Page 20 - Summer 2006

A BRIEF HISTORY OF ROOM ACOUSTIC DIFFUSERS
Trevor J. Cox
Acoustics Research Centre, University of Salford Salford M5 4WT, UK
Peter D’Antonio
RPG Diffusor Systems Inc. Upper Marlboro, Maryland 20774
“What wall shape has the highest possible sound diffusion, in the sense that an incident wave from any direction is scattered evenly in all directions?”
About three decades ago, Schroeder posed this ques- tion in his seminal paper outlining a new type of dif-
1 fuser based on maximum length sequences .
Together, with his later ingenious designs based on quad- ratic residue2 and primitive root sequences3, he provided some possible answers to his question and revolutionized thinking about surface scattering in rooms. This has inspired others to research and develop new diffuser designs, drawing inspiration from disciplines as diverse as x-ray crystallography, optics and mobile telephony.
Concert halls
One application for these types of diffusers is within con- cert halls. The acoustics of a concert hall plays an important part in the music performance, as one of the roles of the hall is to embellish and enrich the sound. Outdoor music and concerts may be popular, but the sound quality is usually poor, because listeners only receive sound straight from the orchestra. There are no reflections from walls and the sound appears distant. Tourist guides to ancient amphitheaters often demonstrate a theater’s remarkable acoustic properties by showing how a pin dropped on the stage can be heard by anyone in the audience. However, put an orchestra on such a stage and one would soon realize the weakness of the acoustics for classical music. In a well-designed, enclosed concert hall, reflections from the walls, ceiling and floor add reverberance and other characteristics to the sound—the sound comes alive. It envelops and involves the listener in the music making process.
Much of theater acoustic design concerns manipulating reflections by treating the surfaces from which the sound is reflected. A little over a hundred years ago Wallace Sabine demonstrated how surface absorption could be used to change the reverberation of a hall. Another wall treatment that controls the dispersion of the sound is surface diffusers. Currently, there is much debate about what role diffusers should play in a concert hall. One eminent concert hall designer regularly claims that too much diffusion is detri- mental to the sound quality of the upper strings, while others have blamed the disappointing acoustics of some major con- cert halls on a lack of surface diffusion.
Treatments
To alter the acoustics of an existing room, some form of treatment is usually applied to the room surfaces. In concert
Fig 1. The spatial and temporal response of sound reflected from a plane flat sur- face and a diffuser. (After Cox and D’Antonio15)
halls, the sound can be altered by placing treatment on the walls and the ceiling (the floor already has seating and an audience). There are three basic forms of treatment—large flat surfaces, absorbers, and diffusers. Absorbers, such as car- pets are not often used in large concert halls, because they remove sound energy from the space. Every bit of energy must be conserved because the maximum sound power out- put from an unamplified orchestra is limited.
The designer of a large concert hall usually chooses between large flat surfaces or diffusers. Figure 1 contrasts the spatial and temporal responses of these two surfaces. The
18 Acoustics Today, July 2006
Fig 2. The time and frequency from a flat (top) and diffusing (bottom) surface. (After D’Antonio and Cox16)