Room Ratio Mode Spread Calculator
A rectangular room's proportions decide how its low-frequency resonances are spread out. When the length, width, and height share simple integer relationships, axial modes coincide and pile up, leaving uneven bass with strong peaks and nulls. Even ratios spread the modes apart for a smoother response. This calculator normalises your dimensions to the height to show the ratio set, lists the first axial modes of each dimension, and reports the closest spacing between any two modes as a fraction of an octave. A larger minimum spacing indicates better-distributed modes. Use it to test or compare candidate room dimensions.
Mode spread method
Ratio = (H / H) : (W / H) : (L / H)
Axial modes f(n, D) = n * c / (2 * D) for each dimension D
Spacing (octaves) between adjacent modes = log2(f_higher / f_lower)
Closest spacing = minimum spacing across all mode pairs
The calculator computes the first three axial modes of each of the three dimensions, sorts them by frequency, and finds the smallest gap between consecutive modes in octaves. Coincident or near-coincident modes (very small spacing) indicate problematic ratios.
Worked example
A 6.4 by 4.7 by 3.4 metre room normalises to 1.00 : 1.38 : 1.88, close to a well-regarded ratio set. The first axial modes are about 26.80 Hz (length), 36.49 Hz (width), and 50.44 Hz (height), giving reasonably even spacing with no coincident modes in the lowest band.
Room ratios: frequently asked questions
Why do room dimension ratios matter?
The ratios of a rectangular room's length, width, and height determine how its axial resonances (room modes) are spaced in frequency. Well-chosen ratios spread the modes out evenly, giving smoother bass; poor ratios let modes pile up at the same frequency, leaving strong peaks and deep nulls. Cubic rooms, where all three dimensions are equal, are the worst case.
What does this calculator compute?
It normalises the three dimensions to the height (so height is 1.00) to give the width-to-height and length-to-height ratios, then lists the first axial modes of each dimension and reports the smallest spacing between any two modes as a fraction of an octave. A larger minimum spacing means a more even, better-behaved low-frequency response.
What is the Bonello criterion?
The Bonello criterion is a method of judging room ratios by counting modes in each one-third-octave band and checking that the count rises smoothly from band to band, with no band having fewer modes than the one below it (except where the count is small). This calculator focuses on the related and simpler measure of axial-mode spacing, which captures the same idea: avoid coincident modes.
What ratios are considered good?
Several published ratio sets aim for even mode distribution. A widely cited example is about 1 to 1.4 to 1.9 (height to width to length). The goal is to avoid simple integer relationships between dimensions, which cause modes to coincide. Use this calculator to test your own dimensions against that principle.
Official sources
- International Organization for Standardization: ISO 3382 room acoustic parameters.
- U.S. National Institute of Standards and Technology: nist.gov (speed of sound reference).
Reviewed by the CalculatorHub team, edited by James Graham, 19 June 2026. See our methodology.