Voicing is the process of adjusting the tone of the piano – it is also sometimes called tone-regulating. This can mean minor local adjustments, so that adjacent keys produce the same volume when struck with the same force, or it might mean adjusting the balance of volume (or tone) among different sections of the piano.
It could also refer to changing the tonal characteristics of the entire instrument. For instance it is very common for the tone of a piano to become significantly brighter with use, as the hammers wear and the felt becomes harder. Some sections, like the melodic area of the 5th octave, tend to become especially harsh with heavy use.
Careful voicing encompasses a variety of techniques to manipulate each individual hammer’s weight, shape, striking surface and texture to maximize the player’s control over the tonal and dynamic range of the piano.