The sound of a real rotary speaker is complex and is created by the Doppler effect, which is why it’s difficult to create a truly accurate digital model of the hardware. The most famous rotary speaker is the Leslie, which had controls for both the upper rotating horns, which handled the high frequencies, and a separate baffle for the low frequencies. This makes it almost impossible to close mic a Leslie (or any other similar rotary speaker). The key to miking a Leslie is to place the microphones out in the room or hall, since that’s where listeners normally would hear the best effect. A stereo condenser mic, such as the Audio-Technica 825 or the Rode NT4, would be excellent, as they are set up to produce an optimal stereo signal. For those with deeper pockets or who require the best possible sound quality may look at the Royer SF-24 stereo ribbon mic or the Manley Gold Reference stereo mic.
If you’re going to use a matched pair of mics, you can use the classic XY stereo configuration, which places two mics at a 90-degree angle from each other. You might want to experiment with different polar patterns, such as omnidirectional or figure 8 (if your mics have variable polar patterns), particularly if you are stuck miking a rotary speaker in a small room. Otherwise a cardioid pattern typically produces the most tightly focused sound. In general, the larger the room or hall, the tighter the polar pattern needs to be so that the direct sound isn’t lost in a wash of reflected sound. For the trademark overdriven Leslie sound, make sure you are using mics that can handle high SPLs and if necessary, insert a pad. That’s it – have fun!