Cinema 7 had a problem that only existed in Row 12. Every other row: thunderous, satisfying bass during action sequences. Row 12: thin, anemic sound, as if someone had surgically removed the low end. Naturally, Row 12 was the premium reclined seating section that cost three times more.

Complaints piled up. Management checked the seats for broken transducers (there were none). They suspected hearing damage among Row 12 patrons (statistically unlikely for all of them). They even wondered if the acoustic panels behind Row 12 were absorbing bass (they weren't — acoustic foam doesn't absorb 80 Hz).

The culprit was SBIR: Speaker-Boundary Interference Response. The subwoofers were mounted behind the screen, and the rear wall of the cinema was 17.2 meters away. Row 12, at 12.8 meters from the subs, sat exactly where the direct sound and the rear-wall reflection arrived approximately half a wavelength apart at 80 Hz. The reflection cancelled the direct sound — a textbook destructive interference null specific to that distance from the reflecting surface.

SBIR nulls are narrow in space but devastating in magnitude. A few feet forward or backward and the problem disappears. The null frequency depends on the listener's distance from the reflecting boundary.

The Moral: Bass nulls are position-dependent. Measure at every seating zone with SonaVyx Transfer Function to map where cancellations occur — then adjust sub placement, timing, or add a rear sub array to fill the gap.