a matter of one approach being universally better than the other, but of obtaining the results that best suit your playing style.

To examine the open-back cabinet, let’s consider the classic models of Fender tweed and blackface guitar combos (correctly speaking, most of these are partly-open-back, as upper and lower panels still enclose between a quarter and three-quarters of the back). Such cabinets provide a wide “surround-sound” type of signal projection, which still pumps the greatest volume straight out front, but also disperses sound out the back of the amplifier, and somewhat to the sides, as well. This factor alone can be useful on club stages where your band uses just a vocal P.A. without miking the guitar cabs, because it helps your drummer and other musicians hear what you’re doing.

An open-back cab also provides interesting miking opportunities in the studio, where a mic placed at the back of the cab captures a tone that is slightly gutsier—albeit also a bit more muted—than a front-of-amp mic position. Sonically, the open-back cab presents a broad, transparent soundstage that might be considered more “realistic” than many closed-back cabs—which is to say the sound is somewhat more linear frequency-wise. High frequencies are shimmering and multi-dimensional—and the midrange avoids being overly aggressive—while lows generally pack a little less “oomph” than those produced with a closed-back cab. This is because when you blend the sound coming out the front of the cab (which is created by the speaker pumping forward) with the sound coming out the back of the cab (where the speaker is pumping backwards), you get some frequency cancellation that softens up the low frequencies more than anything else. As a double-whammy, taking the back off a speaker cab (or never giving it one) makes the speakers work harder to produce the low notes in the first place. This points us to another obvious conclusion: If you want to achieve thundering lows, you’ll get there quicker with a closed-back cab.

Aside from emphasizing low end by keeping those out-of-phase, back-of-speaker sound waves inside the box, closed-back speaker cabs have other sonic characteristics to brag about. While they lack some of the transparency and wide sound dispersion of the open-back cab, they emphasize a muscular midrange, and real kick-in-the-gut bass with superb directionality that makes them extremely punchy out front. They’re also relatively quiet in back, which can prevent the rampant sound reflections that can make a house sound operator’s life absolute hell.

Most of you are probably already picturing a Marshall 4x12 cab—the classic of the genre. When Marshall came along in 1962, Fender had already been using closed-back cabs with its “Professional Series” piggyback amps for more than a year. But Jim Marshall created the most emulated template for the breed when he crammed an unprecedented four Celestion G12 speakers into one chunky cabinet. The box that became the model 1960 speaker cabinet was devised simply as the most logical container for the number of Celestion G12 speakers required to handle the power of the Marshall JTM45 amp head. These speakers were rated at just 15 watts originally, and they were prone to flapping out on low notes when hit hard. Putting four speakers together gave the quartet a fighting chance at handling the amp’s power surges, and enclosing the cab’s back created natural damping—a sort of air-pressure suspension—to help limit extreme speaker-cone travel.

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. 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 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. Some makers also seek to achieve a low-end response that approximates that of a closed-back cab by making their open-back cabs deeper than the norm. A cab’s size affects its tone, with larger boxes allowing more room for bass notes to develop. (Make it too big, however, and you risk creating boomy, overbearing lows.) One way to increase lows and maintain relatively compact overall dimensions is to extend the depth of the cabinet. Boutique maker TopHat, for example, extended the depth of its 1x12 Club Royale combo from 9q" to 11" a few years back, and, as a result, these ripping little amps have had a much bigger bottom ever since.

Although we generally think of the “ported” cab as a thing of the hi-fi world—and, sometimes, an ingredient in bass cabinets—some guitar-amp manufacturers also use designs to help tune a cabinet’s response. A ported cab contains some form of an opening (sometimes called a “vent”) that lets a portion of the sound waves produced by the back of the speaker cone exit the cabinet. Rather than the random blending of the open-back cab, the port’s design is calculated to fine-tune the overall sound without severely squelching the cab’s low-end response. This is usually achieved by feeding a determined amount of reverse-phase sound back into the brew at a slight delay to the signals coming from the front of the cab—either by letting sound pump out through a relatively small hole in the otherwise mostly closed back panel, or through a tube or internal reflection baffle that carries it through a hole or gap in the front baffle of the cab. Fender did exactly this with its Showman cabs in the early 1960s, which used elaborately ported double baffles to help maximize the performance of their 12" and 15" JBL speakers. A number of contemporary makers do something similar. Dr Z, for example, uses a sturdy double-baffle system in its punchy Z Best cabinets.

