Why Are There Rotating Discs Inside a Vibraphone’s Resonators?

Beneath each metal bar of a vibraphone is a resonator tube. A shaft crosses the tube openings and carries a row of discs that act like small shutters. An electric motor turns them: when a disc faces an opening, it obstructs the resonator; when it turns edge-on, the resonator speaks more freely. The bar’s naturally decaying sound is therefore amplified and reduced in a regular cycle, producing the instrument’s characteristic pulse.

Strictly speaking, this is mainly tremolo—a fluctuation in loudness. Vibrato is a periodic change of pitch. The name “vibraphone” does not alter that acoustic distinction. A player can switch the motor off for an unmodulated sustain or adjust its speed. At a slow setting, each change between openness and restraint is audible; at a faster setting, the motion becomes a sheen floating over the note.

What matters musically is not the mechanism as a novelty but the breathing within a sustained sound. The pedal decides whether the bars continue ringing; the rotating discs decide how that resonance alternates between brighter and dimmer. For pianists, the instrument offers a useful reminder: expression can continue through resonance, duration and the amplitude envelope after the initial attack.

https://www.britannica.com/art/vibraphone
https://www.vsl.info/en/academy/percussion/vibraphone