Voicing

Voicing is the process of adjusting the overall tone of a piano to make it sound great.  The felt hammers of the piano tend to harden over time due to repeated impact of the hammers on the strings which can cause the piano to produce harsh and undesirable sounds. 

The goal of voicing is to create uniformity across the piano because the hammers tend to wear unevenly due to their frequency of use.  How forcefully the piano is played is a factor in how often the piano should be voiced.