This aspect of musical instrument design isn’t always talked about, but I encourage instrument makers to take it into consideration, because it’s often instructive and illuminating. For many string instruments and idiophones, it can be useful to think about direction of vibration. In the case of a string instrument, you can ask: in what direction does the string primarily oscillate – toward and away from the soundboard, or sideways relative to the board, or somewhere in between those two? Does the orientation of the vibration change over time? Will it be different depending on how the string is plucked or bowed? And once you have a sense of the direction you can ask: how is the string vibration transmitted to the soundboard — is the direction of the vibration one that will efficiently drive the board?
You might notice that if you pluck a string so that it vibrates in the direction toward and away from the soundboard, it tends to have good volume. That’s because that direction of vibration is good for activating the soundboard efficiently. At the same time, it may also have a bit less sustain than a sideways vibration would, since that same efficiency means it delivers its energy to the sound board relatively rapidly. I have noticed that strings plucked in this direction often have a nice, thumpy attack and a saturated sound, seemingly characterized by a rich fundamental. You can hear this in string bass played pizzicato, when the bass player’s technique entails a heavy toward-the-board pluck. I once made a fretted string bass with the fingerboard and bridge deliberately set at a rotated angle relative to the soundboard in a way that made it easy and natural to pluck in the toward-the-board direction. It was crudely made, not a well crafted instrument, yet had a remarkably full and rhythmic tone, and I credit this in some measure to the plucking direction. I called it Cubist Bass because of the peculiar visual effect of the out-of-kilter fingerboard, as if one were looking at a fingerboard instrument from two perspectives at once. I still have the thing around and play it occasionally; have used it often in recordings; hope to build a better version of it someday.
Pianos are the number-one example of an instrument activated in this direction — the hammer drives the string directly toward the soundboard. That’s one reason (though not the only one) why the piano is the loudest of acoustic string instruments.
As a contrast to the above-mentioned instruments, consider the bowed string instruments. At first glance, the situation here might seem unpromising: the bow consistently activates the strings in a direction almost parallel to the soundboard. This would seem to be just about the worst direction for driving a soundboard effectively. For precisely this reason, the members of the violin family use tall bridges which employ a special pivoting mechanism. One foot of the bridge rests directly over a sound post positioned inside the violin body, braced against the back of the instrument, which more or less immobilized the bridge on that side. The other bridge foot has no such immobilizer. When the string is bowed, the sideways motion is translated into a perpendicular motion against the soundboard in the other bridge foot. This pivoting action activates the soundboard in the ideal direction.
Let’s look again at plucked string instruments like guitar. Here the situation is a bit amorphous, because guitar players, especially finger pickers, pluck in all different directions. Acoustic guitars for the most part are fairly quiet instruments, and that’s partly because it’s difficult to optimize the plucking direction. As a practical matter, there may be an issue with the string buzzing against the fingerboard if you try to pluck forcefully in the direction toward or away from the soundboard. But if you’re familiar with classical guitar technique, you may have noticed that the stroke called the rest stroke produces a richer tone than other plucks. This, once again, is partly because the rest stroke’s plucking direction is more toward-the-board than other strokes.
Let’s jump to a very different application of the direction-of-vibration question. In tubular chimes such as most wind chimes, the direction of vibration is primarily determined by the direction the chime is struck from. Think of a chime hanging vertically, seen from directly above, and imagine how it could be struck from the north, south, east or west. If the chime is struck on the south side, then it will vibrate primarily in a north-south direction. If struck from the east, it will vibrate primarily east-west. If the chime is perfectly cylindrical, then direction of the impulse won’t matter; the chime will produce the same tone regardless. But if the chime is not cylindrical – if, say it’s slightly oval in cross-section — then the direction does matter because the chime will be more rigid for one direction of vibration than another. It will produce a slightly higher tone when vibrating in the more rigid direction; or a lower tone for the less rigid direction. Much of the time it will vibrate in an in-between direction or a little in both directions, and produce a dual tone. If the asymmetry is slight and the two pitches quite close, you’ll then get a beating effect (a steady rise and fall in volume). If the asymmetry is a bit more pronounced you may get a rough or dissonant tone; if quite extreme you may hear two distinct pitches in harmony. You can play with this effect deliberately to get some interesting results – chimes with their own built-in tremolo effects, or with an odd, slightly disconcerting rough tone and slightly conflicted pitch-sense. Be sure to choose your striking direction deliberately when you play such an instrument; it makes a difference in the resulting tone. You can see and hear a chime instrument made with these effects in mind here.
These direction-of-vibration effects come into play any time you have a vibrating body in which rigidity is a major factor in determining pitch. In addition to tubular chimes this includes marimba bars. But marimba bars are usually rather flat and wide, and they are almost always struck in the direction perpendicular to the wide dimension – that is, struck from above with the bar positioned flat in the typical marimba layout. Thus the frequency for that direction of vibration dominates the sound and any effect from side-to-side vibration is negligible. But what happens if you do strike a marimba bar from the side? This is hard to do in the typical bar layout because you don’t have strikable access to the sides of the bar. I’ve played around with unmounted bars a bit though, so I can answer the question. As you’d expect, you get a much higher note, due to the much higher rigidity of the bar across this wider dimension. In fact, the bar may scarcely want to sound in that direction, and may require a harder beater to bring the sound out clearly. Just for fun recently I made a wooden bar with the width carefully adjusted relative to the thickness to make the sounding pitch for sideways vibration one octave above the sounding pitch for the usual up-and-down direction of vibration. It worked quite nicely – a marimba bar with, in effect, a prominent overtone an octave above the fundamental. The blend came out especially well when the bar was struck at a 45 degree angle along the edge (which I flattened a bit for the purpose). Someday when I find the time I’ll make an entire instrument out of such bars. When I do I’ll be sure to mount the bars in such a way that striking at that 45 is natural and easy.
Perhaps you can now imagine similar effects at play in a lamellaphone tine. (Lamellaphone is the generic term for kalimba, mbira, sansa and related instruments in which the player plucks the free ends of rigidly mounted tongues or tines arrayed in a row.) The tines on such instruments are usually flat and wider than they are thick, like marimba bars, and like marimba bars they’re usually activated in the direction that brings out an up-and-down direction of vibration. As a result, sideways vibration doesn’t come into play too much, and any frequencies associated with sideways movement have only negligible effect in the tone … or, at least, that’s the intended result. In practice, sideways modes do sometimes come into play with lamellaphones, introducing extraneous pitches which may be confusing or undesirable. They’re most pronounced when the portion of the tine near the bridge, where it flexes the most, is not much wider than it is thick.
Normally this effect is a problem – the source (often unrecognized) of unwanted frequencies adding confusion to the overall sound. But as with marimbas, you could have some fun with it if you wanted to do a bit of experimenting. For instance, you can start with tines which are perfectly round in cross-section. These tines should produce a single consistent pitch regardless of direction of vibration. (Caveat: if the mounting is noticeably more rigid for one direction of vibration than another, there may still be differential directional effects even with round tines.) You can take this theoretically ideal situation – cylindrical tine with one consistent pitch – and try offsetting it just a bit. Do this by thinning the tine on one side near the base where it’s mounted so that it’s slightly less rigid for one direction of vibration than another. See if you can get some interesting beating tones, rough or dissonant tones, or dual-tone harmonies.
There’s plenty more to be said about this question on orientation of the oscillation, but I’ll stop there. It’s just one more thing to think about if you’re designing musical instruments and wishing to understand them.