When it comes to defining the voice of your guitar, amp selection is absolutely crucial. Characteristics like tubes, gain, wattage, channel switching, reverb, tremolo, master volume and speaker construction typically dominate the conversation. But another absolutely essential factor is often overlooked: whether the back of the cabinet containing the speaker or speakers you use is open or closed.
There’s a world of difference between the two, since cabinet construction defines the shape of the sound wave that reaches the ears of those listening. Myriad other elements of cabinet construction are also relevant, including the type of wood used, the size and depth of the cabinet and the thickness of the baffle — which is the piece of wood the speaker is mounted to — and how the baffle is attached to the cabinet. But whether the back of that cabinet is open or closed is the most dramatic and visibly obvious variation.
Most open backed cabinets are actually just partly open, with upper and lower panels covering half to three-quarters of the back. Open backed cabinets allow some of the speaker sound to emerge from the back of the cab and, to a lesser extent, the sides. On stage, this abundance of sound can be quite helpful when monitors are inadequate or altogether absent.
This allows some excellent sonic opportunities in the studio, too. Microphones can be placed at the back of the cabinet, where slightly dirtier and darker tones are typically available, and in an isolation booth the sound tends to swirl and fill the space in a more complete manner that can be captured with multiple microphones. Without a closed back to naturally compress a speaker’s voice, open back cabinets might be considered more organic or accurate representatives of the sound that a guitar produces. High frequencies in particular benefit from this, but open-backed cabs tend to be gentle to the entire spectrum.
Not so with closed back cabinets, which tend to accentuate and harden midrange and bass sounds. They literally project the sound forward, which means no back spill or side leakage from the cabinet. This makes them a bit harder to hear on stage, but is a gift for soundmen who might otherwise have to contend with the wash from open-back cabinets.
Amplifiers with essentially the same circuitry sound radically different when they are propelling open- and closed-back cabinets. And variations in cabinet size create variations in sound. Generally speaking, closed back cabinets with more open space around the speakers create a more bass driven sound. Too much space and the sound can become flubby, however, which is why Jim Marshall’s tightly constructed 4x12 cabinets became the perfect workhorses of the midrange heavy classic rock era.
There are variations on the closed-back and open-back themes. Some cabinets are ported, with sound holes and baffles designed to accentuate certain aspects of the spectrum. And the wood used in building a cabinet is also critical to its tone-producing nature. Generally the pine and cedar used in classic ’50s and ’60s amp construction gave those vintage beauties a more responsive, superbly projected sound than plywood, which became the typical successor material in mass produced amplifiers. Then there’s the way multiple speakers affect the sound of a cabinet, and even the wiring has an impact.
But the essential thing to remember when considering using open-back or closed-back cabinets is that a closed-back cab will yield a punchier, more mid-range structured tone with crisp projection and definition, and open-back cabs will provide a more organic sound with greater ambience. Play through as many types of open- and closed-back cabinets as possible to find out what appeals to your ears.