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SYMPATHETIC RESONANCE: pIANO HARMONICS
Next time you have access to an ordinary acoustic piano (upright or grand), try this:
- Lightly press down on Middle C, and also on the E and G immediately above Middle C—so lightly that the hammers do not hit the strings.
- Hold down the three keys. The strings associated with Middle C, E, and G are now undamped and free to vibrate.
- With your left hand, hit the note C below Middle C. Give that key a short, hard, quick, unsustained “bonk.”
The vibrating strings of C below Middle C cause the piano's sound board to vibrate only for the brief duration of the “bonk.” However, the C-below-Middle-C bonk sets the open strings of the three keys you are holding down into sympathetic vibration (sympathetic resonance). This causes the soundboard to vibrate and produce sound waves at the same frequencies as some of the overtones of C below Middle C. So that’s what you hear—a series of faint harmonics of C below Middle C.