Room Mode Calculator

Knowledge Base

What Are Room Modes?

Every rectangular room resonates at specific frequencies determined by its dimensions. The same way a guitar string resonates at a frequency set by its length. These resonances, called room modes, boost some bass frequencies and cancel others depending on where you stand or sit in the room.

Axial, Tangential, and Oblique Modes

Axial modes (calculated here) run between two opposite parallel surfaces (floor/ceiling, or the two side walls) and are the strongest and most audible. Tangential modes involve four surfaces and oblique modes involve all six. Both are weaker and left out of this simplified calculator, which focuses on the modes worth treating first.

Reading the Clash Warnings

When two axial modes land within about 5 Hz of each other, their effect compounds. That frequency gets significantly louder at certain listening positions and significantly quieter at others. Rooms with a length, width, and height that are simple multiples of each other (e.g. a perfect cube) have the worst clustering. Rooms with dimensions in a more irregular ratio spread their modes out more evenly.

What To Do About Problem Frequencies

Bass traps in room corners (where all three axial modes overlap in pressure) are the most effective single treatment. Moving your mix position and speakers away from the exact 1/2 or 1/3 points of the room length also helps avoid sitting in a null or peak. This calculator tells you where the problems are. The Reverb Room Estimator tells you how much absorption you need to add.

Why bass is uneven in a small room

Sound reflects between parallel surfaces, and where a half wavelength fits exactly between two walls, the reflections reinforce each other. The result is a standing wave at that frequency, louder in some parts of the room and near-silent in others. The frequency depends only on the distance, which is why room modes follow the room rather than the speakers.

The first mode for each dimension sits at the speed of sound divided by twice that distance. A five metre wall produces one at about 34 hertz, and multiples of it follow. Small rooms have widely spaced low modes, which is why they sound uneven at the bottom, and large rooms spread theirs closely enough that the problem largely disappears.

Position changes what you hear more than treatment does. Every mode has locations of maximum reinforcement and locations of near cancellation, so moving a listening position a short distance can change the perceived bass dramatically. Sitting exactly halfway along a dimension places you in a null for that dimension's first mode, which is the single worst place to judge bass.

Modes need mass to control. Ordinary acoustic foam does effectively nothing at these frequencies, because the wavelengths involved are metres long. Porous bass traps of substantial depth, placed where pressure is highest, which is in corners, are what actually works, and the required depth surprises people the first time.

Treat the room before trusting a mix made in it. A boomy room makes you cut bass that was never excessive, and a mix corrected for a room fault sounds wrong everywhere else. Knowing which frequencies your room exaggerates lets you discount them while listening, which is useful even before any treatment is installed.

Vocabulary

Room mode. A resonance caused by sound reflecting between surfaces a fixed distance apart, reinforcing one frequency. Determined by the dimensions of the room, so it stays put regardless of where speakers are placed.

Axial mode. A mode formed between one pair of parallel surfaces, such as the two side walls. The strongest kind, and the ones worth calculating first, since they cause most of the audible unevenness.

Standing wave. A wave pattern appearing stationary because reflections continually reinforce it in the same places. Produces fixed locations of exaggerated level and fixed locations where the frequency nearly disappears.

Bass trap. An absorber designed for low frequencies, requiring substantial depth because the wavelengths are long. Placed in corners, where pressure is highest and absorption is therefore most effective.

Worked example: a 5 by 4 by 2.7 metre room

An axial mode is a standing wave between two parallel surfaces. Its frequency equals the speed of sound, 343 metres per second, times the order, divided by twice the distance between the surfaces. The table lists the first four orders on each axis for the tool's default room, from lowest to highest.

Two modes clash when they fall within 5 Hz of each other on different axes. Here the second height mode at 127.0 Hz and the third width mode at 128.6 Hz clash, 1.6 Hz apart. Modes that close pile energy into the same band, which makes the bass uneven from one listening position to another. No other pair in this room lands within 5 Hz. The lowest mode, 34.3 Hz, comes from the 5 metre length. The tool lists axial modes only.

Vocabulary

Axial mode. A standing wave that forms between one pair of parallel surfaces, such as the two end walls. It builds up at frequencies whose half wavelengths fit the distance exactly, and it is the strongest kind of room mode.

Mode order. The count of half wavelengths that fit between the surfaces. The first order fits one, the second fits two. Each order sits at a whole-number multiple of the first order frequency.