The ‘batter head’ is the part of a drum you hit. If the drum has a vibrating head opposite the batter head it is called the ‘resonant head’. Not all drums have resonant heads, conga drums and bongo drums for example do not have a resonant head. Some drums such as marching band bass drums and bata drums, have two batter heads, which also act as resonant heads. The cajon’s batter head is called the tapa. Some cajons have thick, stiff backs that are not designed to vibrate, others have thinner backs which vibrate and create resonate. The back of a cajon which is intended to vibrate and produce resonance could be consider to be a ‘resonant head’. There are a few cajon models that have two batter heads – the ‘back’ is thin plywood, usually a little thicker than the primary tapa, and is designed to be played.
Many commercial cajons have thin plywood backs, some slightly thinner than 5mm. Some thin backs even seem to be impregnated with plastic or flexible epoxy, simulating a stiff drum skin. Some are attached very tightly, almost as if the manufacture found a way to stretch the wood across the back. These cajons have more resonance than cajons with thick backs. To the manufacturers benefit these cajons also weigh less, hence are cheaper to ship in bulk resulting in less carbon burden.
Unlike drum kit drums, the back of the cajon is important structurally. The back provides support to the sides, at least under lateral pressures. A cajon with a membranous drum head on the back would need to have thicker sides with extreme corner bracing to be structurally stable. Or it would need some sort of reinforced edging around the back. Some cajon manufactures have used a typical circular drum kit head system inserted in to a solid plywood back to simulate a resonant head on a cajon, but this is costly and rare.
I’ve built about 40 cajons using polycarbonate for the cajon back. I consider these to be cajons with resonant heads, perhaps the most resonant heads found on any cajon. Polycarbonate is very strong, it is typically found in the face shields of motorcycle helmets. It has excellent lateral strength but when flat sheets are attached along the edges, it readily vibrates in the center. Used as the back of a cajon it provides significant lateral support and significant resonance. Unlike a membranous head, polycarbonate does not need to be stretched to produce resonance. Some of my cajons are made from solid softwood and have slightly shrunk over time. The polycarbonate heads are slightly buckled but continue to produce about the same amount of resonance as they did when the cajon was new.
Polycarbonate is not the same as ‘acrylic’. Acrylics have much less strength and can shatter. I would never use acrylic on a cajon back.
Attaching polycarbonate to the edges of a wooden cajon is an issue, you can not simply glue it. Most glues do not adhere to polycarbonate. I’ve been using two part epoxy and screws. This last summer I tried a few other methods hoping to find a simpler method. I tried a variety of double sided tapes and screws. I didn’t like any of them. The tapes gave inconsistent visual results and unless you use a lot of screws, the tape actually seems to slightly mute the polycarbonate.
The simplest method with acceptable visual results was to simply use a lot of screws to attach the polycarbonate, no tape, no epoxy. But this is tedious, the polycarbonate is tough and drilling 32 holes and counter sinks is time consuming. Plus the results around the edge are not as visually appealing as an epoxy edge.
So I spent part of the summer of 2019 trying to simplify attaching a polycarbonate back to a cajon using 2-part epoxy and screws. I’ve updated a previous blog with the most recent methods. It is more efficient than the previous version and is the result of 5 years working with cajons with polycarbonate backs.