Boundary loading.

[LelandCrooks]

From the article
A general conclusion from the measurements is that if your objective is accurate sound reproduction with minimal coloration, it is bad news to place a loudspeaker anywhere near a boundary. The striking exceptions are that of the horn/driver and the in-wall.

http://www.synaudcon.com/site/author/pa ... dspeakers/

[Bill Fitzmaurice]

The author's procedural errors negate many of his conclusions, so I wouldn't put a lot of stock in this document. He seems unaware of what the baffle step is, nor Allison Effect, and if you don't take those into account all that follows is for naught. The only accurate observations made, with respect to horns and flush mounting, are accurate because those remove baffle step and Allison Effect from the equation.