Multiple Strings
All but the lowest notes of a piano have multiple strings tuned to the same frequency. This allows the piano to have a loud attack with a fast decay but a long sustain in the ADSR system.
The three strings create a coupled oscillator with three normal modes (with two polarizations each). Since the strings are only weakly coupled, the normal modes have imperceptibly different frequencies. But they transfer their vibrational energy to the sounding board at significantly different rates.
The normal mode in which the three strings oscillate together is most efficient at transferring energy since all three strings pull in the same direction at the same time. It sounds loud, but decays quickly. This normal mode is responsible for the rapid staccato "Attack" part of the note.
In the other two normal modes the strings do not all pull together, e.g., one will pull up while the other two pull down. There is slow transfer of energy to the sounding board, generating a soft but near-constant "Sustain".
Read more about this topic: Piano Acoustics
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