If you measure the width of the organ you can by using trigonometry decide upon what distance and/or angle (for the true stereophonic field) you'll have to have to the microphones. When you know the total width from the pipe furthest to the left and to the right, you halve that distance (you want to get the microphones in the middle don't you). That number is "a" in the picture to the right.
The letter "b" is the distance to the microphones and the angle "A" is half of the true stereophonic field which you'll find as the +/- angle in the diagrams (The Stereophonic Zoom article)
Trigonometry states the formula: tan A = a / b. This can be substituted into: b = a / tan A. Or into: A = tan-1 (a / b).
So, say that the organ is 8 meter wide and you wish to have the microphones 3 meter away. Then a = 8 / 2 = 4 meters. Your b = 3 meters. Then the angle (+/-) A = tan-1 (4/3) or approximately 53 degrees. Thus, you'll have to get a true stereophonic field of +/- 53 degrees to capture all the pipes in it. Then reference the table (in The Stereophonic Zoom) to get the distance between the microphones and the angle between them. (For cardioid mics approximately 23 cm apart and 75 degrees in between the microphones should be acceptable)
Another example. The ORTF setup calls for cardioid microphones 17 cm apart and 110 degrees in between. This gives a recording angle of 96 degrees in the true stereophonic field, that's equal to +/- 48 degrees. For the same organ of 8 meter width you'll have to be b = 4 / tan 48, or approximately 4.44 meters away to get all the pipes in the stereophonic field.
These measurements and calculations are no absolute truth or unbendable rule, but they can be of help to decide about your microphone placement.
|Recording angle||Approximate distance between microphones||Approximate angle between microphones|
|+/- 30 deg||44 cm||95 deg|
|+/- 40 deg||32 cm||82 deg|
|+/- 50 deg||24 cm||75 deg|
|+/- 60 deg||19 cm||70 deg|
|+/- 70 deg||16 cm||66 deg|
|+/- 80 deg||12.5 cm||62 deg|
|+/- 90 deg||10 cm||60 deg|