Room gain

Below the frequency whose wavelength no longer fits in the room, a loudspeaker stops radiating waves into a space and starts pressurizing a sealed volume. Output rises as frequency falls — the opposite of the driver’s own rolloff. Smaller and better-sealed rooms gain more, and start gaining higher up; a large or leaky room gains little.

This is why the anechoic curve on a datasheet is not the curve you hear, and it is the first place this spoke’s predictive loop (synthesis) runs out of road: the box is designable, the room is not.

The design use

A designer can spend the room’s rise instead of fighting it. A sealed box rolls off gently, and a gentle fall meeting a gradual rise can sum flatter and deeper in situ than either curve predicts alone — the argument svs-sealed-vs-ported makes for sealed subs in small and mid-size rooms. A ported box’s steep 4th-order fall below tuning is harder to meet this way: it drops faster than a typical room rises.

The catch is that it only works if you know the room, which nobody does at design time. In practice the rise is discovered by measuring in place and then trimmed with EQ (audio-dsp) — the same correction layer this spoke keeps finding at the boundary between a designed device and an undesigned environment (in-vehicle-audio is the extreme case, where the load could never be designed at all).

Not to be confused with

Room modes — standing waves at frequencies whose half-wavelengths match a room dimension, which produce peaks and nulls at specific positions. Room gain is a broad rise below the modal region; modes are narrow and place-dependent. Both are room effects, both get blamed for bass problems, and only one of them can be fixed by moving the listener.

Related

loudspeaker-enclosure · group-delay · audio-dsp · in-vehicle-audio · svs-sealed-vs-ported · synthesis