Why Is Piano Voicing Different from Tuning?

A piano can be accurately in tune while a few notes still sound unusually hard and bright and others sound dull. Tuning changes string tension so that notes stand in the intended pitch relationships. Voicing deals with how a hammer turns an impact into sound. Both affect whether the instrument sounds “right”, but they correct different things.

The outside of a piano hammer is compressed wool felt. Repeated striking leaves grooves in the crown and compacts the fibres, changing the contact time and the character of the impact. A technician first makes sure the tuning and action are sound, then listens before reshaping the hammer, using needles to alter the felt's resilience or, in some manufacturing traditions, applying a controlled amount of hardener. Yamaha describes repeated needling to remove unwanted noise and increase flexibility as the main work of voicing. Steinway likewise explains that needling can soften the tone, while hardening treatment can add brightness. The work is judged note by note; the aim is not to make every hammer equally soft.

What a player should listen for is continuity. When a melody crosses registers, does one note suddenly sound as if it belongs to another instrument? Can a soft note speak clearly, and can a loud one retain colour instead of becoming a knock? Good voicing does not decree that every piano must be “warm” or “bright”. It makes a similar touch intention produce a predictable yet varied response across the keyboard. Voicing adjusts the conditions for tone; it does not make tone on the player's behalf.

https://au.yamaha.com/en/musical-instruments/pianos/explore/musical-instrument-guide/piano/manufacturing/manufacturing005.html
https://www.steinway.com/news/features/instrumental-royalty