Dirac Live requires that sound pressure from the room's reflections and the speakers' lines-of-sight are measured with equal gain. A cardioid or bi-directional microphone will measure the sound with different gain depending on sound waves' inclining angles to the microphone. Measuring a surround system with a cardioid microphone pointing at the front speaker will measure lower gains for the speakers behind the microphone, preventing the system calibration from being robust.