Simon, here's part of an article I wrote for EQ magazine. I hope this is of some help.
Pianos and Mics - No Simple Solutions
Just as with most acoustic stringed instruments, the bulk of the sound is produced by the sounding board to which the strings are attached. In guitars and violins, it's the top of the instrument; in pianos, it's the sounding board. You don't mic the picks, the bows, or the hammers - they produce very little sound.
There are several considerations when placing mics for piano recording. Foremost, will the instrument be recorded by itself, or with other instruments playing at the same time? Those two situations require different mic techniques. Is it a grand piano or an upright piano? Each requires different mic techniques. Finally, where will the recording take place? That may also require different mic techniques.
If the purpose of the recording is accuracy, and you're micing a solo concert grand piano, then you'll need some good, small diaphragm condensor mics, placed some distance from the piano, usually around 6 to 8 feet away. You can use an x-y setup for cardioids, or a wider spaced ORTF setup with omnis or cardioids.
The piano lid is used to direct some of the sound towards the mics. IF the piano is part of a group of instruments, you can get better isolation by micing the underside of the instrument, using a sightly wide spacing with omnis or cardioids. Mics placed inside the top of the instrument can also be used, but it's harder to achieve a good balance or isolation since the piano lid will also reflect sounds from the other instruments into the mics.
Large diaphragm mics can also be used, but the response changes as the sound enters from different angles and the larger mics add coloration (which can sometimes add an unexpected richness to the sound).
Upright pianos should be miked from the back of the instrument, but try to avoid having the soundboard too close to a wall. The distance from the wall will create a standing wave which will interfere with the sound. If the piano has to be near a wall, angle the piano so that it doesn't sit parallel to the wall. Be especially attentive to a ringing sound when micing upright pianos.
This ringing is caused by resonances within the piano, and usually can be solved or reduced by moving the mics around till you find a dead area, free of the ringing. Just as with a concert grand, close micing is not advised, but since an upright piano is usually part of a group, it's not possible to mic from a distance and still have isolation.
To sum it up, first choices for recording a piano would be small omni or cardioid condensor mics, but don't be afraid to try large condensors, ribbons, or dynamic mics (if that's all you have). Mic from a distance if possible. Second choice would be under the piano, and finally, from the top of the piano, but watch out for ringing and reflections from that position.
