WebAudio ThereminClick to play, move mouse to change pitch/volume. Pitch is horizontal, volume is vertical.
You can also hover over and press a key.
Behavior is different depending on browser. Try in Chrome.
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This concept can also be done through any MIDI sythesizer that can handle real-time pitch-bends and can set a wide pitch bend range (this setting is specific to each device).
Works better with generated sound than with samples.