Why Are Piano Bass Strings Wound with Copper?

The copper wound around a piano’s bass strings is not mainly there to make the low notes louder. Its central purpose is to increase mass per unit length. A string’s pitch depends on its length, tension and mass. With the same material and tension, dropping by an octave would require roughly twice the string length. Continued across the keyboard, that scaling would make the lowest plain-steel strings far too long for an ordinary piano.

Densely wound copper lets makers produce low pitches without making the strings impossibly long or leaving them excessively slack. The steel core carries the tension, the copper adds mass, and the frame and soundboard help turn the vibration into audible sound. There is a trade-off: thicker, stiffer bass strings tend to have partials that depart from exact whole-number multiples, producing inharmonicity. Piano design therefore balances length, mass, tension, stiffness and tone.

In the lowest register, listen for a broad and slightly complex spectrum rather than a single pure vibration. Copper winding solves an engineering conflict between size and pitch; the resulting bass colour comes from the whole instrument working together.

https://newt.phys.unsw.edu.au/music/people/publications/Acoustics%20of%20the%20Harpsichord%20and%20Clavichord.pdf
https://www.stonybrook.edu/laser/_angelapizzuto/report/
https://www.metmuseum.org/art/collection/search/504676