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- presented by Prof Trevor Cox

Room dimensions for small listening rooms

Over the years, people have suggested certain ratios of room dimensions that minimise the distortion caused by low frequency room modes. These "golden ratios", however, are not necessarily the best dimensions to choose for a room and below some new best room dimensions are suggested.

These web pages gives some more detailed numerical results from an Audio Engineering Society journal paper which appeared in June 2004. The journal paper gives more details of the method used and can be downloaded from the AES web site.

Why choose the right room size?

The sound that is heard in a critical listening room is determined by the combined effect of the electronics of the audio system and the physical acoustics of the listening environment. The tonal balance and timbre of a sound can vary significantly depending on the placement of the listener and loudspeaker and the geometry of the room. Indeed, the modal artefacts introduced by the room can be so influential that they dominate the sound. We will concentrate on the design challenges to minimize these artefacts at low frequencies. Consequently, we are concerned with the interaction between the sources, listeners and the room modes.

Modes in small rooms often lead to extended sound decays and uneven frequency responses – often referred to as coloration. Problems arise at low frequencies because of the relatively low modal density. Designers try to overcome the problems of modes by choosing an appropriately proportioned room, by placing the listeners and loudspeakers in suitable positions and by using bass absorbers. Here we will concern ourselves with the first problem, choosing the right room dimensions.

The determination of appropriate source and receiver location is often undertaken by a trial and error process. Although this is a laborious task, it is possible to do. When sources or receivers are moved, the frequency response changes due to the variation in the modal pressure distribution in the room and the changing radiation resistance of the source. By choosing correct positions in the room, it is possible to minimise the audible effects of the modes within a room . As well as considering the modal (steady state) response, others have considered the effects of first order reflections from boundaries. In particular, the first order reflections from the nearest wall boundaries to the source have been considered along with the effect these reflections have on the frequency response (e.g. [1]). A more complete solution using optimization is possible [2].

modal respose in a small room

At low frequencies rooms suffer from modal problems. The figure to the right shows a typical frequency response. Whatever the shape of a room, there will be a low frequency range where modes are rather sparsely spaced in frequency, leading to a poor sound quality. In particular, it is the excessively long sound decays of modes that make them stand out and more audible. While this problem can be solved by damping the modes (by adding absorption), it is best if the room has dimensions that help minimise the modal problems in the first place, because this makes it easier (and cheaper) to treat the modes.

Next: Methods for choosing correct dimensions

Next + 1: The best room dimensions

Might also be of interest: Another article of studio design, How to design diffusers

[1] Loudspeaker and Headphone Handbook” 2nd Edition. Edited by John Borwick. Focus Press, ISBN 0-240-51371-1.

[2] T J Cox and P D'Antonio. Room Optimizer: A Computer Program to Optimize the Placement of Listener, Loudspeakers, Acoustical Surface Treatment, and Room Dimensions in Critical Listening Rooms", 103rd Convention of the Audio Engineering Society, Preprint 4555, Paper H-6, New York (September 1997).

Trevor Cox's research pages

Re-use of material permitted provided it is clearly labeled "(c) University of Salford, www.acoustics.salford.ac.uk"