Why Does the Sustain Pedal Change Harmony, Not Just Lengthen Sound?

The sustain pedal is often described as a device that simply makes notes last longer. That is only half the story. When the right pedal is depressed, most of the piano's dampers lift away from the strings. Notes already struck continue to vibrate, but other strings that were not struck directly can also respond when their resonant frequencies are related. The pedal therefore changes not only duration, but which overtones emerge and how the components of a harmony blend.

Acoustical measurements have shown that the sustain pedal can lengthen the decay of partials in the middle register and increase sympathetic vibration across the string system. UNSW's musical-acoustics material offers an intuitive demonstration: depress the sustain pedal on an open piano and sing a well-tuned note, and related strings can begin to respond.

This is why pedalling cannot be reduced to “use it wherever the notes need joining”. With the same chord, pedal depth, the timing of a pedal change, and the amount of the previous harmony left resonating can all alter the colour. Good pedalling is not glue for disconnected notes. It is control over a continuously changing resonant space inside the instrument. The ear has to decide how much harmony should remain before the foot carries out the decision.

https://pubmed.ncbi.nlm.nih.gov/17927438/
https://www.phys.unsw.edu.au/jw/musFAQ.html
https://au.yamaha.com/en/products/contents/musical_instrument_guide/piano/selection/index.html