A piano’s sound is often referred to as ‘bright’ or ‘mellow’ by a player. In ‘tech speak’ this would be referred to as the piano voice. Piano Voicing, however, is unique to each individual instrument and is not necessarily confined to the hammers alone.
Every piano’s voice will change over time. Each time the hammer strikes a string, the wool fibers are compacted. Over time, the hammer becomes more dense (harder) and the voice of your piano becomes brighter. You may also notice grooves appearing in the tip of the hammer head. This is a normal occurrence, and how the piano is played is just one of the contributing factors as to when this will happen.
Piano voicing is a delicate and complex procedure, including reshaping the hammers to remove grooves caused by striking the strings, and needling the compacted wool fibers to loosen them. While this may seem a simple task, there is a little more to it. The hammer head is set by the manufacturer to strike the string at a precise spot both on the string and the hammer head. When the hammer is reshaped by removing the grooves, the geometry of the hammer head is changed, and unless additional adjustments are made within the action assembly, the hammer will strike the string in a different place. Voicing often requires adjustment of additional components along with the hammers, such as reseating strings and addressing dampers, just to name a few.