Two subwoofers in opposite mode lobes can cancel room resonances without equalization

Evidence

Employing the logic of Figure 14.17(b) the single sub in the corner (position 1) was replaced with two identical subs located in positions 2 and 3. The subs, driven with identical signals, were in opposite polarity lobes of the width mode and also of the length mode, so for both modes the combined modal coupling factor was very low and mode energy substantially attenuated. The resonant peak was attenuated, and one of the modes was truly cancelled as is demonstrated by the narrow destructive-interference dip in Figure 14.18(b). The other improvements were fortuitous, and never analyzed, following the “leave well enough alone” principle. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1583

Categorization

• DEFINE: Concretely illustrates how modal coupling factor cancellation works in practice for two room modes simultaneously.
• SHARE: A specific, actionable acoustic-design case (two subs in positions 2 and 3 canceling width+length modes) worth sending to someone doing home theater or studio setup.