Room modes: why bass changes with position
At low frequencies a room behaves like a resonant volume. The same subwoofer can therefore sound powerful at one seat and weak a short distance away.
Standing waves
Reflections between room boundaries form axial, tangential and oblique modes. Their frequencies depend on room dimensions. Pressure maxima and minima are spatial, so moving either listener or subwoofer can change the measured response substantially.
Placement before EQ
Equalisation can reduce peaks at a defined seat, but it cannot safely fill a deep spatial null because the cancellation is acoustic. Placement, multiple subwoofers and averaging across seats are often more effective.
Measure the room
A sweep measurement with a calibrated microphone reveals modal decay and seat-to-seat variation. DSP is then used to address what remains after sensible placement and crossover integration.