Resonant room modes

Room modes are the collection of resonances that exist in a room when the room is excited by an acoustic source such as a loudspeaker. Most rooms have their fundamental resonances in the 20 Hz to 200 Hz region, each frequency being related to one or more of the room's dimension's or a divisor thereof. These resonances affect the low-frequency low-mid-frequency response of a sound system in the room and are one of the biggest obstacles to accurate sound reproduction.

The input of acoustic energy to the room at the modal frequencies and multiples thereof causes standing waves. The nodes and antinodes of these standing waves result in the loudness of the particular resonant frequency being different at different locations of the room. These standing waves can be considered a temporary storage of acoustic energy as they take a finite time to build up and a finite time to dissipate once the sound energy source has been removed.

A room with generally hard surfaces will exhibit, high Q, sharply tuned resonances. Absorbent material can be added to the room to damp such resonances which work by more quickly dissipating the stored acoustic energy.

In order to be effective, a layer of porous, absorbent material has to be of the order of a quarter-wavelength thick if placed on a wall, which at low frequencies with their long wavelengths requires very thick absorbers. Absorption occurs through friction of the air motion against individual fibres, with kinetic energy converted to heat, and so the material must be of just the right 'density' in terms of fibre packing. Too loose, and sound will pass through, but too firm and reflection will occur. Technically it is a matter of impedance matching between air motion and the individual fibres. Glass fibre, as used for thermal insulation, is very effective, but needs to be very thick (perhaps four to six inches) if the result is not to be a room that sounds unnaturally 'dead' at high frequencies but remains 'boomy' at lower frequencies, so that it provides absorption across a broad range of frequencies. Curtains and carpets are only effective at high frequencies (say 5 kHz and above).

As a rule of thumb, sound travels at one foot per millisecond (344 m/s), so the wavelength of notes at 1 kHz is about a foot (344mm), and at 10 kHz about an inch (34mm). Even six inches of glass fibre has little effect at 100 Hz, where a quarter wavelength is over 2 feet (860mm), and so adding absorbent material has virtually no effect in the lower bass region in the 20–50 Hz region, though it can bring about great improvement in the upper bass region above 100 Hz.

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