Between two parallel walls sound folds back on itself and stands still. The result is a handful of bass notes that boom and a handful that vanish, fixed by the dimensions of the room and nothing else. Put your measurements in and see exactly where they fall.
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Modal spectrum
length axis
width axis
height axis
tangential (2 walls pairs)
oblique (all three)
Axial modes are the strong ones. Where colored lines coincide or leave a wide gap, the bass gets uneven.
Placement where the source and the listener stand, as a point in the room — slider 0 = wall, 1 = opposite wall
Swept response
Both are point positions; a real cabinet or a real head averages over a small volume and fills in the very sharpest nulls a little.
The first three plots sweep each axis on its own. The fourth adds all three axial families together as complex pressures — not as three magnitude curves laid on top of each other — so modes that fall close together reinforce or partly cancel. Its tick marks are colour-coded, so you can see which axis each peak came from. Peaks land on the mode frequencies; the deep nulls appear where either the source or the listener sits on a pressure node for that mode.
Length axis
Width axis
Height axis
How to read the plots
The spectrum
One line per resonance. The tall coloured ones are axial modes, bouncing between a single pair of walls — these are the ones you hear. Grey lines involve two or three wall pairs and are progressively weaker. Above the Schroeder frequency the modes overlap so densely that the room stops behaving modally and starts behaving statistically.
The node maps
The curve is sound pressure along one axis. Pressure is always at maximum against a wall, which is why a corner is the best place for a subwoofer and the worst place to sit. The dashed lines are nulls: at that spot, that frequency is simply not there. The first mode has its null at half the dimension, the second at a quarter and three quarters, and so on in odd quarters of a wavelength.
The swept response
What a microphone would actually record if you swept a sine through the room. Peaks land on the mode frequencies and nulls appear whenever the source or the listener sits on a node — the mode then cannot be driven, or cannot be heard. Slide the listener to the middle of the width and watch every odd width mode collapse. A shorter RT60 damps the peaks; the geometry of the nulls does not move.