The client called me because his home theatre had 'magic bass.' His words, not mine. When seated in his recliner, the bass was extraordinary — deep, even, enveloping. When he stood up — which he demonstrated repeatedly with the enthusiasm of a man showing off a card trick — the bass disappeared. Sit down: bass. Stand up: no bass. He had been showing this to dinner guests for months.

The explanation was delightfully simple. His subwoofer was positioned on the floor against the front wall. The listening position, when seated, placed his ears at approximately 1.1 meters above the floor. The direct sound from the subwoofer and the floor-bounce reflection arrived at his ear height with a path length difference that produced constructive interference at his preferred bass frequency range (40-80 Hz).

When he stood up, his ears rose to approximately 1.7 meters. At this height, the path length difference between the direct and reflected sound shifted, and the constructive interference moved to a different frequency range. The 40-80 Hz range that had been boosted was now experiencing partial cancellation. The bass hadn't disappeared — it had moved to a frequency range above what he perceived as 'bass.'

His room had a vertical interference pattern, like an acoustic standing wave in the height dimension, with nodes and antinodes at specific heights. Sit at a pressure maximum: bass. Stand at a null: silence.

The Moral: Speaker-boundary interference depends on listener position. SonaVyx's transfer function measurement at multiple heights reveals these spatial patterns — and subwoofer placement optimization can minimize the variation between sitting and standing. Or just never stand up. That works too.