The frame with its bars is mounted on a metal stand (very occasionally on a wooden one) with wheels.
The modern marimba is equipped with chromatically tuned wood bars arranged in two rows, usually on two levels with one about 4 cm higher than the other. The bars are ordered according to size and have holes drilled at their nodal points through which a string is threaded and held taut. The bars are suspended from this string, which rests on pegs mounted on the frame. This prevents the bars from sagging and at the same time ensures that they can vibrate freely. The number of bars depends on the instrument’s compass.
Unlike vibrating strings, halving the length of a bar raises its pitch by two octaves. It is for this reason that the difference in length between the lowest bar and the highest is relatively small.
The bars used on the marimba are made of rosewood and are somewhat softer than xylophone bars. In addition, the bars are thinner than the xylophone’s and wider and longer at the bottom end. Marimba bars can be broken by very hard marimba mallets and especially by xylophone mallets.
The pitch of each bar is determined by its length, thickness and the density of the material; the width has no influence on pitch. The longer, thinner and denser the bar the lower the pitch. The shorter, thicker and less dense the bar, the higher the pitch.
The bars can be tuned by adding or taking away material. When tuning, the following rule applies: if material is filed off the ends of the bar, the pitch of the fundamental note is raised. If, on the other hand, material is carved out of the center of the bar (either from the top or the bottom), thus making it thinner, the pitch of the fundamental note is lowered. By removing material from different parts of the bar it is even possible to tune single partials. If it is necessary to improve the tuning quickly, the fundamental note can be raised by adding lumps of wax, a practice common in Africa.
Modern marimbas are generally tuned to 442 hertz equal temperament. However, marimba makers produce instruments in various tunings, because of the differences in tuning pitch used by orchestras in different parts of the world.
Marimbas are equipped with resonator tubes on the underside, the resonators for the lowest notes being rectangular. Each bar has its own resonator.
Resonators amplify the partial to which they are tuned by means of resonance: according to tradition this can be the 1st, 2nd or 3rd partial. The resonators of modern orchestra marimbas amplify the 1st partial (= fundamental). The plugs that close off the resonators at a certain depth can be moved, allowing precise tuning of the instrument. Resonators also have a mellowing effect on the timbre.
To improve the instrument’s appearance there are often ”false” resonators between the ”black keys”, which are more visible to the audience than the ”white keys”. These extra resonators have no effect on the timbre.