What is voicing?
Most people understand that a piano needs tuning regularly to set the pitch of each of the individual strings. Voicing or Tone Regulating is the process of change the tone of the piano to make it more pleasing and more even from note to note. Descriptions of voicing are words like tinny, bright, mellow, dark, muffled, brassy, and sweet. Regulation of the action can have an effect on the tone of the piano, but for the most part voicing involves working with the felt in the hammers. Piano hammers are made of wool felt which is highly compressed on the inside of the hammer and highly stretched on the outside. This makes the hammers compress and bounce well. As the hammer uncompresses it springs away from the string. If the hammer stays on the string a long time as it strikes the string it tends to stop some of the motion it has created in the string. A hammer that bounces away quickly gets the string moving and then gets out of the way.
As hammers wear they compress and get harder which tends to make the tone brighter. As they wear they get flat on the surface that strikes the string. This condition tends to press a square shape into the string as it starts it vibrating. This makes the string create more high harmonics which the ear hears as brightness. By careful filing of the hammer felt, the original shape of the hammer can be restored and the tone improved. Sometimes the grooves in the hammers become so deep that the hammer tends to stick on the string when it hits which dampens the tone.
As hammers become hard from use sometimes it is necessary to soften the felt by driving needles into the felt to break up excessive compression. This is a highly refined art. The effect of needling hammers differs greatly with where the needles are placed. Different areas of the hammer affect the tone on loud and soft blows. Voicing is setting the overall brightness of the piano as well as evening out the tone from note to note.