Aside from tin foil, there are other effective ways to prepare a vibraphone (or other keyboard instrument!) One of my personal favorites is to use coins. By adhering a lightweight coin (for Americans, I favor the dime) to the nodal point of a bar with tape, the coin will vibrate quickly when the bar is struck, creating an exciting sizzle-like effect that can bring out some interesting overtones. Placing the coin near the middle of the bar will cause the pitch to be mostly muted, with the predominant sound being an aggressively bouncing coin.
I find this to be most effective on the vibraphone, given that the sustain will create a much greater effect than any dryer counterpart. My personal favorite sound comes with continuously bowing the prepared bar, as seen in the first part of the video below. The second part of the video demonstrates coins taped to a marimba. Given its comparably short resonance, the effect is less effective than it is on vibraphone, though it does give the marimba an interesting sound reminiscent of West African gyil or balafon.
It is of course completely subjective which preparation causes the most exciting effect, though it is worth pointing out that using coins is probably the best option for preparing a sound for a single pitch, and perhaps a little labor intensive when preparing more of the instrument.
Coins can be removed or adhered mid-performance, though it will take a few seconds to take them from point A to point B, and may cause a bit of extraneous noise.
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