Standing waves form when a sound's wavelength matches a room dimension, creating fixed nodes and antinodes

When a sound wave at a specific frequency reflects off a parallel surface and combines with the incoming wave, it creates a standing wave: a fixed pattern of pressure maxima (antinodes) and pressure minima (nodes). Standing waves form at frequencies where the room dimension is a multiple of a half-wavelength — these are room modes or room resonances. They cause severe frequency response variation within a room: at an antinode the level of a modal frequency is boosted; at a node it is nearly absent. Room modes make the acoustic response highly position-dependent. In live sound, modes cause feedback to ‘lock on’ at specific frequencies and alter the perceived balance at the mix position.

Examples

A room 17 feet long has its first axial mode at approximately 33 Hz (speed of sound ÷ 2 × length = 1130 ÷ 34 ≈ 33 Hz). A subwoofer placed at the corner excites all three axial dimensions simultaneously, maximizing mode energy.

Assessment

A mix position exhibits an audible 80 Hz boom that disappears when the engineer moves 3 feet back. Name the acoustic phenomenon and explain why moving position changes the perception.