The frame with the chromatically tuned steel bars is mounted on a metal stand, which in turn rests on small wheels so it can be moved easily. The steel bars are arranged in two rows in the same way as the keys of a piano. Unlike the xylophone and the marimba the bars are on one level. The bars – steel, aluminum alloy – have holes drilled horizontally at their nodal points (near the end) through which a string is threaded on which the bars are suspended. Each bar is separated from its neighbor by small isolating plates, which stabilize the bars and allow them to vibrate freely. If these plates are too close to the bars, a buzzing sound results.
Resonator tubes and vibrator
1. Resonator tube
2. Vibrator disc
4. Sound bar
Under every bar and at ninety degrees to it there is a corresponding resonator tube which amplifies the fundamental tone. On the upper edge of the resonator the vibrator is fixed; this is a disk which is connected to a motorized spindle. When the motor is switched on the spindle periodically moves to and fro while the disks covering the resonators open and close, on all resonators simultaneously. The air column inside the resonators moves, and an alternate increase and decrease in the pitch of the note is created, the well known vibrato effect. The rotation speed of the motor is adjustable and ranges from about 0–12 rotations per second.
At the bottom the resonators are closed; the length of the air column inside corresponds exactly to the fundamental tone of the bar above. The same acoustic principles apply as to stopped organ pipes (i.e. tubes closed at one end). The fundamental note is determined by the length of the air column inside the tube.
The wavelength is the speed of sound (= 340 m/sec) divided by the frequency. The greatest possible wavelength (= fundamental tone) of a perfect tube open at both ends is half the length of the tube; in a tube like this there is therefore room for half a wave. The greatest possible wavelength of a tube that is closed at one end and open at the other (as a stopped organ pipe) is only a quarter of the tube’s length. So in a tube like that there is room for only a quarter of a wave. An example? How long must the resonator for the note C4 be? (Frequency = 260 hertz (= 260 vibrations/sec).
340 m : 260 = 1.30 m
130 cm : 4 = 32.5 cm
The resonator for C4 is 32.5 cm long. In practice, however, the tube must be shortened by the length of the “end correction”, which is 5:3 of the tube diameter. For every octave upward the tube length is halved.
However, to make the instrument look more attractive the resonators are often arranged in a semicircular curve. The two end resonators are the same length, and the tubes get shorter from both ends toward the middle, forming a symmetrical pattern. The resonators of the higher bars, on the left from the audience’s point of view, should, of course, be the shortest. But the tubes are closed inside at the point necessary for the pitch of the corresponding bar. In other words, the instrument’s appearance does not reflect its acoustic properties.
- Construction of vibraphones
- Acoustic reality: actual length of the resonance tubes
The vibraphone owes the great resonance of its notes to the damper pedal. This pedal operates a bar of felt, which is removed from the bars when the pedal is depressed. When the pedal is let go, the felt strip presses against the metal bars, damping them. So the pedal works the same way as on the piano: to achieve resonance it must be depressed.
Generally, modern vibraphones are tuned to 442 hertz equal temperament. However, vibraphone makers produce instruments in various tunings, because these are required by orchestras in different parts of the world.