There is a lot of confusion as to what we mean when we refer to a 'voice'. Perhaps you are under the impression that the piano as a whole is equivalent to one voice. However, you would be mistaken.
The voice doesn't refer specifically to an instrument so much as it does to part of a piece of music, and if you are learning or performing a piece of music without recognising potential voices then you are not necessarily doing so to the best of your or the piece's capability.
Let's investigate...
What is a Voice?
The voice is an overall part of the music that gives a different texture to the rest of it.
To better understand how it might work, consider the idea that - most of the time - different voices play different durations of notes or different rhythms.
It is important, therefore, to not fall into the trap of assuming that a triad chord is three separate voices. It can be considered the same. Harmony alone does not create the voice. However, if a fourth note is vamping crotchets underneath - for example - a semibreve triad chord, we now have two voices i.e. two different things happening together.
These can happen in the same hand. The minimum number of voices you can have whilst playing hands together is, of course, two! The most basic example of this would be if you were playing one note at a time in each hand.
But if we now assume that our aforementioned example of the semibreve triad chord and crotchets were to happen in the left hand, we now have two voices in the left hand and one voice in the right - three voices.
And what's to stop lingering notes singing out above vamped notes or chords in the right hand in addition? Another voice in the right hand. Two in the right. Now it's a four voice piece!
If you are ever struggling to identify voices, one of the best things to think about is how a piece of music would be orchestrated.
Which notes or group of notes would be performed on, for example, a flute? A violin / violins? Cellos?
How many instruments could pick out notes in the piano arrangement?
Identifying Voices
For my example - and the inspiration for this post as I was playing this earlier this morning - I am going to reference Robert Schumann's spectacular second Romance ('Romanze II' from '3 Romanzen').
Take a look below and see if you can identify how many voices are in the right hand of the first couple of bars ('Rechte Hand' is German for right hand!)
The answer is - two!
However, it's a little bit more complex to assume that it's two purely because it's written over two staves. If you used that technique to get to your answer, well done on getting the correct answer - but unfortunately your reasoning is flawed and just happened to be right this time!
Take a look in the left hand (the third stave, on the bottom).
How many voices can you see here?
The answer, again, is two!
Pay close attention to the stem directions. Yes, the whole thing is kept moving largely as semiquavers if you look at the bottom, but the stem directions going up on certain notes implies that these notes are to 'sing above' The notes that descend arpeggio-like from them.
This exact same principle can be applied to the right hand as well, but it does beg the question - why would the right hand be written across two staves? The notes don't go significantly lower enough to justify that bass clef, after all.
The answer is because here the composer (or, at the very least, transcriber) has picked out our main focal notes for us. Yes, we would be able to do that ourself to an extend because we can identify them as separate voices away from the semiquavers on the top stave, however having them clearly separated allows for the performer - especially in the early stages of reading the music - to clearly pick out which notes ought to be made something of.
If this were a song, you can consider that the notes on that middle stave would be the vocal line.
How to Distinguish Voices
Now we know how to identify the voices, but we need to know how to distinguish them.
Continuing with Schumann's piece above, we are given a dynamic marking of 'p' (piano - i.e. softly) in both hands. However, we as performers - particularly when it comes to music from Schumann's era (romantic) must be wary of taking dynamic markings too literally. The overall dynamic needs to be gentle, therefore, but things need to stand out;
Split the Dynamic
This is the easiest and most natural thing to do. Given that the middle stave, we have established, is our lead line, we want to ensure that we give a touch more velocity to the notes to make them ring out.
However, there are two things that may make this a little tricker that ordinarily would be the case:
Our right hand thumb is our main finger for the job here, and the natural instinct would be for dynamic to flow from left to right. Bearing in mind that the thumb is on the bottom playing the melody, whilst the rest of the fingers actually rise above the note, it does require a little attention to ensure that you play the top notes much, much softer - especially if you struggle to get a good velocity from your thumb. Obviously it can be achieved with practice, but it needs to be recognised early on in the learning process so you don't learn it one way and have to unlearn it!
The more dominant voice in the left hand should be - by the same logic - the top notes. Given that these notes are merely a third below the right hand's main voice, playing at a similar velocity could mean that the top note merges in and creates a not-unpleasant harmony, but one that disguises the melody. Practising getting a softer but still fairly prominent dynamic in the left hand will reap great benefits for a piece like this. However - not as an alternative - but as an extra technique you could adopt to further separate the two, you could try the following technique;
Splay the Notes
Approach with caution! This isn't something you necessarily want to apply to every single group of two notes here, as it may start to get a bit tedious on the ear and may even interrupt the flow of the music at times.
It is exactly as it sounds - create a rapid ascent upwards between the notes so that the ear is tricked into taking note of the last note you play (in this case, our lead line) and tuning into that one!
It's a lovely technique and it works well in a piece like this that is performed quite slowly. However, you should try to focus more on dynamic and use splaying and arpeggiation as carefully plotted colourful techniques to add your own take on a performance, rather than as a means to enhance a melody or a part that should already be there.
Recognise the Note Value
This is one that I have been known to mention quite a lot. It is true - particularly of pieces in the Romantic era - that you can express much with the pedal. However, because the pedal can do so much of the holding on for you, it can lead your fingers to being lazy.
In a piece such as this Schumann Romance, for example, you may be tempted to just play semiquavers entirely throughout and use the pedal to create the sustain.
Minimise the pedal use!
Learn to perform a piece of music as close as its transcription as you can, as when it is written in voices then it will really enhance them. Once you are ready to incorporate pedal, will find that you need less because you are holding on the important notes yourself and any additional sustain you wish to add to your performance can be worked on afterwards.