When I was young and naïve, I liked to program unaccompanied pieces, because I thought that I didn’t have to pay as much attention to intonation. If there was no other instrument to compare my pitch to, nobody would notice if I was a little (or a lot!!) flat on the low notes or just a tad (or a mile) sharp on the high notes? After all, only those extremely rare people with perfect pitch could tell the difference and even that was about the notes, not the intonation, right? HA! I was so wrong, and if you think this way, too, so are you!
Learning to play in tune isn’t just about learning to play in tune with others, it also about learning to play in tune with yourself. If you are playing without accompaniment, the pitch of the first note you play determines the placement and intonation of every note that comes after. EVERY NOTE.
In the listener’s ear, the intonation is always relative to some reference point. As a piece progresses, the reference might shift a little, but the listener’s brain is constantly measuring and comparing each new pitch to pitches that came before. The proportions of the intervals are dictated by mathematical proportions that occur naturally. Your brain recognizes them, even if you don’t know specifically what they are. Even if no one in your audience has perfect pitch, there is something in each of their brains that says “Hm, the notes seem like they fit, but something is a little off.” If the intervals between the reference pitch and succeeding pitches are too big or too small, you run the risk of sounding ‘off-key’ if it’s really bad, or just not quite right if it is only a little. So, yes, your intonation matters even when you are playing unaccompanied pieces.
To address this, practice your unaccompanied pieces against a drone pitch, usually the tonic pitch of the piece or section you are working on. I advise you to practice this way for two reasons. First, so you can work on keeping a consistent pitch center, not letting your overall pitch wander around. Second, so you can work on learning the sizes of the intervals between the tonic and the other notes in the piece. It’s not enough to know the notes and which keys to press and how to make each note sound pretty, you have to learn the actual sizes of the intervals.
Just like when you sing and you don’t have keys or finger positions to help you find the pitches, you have to learn the size of each interval from one note to the next and from the tonic to each note. (Knowing the pitch tendencies of your instrument and how you play it is very helpful in this endeavor, hint, hint) This is what ear training courses are supposed to teach you, though most forget to teach you why you might want to know how to do this.
I’m starting to ramble now, and maybe rant a little, so I’ll summarize by saying this. Yes, your intonation does matter when you play unaccompanied pieces, so you have to subject them to the same careful intonation practice that you would anything else. Your audiences will thank you for it!