Originally Posted by just.sounds
First thing is to get a pretty omnidirectional loudspeaker. A speaker with dull off axis response will dull more than 75% of the impulse response. And that is the same for mic's.
+1 on this, too! We're currently using an eight-speaker (2x4), 32-driver system with bi-directional and omni-directional speakers (all custom builds). This is a great system to get a full-range test signal, but very dangerous if the system is not put together properly. If the crossover points aren't smooth, your IRs will reflect the peaks and dips in your source frequency response. If you do feel that you must acheive true, full-bandwidth testing and don't have access to a well-tuned multi-driver system, you could try to approximate the system's frequency response and then use it as a "correction curve" to be deconvolved from your IRs...