Room Acoustics Calculator — Room Modes & Dimensions

Room modes are resonant frequencies where sound waves bounce between parallel surfaces and reinforce each other. Axial modes occur between two surfaces (e.g. front and back walls), tangential modes involve four surfaces, and oblique modes involve all six. Axial modes are the strongest and most problematic.

The ideal room has dimensions where no two sides are the same length or simple multiples of each other. Published ratios that spread the room modes evenly include Sepmeyer's (1 : 1.28 : 1.54, from Sepmeyer 1965) and Louden's (1 : 1.4 : 1.9). The Bolt area is not a single ratio but a region on a length/width/height ratio chart within which the modal distribution is acceptably even — any ratio inside it works well. IEC 60268-13 lists similar listening-room recommendations. Cube-shaped rooms are the worst as all modes occur at the same frequencies.

The Schroeder frequency marks the transition between individual resonant modes (below) and statistical sound field behaviour (above). Below this frequency, room modes dominate and acoustic treatment needs to target specific frequencies. Above it, broadband treatment is more effective.