Of course, classic open-back and closed-back cabs often sound different for reasons other than what’s happening around their blind sides. To use the most obvious examples, early Fender and Gibson cabs, and early Marshall and Vox cabs are constructed very differently.

The cabinets in 1950s tweed and 1960s blackface Fender amps were made from glued, finger-jointed solid-wood boards, usually of yellow pine, red cedar, or a similar sturdy softwood. This element contributes a warm, round, slightly soft resonance to the sound of the speaker itself. It’s a factor that can be somewhat unpredictable, too, but when it comes off right, it becomes a big part of an amp’s voice. The ’50s Fenders in particular had thin, “floating” plywood baffles mounted in these cabs—which is to say the baffles were bolted in at their four corners only (with extra bolts center top and center bottom in the big amps), rather than firmly all across all four sides. When such an amp is cranked up, this floating baffle vibrates considerably, and it contributes its own resonance to the sonic brew.

Using quality plywood and more rigid construction techniques—which often include a fully secured baffle—creates a stiffer cab in which the wood itself contributes less resonance. This was the Marshall and Vox standard. Numerous top-notch boutique makers use plywood cabs these days (usually made from high-grade, 11-ply Baltic birch ply or similar) in order to produce consistent and predicable results in a punchy, powerful speaker cabinet. The stiffer box allows the speaker to project its sound a little more immediately—and to retain its own character while doing so—and is often the choice of amp makers who want to favor muscle, articulation, and a quick response more than a compressed and somewhat velvety vintage tone. Decent “firm” cabs have even been constructed from MDF and particle board—although these are typically considered low-budget options.

We don’t have space here to deal with another of the biggest variables in any speaker cabinet—the makes and models of the speakers themselves. But let’s at least consider the ways in which different speaker types and configurations affect tone.

The purest, simplest format is the single-speaker cab, where the lone driver presents no interference with its own tonal character. No two speakers of even the exact same make and size sound exactly the same (these are electromechanical components with multiple moving parts and numerous variables within their own makeup), and putting two or four speakers into the same cab will introduce a degree of phase-cancellation that affects the sound the cab produces as a whole. In fact, the slight out-of-phase issues presented by cabs with two or four identical speakers often work to improve a cab’s overall performance by smoothing out any harshness or woofiness that the same speakers might present individually.

Multi-driver cabs are clearly very popular, so this isn’t always perceived as “a bad thing” in sonic terms—it’s just a factor to be aware of. Some makers even emphasize this affect by mixing and matching entirely different speakers in the same cab. For years, Matchless used slightly modified versions of the Celestion G12M Greenback and G12H-30 in its DC30 cab. Mojave, 65 Amps, and TopHat are all enamored of the G12H-30 and Celestion Alnico Blue pairing in their 2x12 cabs.

When mixing speakers in this way, however, amp designers put in a lot of thought and testing to find pairings that complement each other. For example, Celestion’s famed Alnico Blue has gorgeous, sweets mids and highs, but a rather soft low end, which the G12H-30 fills in perfectly. Going further, two speakers of different sizes sometimes work well together. I recently played through a custom-made cab with one Eminence Legend alnico 10" and one Red Fang alnico 12", and the sound was absolutely ripping. However, the wrong mismatch could create a sonic blur far worse than the sound of even two of the same rather mediocre speakers paired together.

As often as not, of course, amp manufacturers put more than one speaker together simply to help the cabinet handle more power. The speakers in a cabinet divide the amp’s output power between them equally, so two speakers that can only handle 30 watts individually band together to blast the full fury of a cranked 40-watt amp, while four can handle 100 watts.

The ways in which multiple speakers are wired together also affects a range of factors in their performance. Many players are aware that wiring two speakers in parallel yields a total impedance half that of each of the individual speakers, and four in parallel results in a load a quarter that of each speaker (two 8-ohm speakers = 4 ohms; four 8-ohm speakers = 2 ohms). On the other hand, wiring two speakers in series yields a load double that of each individual speaker (two 8-ohm speakers = 16 ohms).

But fewer players are aware of the fact that these different wiring schemes contribute to different-sounding cabs, too. Wired in parallel, speakers in a pair or quartet will dampen and restrain each other somewhat, yielding a slightly tighter response, and a smoother breakup. Multiple speakers wired in series (usually no more than two) run a little looser, giving a slightly more raw, open and edgy sound. Some cabs with four speakers have two series pairs wired together in parallel to yield a total load that is equal to that of each individual speaker, and a blend of smooth and open performance, as well.

There’s no “best approach” here—just a range of enticing variables that, once mastered, can help you further fine-tune your tone. So plug in, and turn on.

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