Room modes are determined by the linear dimensions of opposing surfaces

Evidence

Another name for these standing waves in a room is resonant modes, and the frequencies at which they occur are known as “modal frequencies.” Because the modes are spatially static, there will be a strong variation of sound pressure level as one moves around the room, which is undesirable. — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1953 (#expands, via linked claim)

Categorization

• XREF: connects to: Hard-walled rooms behave as three-dimensional resonators with many audible modal frequencies — Ville Pulkki, Matti Karjalainen, Communication Acoustics- An…, 291; Dimensionally symmetrical rooms are acoustically asymmetrical due to construction details — , Toole, Floyd E - Sound reproduction-…, 631; Axial mode frequencies are computable from room dimensions by a simple formula (+9 more)

Broader Connections

• Avoiding commensurate room dimensions prevents overlapping modal frequencies that cause acoustic problems #expands (0.95)
• A room’s critical frequency depends on absorption and mode bandwidth, not just room dimensions #qualifies (0.85)
• Spatially static resonant modes cause sound level to vary across a room #expands (0.85) — David M. Howard, Jamie Angus, Acoustics and Psychoacoustics, 1953
• A transition frequency separates modal from ray acoustical behavior in rooms #qualifies (0.75) — Toole, Floyd E, Sound reproduction- the acoustics and…, 475
• Room mode coupling depends on source and listener positions, not frequency #qualifies (0.75)