Pianos are defined by their tone. Piano voicing is ensuring that the tone of the piano is consistent from note to note. Achieving abalanced sound, whether it is bright, medium, or mellow, is the ultimate aim in piano voicing. Unlike tuning, which deals primarily with adjusting the tension of the strings, voicing deals with the piano hammers and their relationship to the strings.
Hammers are made of dense layers of wool felt that are wrapped and fastened to a wooden molding. The density and tension of these felt layers determine the tonal quality of a note when the hammer hits the string. Hard hammers will give a brighter tone, where soft hammers give a mellower tone.
Piano voicing is a process of manipulating the felt layers of the hammers through various means. Hardening hammers can be accomplishedby: 1) filing away softer top layers of felt, 2) using heated hammer 'irons' to drive out moisture and increase density of the felt, or 3) using chemicals on the felt to harden the felt fibers. Softening hammers can be achieved by 1) using various needles on the hammers to release tension in the internal layers of felt, 2) introducing moisture into the felt through steam or water/alcohol mixtures, or 3) reversing chemical hardening procedures using solvents or thinners.
Voicing the hammers is dependent on the piano being in good regulation, with all the hammers hitting the strings evenly. Voicing is also dependent on the piano being in tune. Piano voicing changes as hammer felt changes, through playing, which flattens and grooves the felt, or aging, where the felt loses moisture and becomes hard and brittle.