One of the reasons I've been behind on the fiddle practice is I've been using my breaks to tinker with an acoustic experiment. I bet I could get it declared a "20 percenter." Considering we do, well, acoustic design at work. Or rather, the company does. I reload coffee machines.
So what I built was an experimental Cajon. These are drums made in the form of (and historically, from) wooden boxes. Because of the nature of the sounding surface (or tapa), there are a variety of different sounds that one can get from one, including a reasonable approximation of the basic kick-tom-snare setup.
I was cutting from scrap wood, so I used slightly smaller and non-standard dimensions. But what I really wanted to explore was the idea of porting.
See, the box itself functions as a Helmholz Resonator. Not the perfect spherical one, however. Like a guitar body, the acoustics are a complex blend of the air mass inside the volume of the cavity communicating with the outside through the tone hole and modified by the flexible materials making up the body itself. This is even more complex in a cajon as one side of the box is the drum head itself -- which has specific resonance modes itself (multiple modes, in fact, with different combinations of strength of the various nodes excited depending on where the surface is struck).
According to a university acoustics lab experiment I read, even though the 0,0 node of the tapa is around 110 Hz, there exists a second peak of acoustic energy of the cavity. They were studying how this is modified by changing the diameter of the tone hole.
Well, I thought I'd see if I could emphasize low frequencies by using a cabinet design trick; by porting the hole. Adding a tube extension essentially lowers the emphasized frequency. This, at least, can be readily calculated. I didn't bother, as I was making this from available scrap. Instead I simply experimented.
Adding the port instantly cut much of the supporting resonance in the 200-400 hz range. Which is where the strongest most characteristic strike tone had been. It brought in a new peak of strongly boosted frequencies centered at about 50 Hz for the tube length shown above. The wadding (which I added to before closing the box) was designed and effectively did muffle most of the original "box" tone, leaving almost nothing but the "slap" of hand on wood and a deep powerful thump much like that you'd get from a good kick drum.
Unfortunately another part of the experiment was not as successful. It did not seem possible to selectively reduce the damping (and the effect of the porting) to allow richer, more tom-like tones in other strike zones. Nor was I entirely happy with the "slap" of the loose edge I designed for a snare-like effect (too woody, although it did have a good slap. I can put more sizzle in by adding guitar string under it, but I'm afraid this might be audible in the "kick" as well).
If and when I get back to this (I saved a few other pieces of scrap wood) the next experiment is going to be making a bongo-cajon but using partial baffles instead of airtight partitioning. I'll see how the two air volumes communicate and interact.
A little more on-line research and I found some good technical discussion at a Cajon builder's forum. And, yes, the porting trick is well known -- there's a pair intersection between Cajon builders and speaker builders. There's also a style of Cajon playing (and building) that aims for a close approximation of kick-and-snare (and, somehow, hat).
But I find I side with the larger community in that I miss the "wooden" tone of the classical Cajon. That is, the 200-400 Hz range which my ported and damped experiment specifically reduced. However, based on a slightly better understanding of the underlying math (one day I'll get around to reading the rest of that book I have on musical acoustics) I've decided to pre-calculate the dimensions (particularly the critical sound hole dimension) of the planned Bongo Cajon.
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