Small green room 3 x 4 x 2.5 m: only a handful of modes under 100 Hz, big gaps, bass booms at 40 Hz and vanishes at 55 Hz.
Large hall 30 x 20 x 8 m: hundreds of modes under 100 Hz, packed tight, bass sounds even everywhere.
A 5 m wall rings at 34/69/103 Hz; move the mic 1 m and a kick drum note jumps 10 dB.
Cube-shaped room (equal L/W/H): modes stack on identical frequencies, worst possible boom.
Standing in a corner: every mode peaks there, so subs and bass measure much hotter than mid-room (often +9 to +18 dB), because all the modal peaks stack at once.
How it helps in live sound
Never put the sub in a corner unless you WANT big boom; a corner can add up to ~+18 dB (often +9 to +12 dB in real rooms) and excites every mode at once.
Walk the room before doors: bass that booms in one spot and dies in another = low modal density, sparse modes.
RTA at the mix position, then measure 3-4 spots; big bass swings between spots flag the modal region.
Park your mix desk away from dead-centre and away from walls (centre sits in nulls for odd modes).
Notch the boom peaks (often 40-80 Hz) with EQ; do not try to boost the nulls.
In small rooms cross subs lower (e.g. 80 Hz) and add corner bass traps to tame sparse-mode lumps.
Everyday analogy
Like guitar frets: fat low notes sit far apart and ring obviously, but high up the frets jam together and blur into one another.
Watch out
Myth: a bigger sub fixes lumpy bass. No - lumps come from sparse room modes; a bigger box just drives the same peaks and nulls harder.
Fun fact
A perfect cube is the worst room shape for bass: its three dimensions ring at identical frequencies, so modes pile up instead of spreading out, tripling the boom.
Key takeaways
Modal density = how tightly packed a room's bass resonances are.
Density rises with frequency AND with room size.
Below the Schroeder frequency bass is lumpy; above it bass is smooth.
Small rooms = sparse modes = boom-and-null; big rooms = dense modes = even bass.
Corners stack all modes (up to ~+18 dB, often +9 to +12 dB); nulls can drop 15-30 dB.
Fix peaks with EQ and bass traps, not by boosting the dead notes.