The timbre of the piano is determined by the density of the felt on the hammer head.
The technique used to vary the density of the hammer felt is call voicing. A special tool with three needles that prick and penetrate the surface of the hammer is used to ‘voice’ the piano.
This, the final stage, is carried out after all of the regulating and tuning is completed. It is of fundamental importance to achieve the finest sound and requires great sensitivity and patience from the master technician.