Surface wall mounting creates a comb-filter cancellation at the woofer-to-wall round-trip wavelength

Evidence

The approach offered by Allison (1974, 1975) and Ballagh (1983) involves choosing the position of the loudspeaker with respect to the boundaries in a manner that minimizes the variations in frequency response at the listening locations. Obviously, the goal is to arrange for the woofer-to-boundary distances, floor and walls, to be as different as possible, while not being multiples of each other. The more different these distances are, the less will be the effect. Finding an optimum result may involve some trial-and-error in practical listening. It may also result in visually unappealing, asymmetrical, or incorrect (in terms of stereo or multichannel imaging) locations for the loudspeakers. — , Toole, Floyd E - Sound reproduction-…, 1068 (#expands, via linked claim)

Categorization

• XREF: connects to: Delayed correlated sources produce frequency-dependent comb filtering; Console-mounted near-field monitors suffer comb filtering from coaxial reflections; A finite source-to-wall distance creates a destructive interference null at the half-wavelength frequency (+2 more)

Broader Connections

• Optimal woofer placement makes boundary distances as different and non-multiple as possible #expands (0.95) — , Toole, Floyd E - Sound reproduction-…, 1068
• A reflecting wall doubles total sound power output from any source #qualifies (0.72)