With an upright piano, the best results come from using close mic techniques.
One option is to open the top lid and to remove the front panel of the piano. Position 2 microphones from above (remembering to use 3-to-1 spacing), around 2-3 feet away. This should give your sound a direct and full-bodied quality.
A second option is to keep the front panel removed (but close the top lid), and to have a microphone either side of the pianist - one pointing towards the low-end strings, the other towards the high-end strings.
You could even remove the piano's backboard and have two condenser mics pointing towards the piano, at a distance of around a foot. Large-diaphragm models are the best option here, but small-diaphragm mics still work really well.
When it comes to judging the space between your two microphones, remember that the overall goal is to produce a panoramic stereo spread of the piano. If the mics are too far apart, you'll hear a 'hole-in-the-middle' - the left and right channels will sound like two separate signals instead of one 'signal' (the piano) spread across two channels.
But don't put the mics too close together either - you'll start to lose the stereo picture, which is the whole point of using two mics. And remember the 3-to-1 rule to help deal with any possible phase issues. If you do hear the effects of phase cancellation between the two mics (like losing the low-end, or mid-range filtering):
The frequency range of a piano is potentially very wide - the lowest note on a grand starts at 26 Hz, and it's upper harmonics can go way over our own 20 kHz hearing limit. It's one of the main reasons why condenser microphones, both large-diaphragm and small-diaphragm, are much better suited to recording piano than dynamic models (ribbon mics will also sound nice).
If you're in a room that has nice acoustics, using the omnidirectional polar pattern can help you capture a nice low-end from the piano - omni mics generally have a better low-frequency response compared to cardioids, and they also don't suffer from the proximity effect.
But it's often better in smaller rooms and studios to record as much of the direct sound as possible, which favours the use of cardioids. Placing the mics closer to the piano will capture more of this direct sound, and reduce the effects of the off-axis spillage from the room's environment.
You want to avoid putting the mics too close to the piano's strings, though, as the mid-range frequencies may start to overwhelm the lows and the highs. It's all about catching the right balance - putting the mics at a distance of 1-3 feet away from the piano usually does the trick, but you may need to experiment a bit.
Recording piano in a small room can often result in a slightly unnatural sound. A small room will mean plenty of reflections entering the microphones which could result in some phase issues, with standing waves and resonances also potentially causing problems.
But however big or small, it's always best to position the piano somewhere near the middle of the room. Try to avoid playing up against walls or in corners - it can easily affect the balance of frequencies picked up by the mics.
Hanging absorbent materials (like duvets or sleeping bags) on the walls can also help reduce a lot of the reflections, especially in the higher frequencies.
Pianos can produce beautifully rich tones when they're played well, and a well-recorded piano can add another dimension and a human quality to your music.
Just like any other instrument, recording piano is easiest and most effective when a few simple guidelines are followed. A piano is a notoriously difficult instrument to record, so sticking to the basics will help you capture great material to work with when it comes to mixing, processing, or adding effects.
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