Martz tells Vulture that microphone readings are taken at multiple locations in a theater and averaged across the audio spectrum, in order to give every seat the best possible experience, but that primary mic sits in that spot, meaning that the system is calibrated very well for that particular spot.
The reason that particular spot works, according to Steve Martz, the director of global technology at THX, is because that’s where the primary microphone is placed when the audio is calibrated in a theater.