The vibraphone resembles the marimbaphone and steel marimba, which it superseded. One of the main differences between the vibraphone and other keyboard percussion instruments is that each bar suspends over a resonator tube with a motor-driven butterfly valve at the top. The valves connect together on a common axle, which produces a tremolo or vibrato effect while the motor rotates the axle. The vibraphone also has a sustain pedal similar to a piano. With the pedal up, the bars produce a muted sound. With the pedal down, the bars sustain for several seconds, or until muted with the pedal.