Take advantage of free acoustical power: always place subwoofers on the floor behind the screen – NOT on a platform with the screen loudspeakers. Floor-positioned subwoofers take advantage of “half-space” loading, which increases radiated power by about 3 dB, WITHOUT adding any extra amplifier power. Even better: place the subwoofers on the floor AND push them all the way back against the solid wall behind the screen. That is called “quarter space” loading, and results in about 6 dB greater SPL, without using a larger amplifier. The very low frequencies produced by the subwoofers have no problem going right through any draping or masking below the screen.
Free Loading
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Pushing the subwoofers back to the rear wall MAY be beneficial. It depends on how far back the wall is and how well damped the behind-screen space is. One could push them into the corner too and pick up yet more boundary conditions/efficiency but often not improved performance (response). Moving the speakers off the main speaker plane adds in a time delay element and would preclude decent bass-management (if employed). You don’t want to delay the main speakers back to the subwoofers.
A better solution for ALL of the speakers is to construct a proper baffle wall so all speakers pick up that 2-Pi space, improved bass, better clarity (no more bouncing the audio off the screen, 2-3 feet back to rear wall and so forth. The subwoofers, floor mounted pick up that extra boundary of the floor as well for the additional 3dB.