Among my more pleasurable activities is teaching small groups of people to sight-sing in tune. You might say that tuning, whether vocal or instrumental, is what I do. I am fascinated by all the various aspects of tuning that affect musicians and listeners alike.
I am struck by how different it is to tune voices singing a cappella than to tune instruments. An instrumentalist reading music uses the notation as a kind of navigational map: you learn the instrumental equivalencies of the dots on the lines and spaces, then you master the physical coordination that produces an appropriate sound.
But in singing intonation is a function of vocal resonance and memory almost more than anything else. If you listen to Renaissance vocal counterpoint well sung, as I make it a point to do on a regular basis, you notice that there is almost always a common tone sounding -- that is, a single pitch traded off between the voices, reliable, stabilizing. It is the principle on which singing works.
If you are curious to pursue this further and if you are in commuting distance of midtown New York, I am starting a new group. If curious send me an email.