A piezo disk driven through a step-up transformer resonates physical objects, turning them into sculptural reverb units

Running a piezo disk as a speaker (driver) rather than a microphone requires high voltage but low current. A small audio output transformer wired backwards steps up amplifier output from ~6V to 200V+, increasing the disk’s vibrational efficiency. The energized disk, clamped to a physical object (metal plate, guitar string, spring, balloon, bowl of Jell-O), makes that object resonate and filter the input sound. A contact mic on the same object picks up the resulting vibrations, recovering a ‘physically filtered’ version of the signal — a form of convolution reverb whose frequency response is the resonant character of the object. This is the principle behind David Tudor’s ‘Rainforest’ installations. The technique creates self-contained feedback when the driver and pickup are on the same object, making it a playable instrument.

Examples

Clamp piezo driver to a coiled spring, contact mic at the other end: spring reverb. Drive a license plate with a piezo driver and pickup with another disk: plate reverb.

Assessment

Explain why the transformer is wired ‘backwards’ in the piezo driver circuit. What is the difference in how physical filtering and digital reverb process a signal?