Often, a young engineer will start to position microphones based on what they see done by others or read in a magazine.
Sometimes they experiment and move the mics to see if the sound improves, but usually once someone ends up with a mic setup they like they stop trying to improve it.
There are certain standard approaches that have been successful, but even these approaches should never be considered “etched in stone”.
Always experiment, especially if it just means putting up a second mic to try a new position without moving the mic you are already happy with.
Once upon a time I had fallen into a typical routine of going with what I was told worked or what I watched the engineers I had assisted use.
I was recording piano with a pair of matching mics in an XY pattern around the hammers. I knew of many approaches (another mic at the far end of the piano and then pan that mic over to the bass side of the stereo spread, pair of PZMs taped to the piano lid, throwing mics under, over, and in the holes, etc).
Sometimes I would use a pair of mics just outside the lid but only when I could get away with more warmth and less percussive clarity.
One day I was working with the talented pianist Warren Wolfe. I was setting up my mics and he said, “You know, nobody ever wants to hear my advice to get the best piano sounds, they always just put mics in the same places.”
I stopped what I was doing, looked him right in the eye and said, “OK, tell me.”
He then said, “All you have to do is to put your head in the piano and listen. Where it sounds good is where you put the microphones.”
So, I moved the mic stands out of the way and listened while he played. Fortunately he played in a way that allowed me to hear how the different sounds from the piano at different ranges and volumes bounced around the piano box…the resonating chamber.
I then put mics where my right and left ears where (very different from the tight XY I usually used) and played with the angles until I felt they were closer to my actual ear positions. When I threw up the faders, I was blown away.
The sound was full, and had a more intimate sound than when I used outside mics (click here for an example). Now I always move my head around inside the piano while the musician played not only wide range material but the actual parts and ranges they would be playing that day.
Sometimes I went back to the XY over the hammers or pair just outside the box, but in general I always found places in the piano I liked.
I now find it especially helpful to listen to all instruments before placing the mic, often getting weird looks from the musician while I walked around them getting closer and farther and moving my head up and down searching for the sweet spots (you would be surprised there can be more than one, each slightly different).