Opposing in-phase subwoofers cancel odd-order axial modes but amplify even-order ones.

Evidence

In Figure 14.19(b) the addition of a second identical in-phase subwoofer at the center of the opposite wall cancels the first-order length mode (1,0,0) at 23.5 Hz (as shown in Figure 14.17(b)) and the accompanying odd-order mode (3,0,0) near 70 Hz. Eliminating these nulls in the listening area results in a dramatic reduction in the MSV. However, the two woofers now amplify the second-order length mode (2,0,0) at 47 Hz giving it more amplitude. — Toole, Floyd E, Sound reproduction- the acoustics and…, 1590

Categorization

• XREF: Core psychoacoustic mechanism: how symmetric drive phasing selectively acts on mode order parity — connects to the reader’s interest in how physical stimuli cause perceptual responses.
• QUESTION: Worth chasing: why do second-order mode pressure nulls sit at 25% and 75% between walls, and how that exact geometry protects a central listening area.