The soundboard in a piano transmits the vibration of the strings in to the air as sound. As such, you can get a very warm and direct sound by miking up the soundboard directly. There are a number of ways of miking a piano soundboard, each with its own pros and cons, and each with a different sound.
This is by far the easiest way to access the soundboard, as on grand pianos it is left completely open underneath apart from the actual framework supporting it. It is an unobtrusive way to mic up a piano and can very very useful if you are short of space. Since mics here are not anywhere near the strings, they will sound very dull on their own but combined with other mic techniques, they will add a very direct and warm tone to the piano sound.
If you do combine mics underneath the piano with mics positioned above, they will need to have their polarity reversed, otherwise they will sound very strange. This is usually done very simply with a button marked Φ on a mixing desk or in an eq plugin.
At the tail end
Near the tail end of most grand pianos there is a space between the bass strings and the side of piano, where the soundboard is exposed just enough to get a mic in. You can make this easier by taking the lid off the piano, but this may not be appropriate if you are using a pair of mics outside the piano. An omni mic would be a good choice to optimise the natural low end available to you here, but if you can’t take the lid off the piano then there may be too much sound reflected off the lid which can be problematic, so a cardioid mic may work better. In this position you will need to get the mic very close to the soundboard to avoid picking up a lot of string noise.
The tone in this position will be brighter than below the piano, but it is harder to control as you have a lot of other factors to content with such as the sound coming directly from the bass strings.
Through the frame
The metal frame that the strings are mounted on (pictured) usually has holes in it to help the sound from the soundboard get through to above the piano, and they also give us a handy access point. This is a great position to experiment with as you can change how near to the hole you put the mic, which hole(s) you use and which mic to choose. Changing how close to the hole you mic is which will affect how much sound you get from the strings, sound reflected from the lid and ambient sound or reverb.
I suggest you try an omni mic to start with to see how all the different elements balance against each other. Swapping the omni for a cardioid will give you a lot less string and ambient sound, and will be much more focused on the soundboard. Figure 8 mics will pick up a lot of direct sound from the soundboard through the hole, with a nice balance of ambient sound but very reduced string noise. Figure 8 mics over the soundholes mix particularly well with other mic techniques as they have the warmth and directness of the soundboard combined with some ambience and space.
[image via GBHPhotoArts]