A friend of mine who is a full-time professional musician has recently bought a new PA and now wants to fit permanent microphones inside his Leslie cabinet. The plan is to power them from the desk. Which microphones would you use?
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Technical Editor Hugh Robjohns replies: Mounting mics inside a Leslie might seem a convenient solution, but there are a number of problems. I've been through this myself and, despite my best efforts, could never achieve a sound as good as that from three or four mics on stands outside the cabinet.
The first major problem is the turbulent air, so very good windshields are essential. The top rotors, in particular, travel very fast and whip the air up enormously! The cabinet louvres are there for a reason, and do a fine job of keeping the disturbed air inside while allowing the sound out. They also help level the sound out and reduce on-axis/off-axis variations as the horns rotate.
Perhaps a more important consideration is the actual sound. Moving mics in close to the horn rotor emphasises its movement considerably, particularly the tremolo aspect, and you will get a very exaggerated, choppy effect. Multiple mics (three or four) can mitigate this effect, but add complexity.
Something else to be wary of is the magnetic field radiated by the motors (especially the slow motor). This really rules out the use of any dynamic mics or capacitor mics with an output transformer (although there are few suitable ones around these days). However, if you fancy giving this method a try, mount the mics on the opposite side, away from the motors, to keep them away from the magnetic fields, but angle them as far as possible to avoid the mic 'hearing' the motor clutches engage when changing speeds.
Position the bass mic so that it is not directly on-axis to the curved reflector part of the drum. If you place it on-axis, you will get a very exaggerated effect again, and the bulk of the sound (basically anything below the 800Hz crossover frequency) will sound unnaturally choppy and modulated. Keeping the mic off-axis reduces this effect, but distance is what is really needed. Let us know if your attempt is successful!