Room modes

In a room this size, the walls shape the bass far more than the speakers do. Sound bounces between each pair of facing surfaces, and at the note whose half wavelength fits the gap exactly, and at every whole-number multiple of it, the reflections stack up into a standing wave, a room mode. The air in it does not travel across the room: it swings in place, pressed together in some places while it is pulled apart in others, and along the nulls between them its pressure hardly changes, though that is where the air itself moves most. So the note booms in some seats and all but vanishes in others. The from the front wall to the rear wall puts those notes at about , and by the last of them the room’s modes have blended.

That is one pair of surfaces. The between the side walls has a ladder of its own from , and the from floor to ceiling another from . A mode between one pair of surfaces is an axial mode. Sound can also settle into a loop round four surfaces at once, like a ball played round all four cushions of a snooker table, which makes a tangential mode, or off all six, the walls, the floor and the ceiling, which makes an oblique mode. Every bounce costs a little energy, so axial modes are usually the strongest and oblique ones the weakest, but the higher the note, the more of the others there are. Below about this room has axial, tangential and oblique modes, in all, and the figure draws every one.

They crowd together as the pitch rises. Between the room has modes, most far enough apart to boom on notes of their own, though some sit close enough to pair up; between it has . By about they lie so close that they blend, and the room stops behaving like a set of separate notes: where that happens is its transition frequency. Below it, the bass you hear belongs to the room as much as to the speakers.

Every mode is strongest in the room’s eight corners, which is why this room’s bass absorbers stand in two of them. Everywhere else, where you sit, and where the speakers stand, decides how strongly each mode reaches you. The figure shows the seat’s side of it, and marks your ears in both of its views. A seat on a mode’s null loses that note, and no correction can put it back, since whatever the speakers add there the room takes away; a seat near its peak gets it too loud. No seat escapes both.

Bad bass does not stay in the bass, either. Loud low sounds mask quieter sounds just above them in pitch and just after them in time, a well-known effect of how we hear, so a note that booms and hangs on also blurs the lower voices and instruments around it and whatever comes straight after it. Tame the bass and the rest of the range has room to come through. So the room is treated first, and the signal is corrected after.