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What would be a good way to actively control the loudness of an acoustic piano note over time after the note has already been struck by the hammer, such as by actively changing the amplitude of a piano string's oscillations over time instead of having it naturally die down?

features wanted:

  • Ideally, the sound quality remains the same, and the only discernable difference between the sound produced by this method and a natural acoustic piano sound is the fact that the amplitude of a natural piano note dies down over time while the amplitude produced by this new method can follow any arbitrary function of time.
  • minimal change or add-on to an actual acoustic piano, minimal cost, complexity, power used, and minimal moving parts for less maintenance.
  • Additionally, since I have heard that the presence of harmonics and exact waveform of a note is different between different musical instruments, I'm thinking it might be more "faithful" to the original sound of a piano to not generate the waveform artificially but amplify the piano string with its own preexisting oscillations after it has been struck by a hammer. This would be like having a speaker and microphone right next to each other so that the microphone picks up the speakers sounds and amplifies the speaker's own preexisting oscillations in a usually very bad feedback loop. Would this be a good idea, and if so, how could it be implemented?

A couple existing ideas:

  • use electromagnetism, perhaps pass AC electricity through the string and immerse it in a magnetic field like in a microphone. Problems/questions: would this produce enough force to make a difference? Can you take advantage of coils of wire instead of a straight wire? Wouldn't the wire heat up and thus change pitch (not wanted)? Or have the wire be magnetized and have changing magnetic field to cause it to oscillate?
  • mechanical wave oscillator, just attach one end of the string to some sort of oscillating mechanism. Problems/questions: what would this look like?
  • just have a computer digital reproduction of a piano sound whose amplitude can be controlled. Problems/questions: kind of a cop-out, as the sound wouldn't be from a "real" piano string and soundboard.
  • somehow change the soundboard itself?
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What you describe has been on the market for over 40 years for electric guitar use; it is called an E-Bow. It contains a sense coil, a drive coil, a battery, and an amplifier circuit inside a case you can hold in your hand. When held near a vibrating metal guitar string, the vibrations are picked up by the sense coil, amplified by the amp, and sent into the drive coil which forces the string's vibrations to increase in amplitude. To modulate the strength of the coupling, the guitarist changes the spacing between the E-Bow and the string.

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You need to put energy into the wire, and probably need to match the frequency of the wire. Adding energy at a different frequency will affect the harmonics of the string/wire and likely wouldn't sound right.

So we need some sort of transducer that vibrates the wire at its own frequency, attached at one end of the sting. You really need to measure the loudness of the string to vary the energy input, otherwise the wire will get louder or softer according to the energy added, compared to how hard the pianist hit the key. An electric guitar pickup would probably work for this. We also need a way to stop energy input when a damper hits the string, probably you could program this into the loudness loop. Do you want undamped strings that vibrate sympathetically to also maintain their sound? If not, some type of low cutoff would be in order.

This blows minimal change or add-on to an actual acoustic piano, minimal cost, complexity, power used, and minimal moving parts for less maintenance out of the water, though. I think this spec is unreasonable. People built organs to do this, trying to rework a piano into it will be tough. I would also expect that this will make a piano sound much less expressive.

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