Stereo microphone technique - the coincident crossed pair
It is possible to record any group of acoustic instruments, or whatever size, using just two microphones in the coincident crossed pair configuration.
In this configuration, one microphone points to the left of the ensemble, the other points to the right. The two microphones are recorded to separate channels.
The microphones should be of figure-of-eight, hypercardioid or cardioid pattern. If the mics are figure-of-eight there should be a 90 degree angle between them. If the mics are cardioid, in theory there should be a 120 degree angle, although in practice this is often narrowed.
When the resulting recording is played back over conventionally situated stereo loudspeakers, the result will be an accurate image that portrays the positions of the instruments in the ensemble very well. Instruments that were on the left will appear on the left, instruments that were on the right will appear on the right. Instruments that were in between will appear as sound images with correct placement between the two loudspeakers. You should be able to close your eyes and hear where the instruments are.
The coincident crossed pair always works and always produces an accurate stereo image. Also, if the two channels are combined into mono, the result will be just as good as if one mono microphone had been used (this is not the case for other stereo microphone configurations).
Although the coincident crossed pair works well, it relies on being positioned precisely. Too close and there will be too much direct sound; too far away and there will be too much reverberant sound. This is not a fault, just a consideration.
If the coincident crossed pair configuration does have a fault, it is that the sound is lacking in 'spaciousness'. Although the image is very precise, it doesn't feel like being in the auditorium. For this, the microphones can be separated by around 10 cm or so, becoming a near-coincident crossed pair. Alternatively the microphones can be spaced further apart and not necessarily crossed. This is called a 'spaced pair', which is similar to the 'spaced omni' technique.
The coincident crossed pair is recommended for recordings of acoustic instruments in groups of any size. The sound in the auditorium must be good in itself and optimum positioning should be sought. The result will be an accurate sound image, with full mono compatibility, but lacking in perceived spaciousness.