Standing Wave

Standing waves form when two waves of the same frequency travel in opposite directions and interfere constructively and destructively at fixed positions. In a room, sound reflecting between parallel walls creates standing waves at frequencies whose half-wavelengths fit an integer number of times between the walls. These resonant frequencies are called room modes, and they dominate the low-frequency behavior of any enclosed space.

The fundamental mode between two walls separated by distance L occurs at f = c/(2L), where c is the speed of sound (approximately 343 m/s). For a 5-meter dimension, the first mode is at 34.3 Hz, with harmonics at 68.6 Hz, 102.9 Hz, and so on. Real rooms have three sets of axial modes (one per dimension), plus tangential modes (involving two wall pairs) and oblique modes (involving all six surfaces). Below the Schroeder frequency, these discrete modes dominate the acoustic response.

Standing waves create practical problems: at antinodes (typically against walls), bass frequencies can be boosted by 10-20 dB; at nodes (typically at specific positions in the room), those same frequencies nearly cancel. This means a subwoofer placed in a corner (triaxial antinode) excites room modes maximally, while a listener at a node position hears almost no bass at that mode frequency. Moving either source or listener by half a meter can dramatically change the low-frequency experience.

SonaVyx provides several tools for standing wave analysis. The room scan calculates room modes from dimensions and highlights problematic coincidences. The RTA reveals standing wave peaks and nulls in the measured frequency response. The transfer function at multiple positions maps the standing wave pattern. The treatment calculator recommends bass traps and Helmholtz absorbers to damp standing waves. AcousPlan predicts room modes at the design stage.