Room acoustics
Bass trap
A thick absorber, usually placed in corners, that damps low frequency room modes and shortens bass decay time.
A bass trap is an absorber designed for low frequencies. Porous types are thick panels of mineral wool or fiberglass, often corner-filling triangles or panels straddling a corner with an air gap. Tuned or membrane types resonate at specific frequencies to target one problem mode.
Room modes cause the largest response errors in any home theater, and thin foam does nothing about them. Corners are where every axial mode has maximum pressure, so treatment there works on all of them at once.
Porous absorbers work down to roughly the frequency whose quarter wavelength equals their thickness: a 4 inch panel starts near 300 Hz, while corner traps with 12 to 17 inches of effective depth reach 80 to 100 Hz. Treat vertical corners floor to ceiling before adding wall panels.