A couple of years ago Experimental Musical Instruments put out a book called Making Marimbas and Other Bar Percussion Instruments. My co-authors and I tried to cover as much of the world of marimba making as we could, including the various systems for mounting percussion bars. But it has since occurred to me that there is one bar-mounting system we neglected. I’ve seen a few makers use it; it’s a very easy one to implement, and it’s quite effective in its way. I say “in its way” because this system produces a distinctive sound, suitable for some applications and not others.
The system consists simply of laying the bars out flat on a surface of soft eggshell foam. That’s the kind of foam rubber that has a pattern of peaks and valleys on one side, reminiscent of an egg carton. In other circumstances it’s used for mattresses and sometimes as a wall covering for sound damping. With a suitably sized rectangle of this foam spread eggshell side up on the floor or on a table top, you can lay the bars out in whatever arrangement you wish. With vigorous playing the bars may dance around a bit on the foam, causing them to eventually get out of position, but the problem turns out to be minor. They don’t move all that much and they’re easily repositioned if they do.
Naturally, the bars are heavily damped in this arrangement – but not hopelessly so, as the crests of the foam hold the bars in a rather yielding, non-rigid sort of way. In fact, the tone of a bar on eggshell foam can be quite appealing. It has relatively little sustain, giving it a rhythmic, percussive quality. The overtones tend to damp out more rapidly than the fundamental, so the overtones aren’t as dominant as they might otherwise be.
The system has most often been used with metal bars, but it’s worth trying with other materials, and in a variety of sizes. It’s particularly practical for assemblages of found objects, since you can try resting just about anything on the foam to see how it sounds. In fact, foam mounting can be effective not only for bars, but for un-bar-like objects as well (think of pot lids, bells, and other oddly shaped objects from the scrap yard or hardware store).
In its typical usage, the arrangement isn’t set up in a permanent way. More often, someone shows up with a rolled up piece of foam and a box of bars or other sound objects, and sets them up on the spot. It only takes a minute or two.