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DRIFTWOOD MARIMBAS by Bart Hopkin _________________________________________________________________ First published in Experimental Musical Instruments Volume VII #1, June 1991. Copyright 1991 Experimental Musical Instruments. _________________________________________________________________

Sometime when you find yourself at an ocean beach, or at some stream or river which has had its way with a lot of downed wood, try making a driftwood marimba. Nothing could be simpler.

To begin, find a place where there is an abundance of beached wood scattered about. Find two relatively long, straight-ish pieces to serve as the cross-supports for the sounding bars. They will be laid down alongside one another, a little out of parallel, and the bars laid across them. Find a couple of sticks to use as beaters. These can be anything reasonably solid that seems comfortable in size and weight and balance (later you can look for other beaters more carefully chosen to bring out the best in the completed marimba).

Then begin auditioning driftwood pieces to serve as sounding bars. An easy way to test a piece of wood for its musical properties is to toss it in the air and strike it near the center in free fall. Some pieces will ring with more resonance than others. Cast off those that don't have a particularly good sound, and select the pieces that work well in terms of clarity of tone, volume and pitch relationships. Some driftwood rots and softens after long exposure to the elements, and other pieces maybe waterlogged. These will probably sound dull. Some become dry and brittle, and produce a correspondingly bright sound. Others may be rich and mellow. Some pieces have checks, visible or concealed, which cause rattles and snaps and buzzes. In spite of these general expectations, many pieces will surprise you, revealing an acoustic personality you would not have guessed at based upon appearance. _________________________________________________________________ [Picture] Driftwood marimba made by the author and son, California north coast, 1989. _________________________________________________________________

When you have plenty of bars, begin assembling the instrument by laying the sounding bars across the two support pieces. To produce their best tone, each bar should rest on the support pieces at the nodes for the bar's fundamental mode of vibration. These are the points that don't move in the desired vibrational mode, but only pivot as the bar flexes. Resting the bars at these points will cause less damping than putting the point of contact at more vibrationally active points. For flat bars of uniform mass, rigidity and cross section size and shape, the nodes of the fundamental actually take the form of two nodal lines crossing the bar about 2/7 of the way in from each end. With natural driftwood bars such uniformity is unlikely, so for our purposes the 2/7 figure serves only as an approximation. You will find that if you lay a sounding bar across the supports in such a way that anything roughly approximating 2/7 of the bar length overhangs at the ends, the bar will show its voice. If it moves far from position you will notice the deterioration in tone. With the two support pieces laid out in their not-parallel arrangement, you can place a set of bars on them in graduated order from longest to shortest, in such a way that each bar lies where the supports are the right distance apart to give it about the right amount of overhang. Try laying a few out and sounding them: adjust the position or that of the supports if it seems to help. Try different combinations of bars for different scales or tumbrel qualities. Look for more or better bars if the need arises. Try different sticks as beaters; the mass and hardness of the beater makes a big difference in timbre and volume. Since the sounding bars are not fastened down, they'll dance around as you play, and inevitably get out of position after a while. Stop then, put them back, and play some more.

Any driftwood marimba you make will have its own musical personality, based its particular set of pitch relationships, tone qualities and spatial layout. It will give rise to its own characteristic music. The sound, especially in its native environment, can be very appealing.