Resonators
Part of the key to the marimba's rich sound is its resonators. These are metal tubes (usually aluminium) that hang below each bar. The length varies according to the frequency that the bar produces. Vibrations from the bars resonate as they pass through the tubes, which amplify the tone in a manner very similar to the way in which the body of a guitar or cello would. In instruments exceeding 4½ octaves, the length of tubing required for the bass notes exceeds the height of the instrument. Some manufacturers, such as DeMorrow and Malletech, compensate for this by bending the ends of the tubes. This involves soldering smaller straight sections of tubes to form "curved" tubes. Both DeMorrow and Malletech use brass rather than aluminium. Others, such as Adams and Yamaha, expand the tubes into large box-shaped bottoms, resulting in the necessary amount of resonating space without having to extend the tubes. This result is achieved by the custom manufacturer Marimba One by widening the resonators into an oval shape, with the lowest ones reaching nearly a foot in width, and doubling the tube up inside the lowest resonators.
Resonator tuning involves adjusting "stops" in the tubes themselves to compensate for temperature and humidity conditions in the room where the instrument is stored. Some companies offer adjustment in the upper octaves only. Others do not have any adjustable stops. Still some companies (Malletech and DeMorrow) offer full range adjustable stops.
On many marimbas, decorative resonators are added to fill the gaps in the accidental resonator bank. In addition to this, the resonator lengths are sometimes altered to form a decorative arch, such as in the Musser M-250. This does not affect the resonant properties, because the end plugs in the resonators are still placed at their respective lengths.
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