Double bass array, aka Controlled Acoustic Bass System (CABS), is a specific case that utilizes bass drivers to turn the rear wall in effect into an absorbing boundary. It "makes" the acoustic impedance of the rear wall the same as the room air, and to the sound waves the rear wall will behave just like a continuation of the room, and therefore the sound waves will be absorbed with no reflection.
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The original CABS paper is behind the AES paywall.
AES E-Library » Controlled Acoustic Bass System (CABS) A Method to Achieve Uniform Sound Field Distribution at Low Frequencies in Rectangular Rooms
A new method for improving the low-frequency response in small- and medium-sized rooms uses a pair of front loudspeakers for initiating a traveling plane wave and a canceling rear pair of loudspeakers that effectively remove the acoustic reflection of the back wall. The rear loudspeakers receive...www.aes.org
A free (slightly earlier) version of this AES paper can be found in the Ph.D thesis of Celestinos (PDF), listed as "Paper D".
