So as not to hijack other threads where this would be only tangential, I would like to start this new thread for dedicated discussion on the subject. I'll ante-up by just putting on the table that there are two meanings to this word. Used as a verb, "to voice" can either mean the act of adjusting the timbre of the piano itself by adjusting the hammer felts, or it means the technical and artistic phenomenon of adjusting the various sonorities being played on the piano, such that greater degrees of loudness are bestowed on those notes that the player wishes to emphasize. Most commonly, it is done in succession to those notes comprising the "melody" that the player wishes to be most pronounced. Used as a noun, it means the act of having or not having drawn attention by volume to a part deemed to be entitled to it. Having said this, it is exactly because of this performing feature that a piano[forte] (or fortepiano) has its name. In particular, the harpsichord and organ are not capable of doing this except in a binary fashion of sounds limited to an entire manual, and never require the performer to differentiate differing degrees of pressure to the playing of the keys. Herein is where our magnificient instrument transcends all others!
The course of becoming an advanced pianist largely shadows that of learning to contextualize the various sonorities being played and is broken into the technical side of gaining the physical ability to do so with independence of the fingers (e.g., so that any note/finger in a chord can be made to be the prominent tone), and the musical artistry to know how to apply such ability. Herein is where the advanced pianist performs most closely to that of a conductor who is constantly managing the sound of the individual performers.
In the course of developing, the beginning student plays works that are largely two dimensional in that one hand may have a more prominent musical feature (melody) and the other a supportive role (e.g. Alberti bass). Take the famous easy sonata in C major of Mozart. The elementary pianist will play all the notes with equal intensity (like a harpsichordist or organist), but an advanced pianist would play the same work with a singing or prominent melody (RH) and a subdued, under-stated Alberti bass (LH). A conciencious teacher will introduce the matter of voicing early on, in fact the first real "pianistic" achievment of a student is to develop the skill to play one hand louder than the other.
With some advancement, the pupil will meet more 3-dimensional works that require greater technical and musical skill in execution. This is the greatest value of Bach's Sinfonias (3-Part Inventions). Here for the first time (usually) the piano student must regularly struggle to exercise the pianistic skill of managing sounds of different loudness in the same hand. This ability progresses in degree and difficulty with the use of the WTC as well as pretty much all the piano literature. The pianist soon enough will meet with chords in works that should be dealt with by individual notes as to proper voicing. All the while this student is advancing in this skill, she is becoming an artisitc listener (which is one of the chief skills of pianism) and is absorbing the very musicianship so important to being a conductor.
In my return to the piano, I was amazed to find the most difficult voicing requirements that I had ever seen in Rachmaninoff's Prelude in D major, Op 23, No. 4. (see at page 16 of http://conquest.imslp.info/files/imglnk ... theil_.pdf
) Though there are some difficulties along the way, in particular maintaining the single-note melody feature even when the melody note occupies the top of a 3- or 4-note chord (often), the first difficult test arrives at bar 21, where the melody is in the alto (8th and two 16ths) and the soprano continues with triplets above, both in the RH. This is similarly performed in bar 26 last beat. Note here how the melody is first in the low notes of the RH, but across the bar it is in the top note of the RH (remember this). After the "B" section (35-52) the melody returns with fuller instrumentation of an octave and some inner notes to boot. In measure 55 one meets with what is the MOST DIFFICULT voicing I personally have every had to tackle. Beginning on the last beat, the melody to be voiced includes the notes D, C#, D, E with the fingering 5,1,5,5 with the RH doing plenty more simultaneously and the difficulty being compounded by a very fast hand position change.
Can you remember the first time your teacher told you that you had to play one part of something louder than the other? Or can you think of a testy passage that required a lot of practice to get down?