Page 21 - Summer 2006

responses shown assume that there are no other surfaces present. A flat surface behaves in the same way that a mirror reflects light; the sound energy is preserved and concentrat- ed in the specular reflection direction, where the angle of incidence and reflection are equal. The time response shows the similarity between the direct sound and the reflection. A flat surface does little to the sound except to change the direc- tion in which the sound propagates. Figure 2 shows the resulting uneven frequency response (called comb filtering) leading to “coloration” of the sound. The timbre of notes is altered due to the emphasis and de-emphasis of the different frequency components.
Alternately, diffusers disperse the reflection both tempo- rally and spatially. The time responses in Figs. 1 and 2 show reflections arriving over a longer time period. The frequency response shows less evidence of comb filtering than the flat surface and the peaks and troughs are uneven and randomly spaced. The sound is now a more faithful rendition of the original sound produced by the instrument and less col- oration will be heard. Any non-flat corrugated surface will have some diffusing ability, but Fig. 1 shows a cross section through one of the specialized surfaces designed by Schroeder—in this case a quadratic residue diffuser.
Diffusers are used in a variety of ways, but most often they are used to avoid a particular acoustic defect. Their abil- ity to spatially disperse sound is illustrated in Fig. 1, and might be exploited to overcome problems of uneven sound distribution over sections of the audience. A diffuser’s ability to disperse sound temporally can be used to reduce echoes from the rear walls of auditoria. Sound often takes a long time to travel from the stage to the rear wall of a concert hall. If a strong reflection comes back from the rear wall to the front of the hall, this can be heard as an echo, especially if the rear wall is concave and focuses the sound. In older halls, the echo problem would have been mitigated by placing absorbent material on the rear wall to remove the offending reflection. However, the absorption removes acoustic energy and is undesirable. A modern solution is to use diffusers to disperse the troublesome reflections because this can be achieved without loss of acoustic energy. An example of using Schroeder diffusers on the rear wall of Carnegie Hall can be seen in Fig. 3.
Fig 4. One period of two different Schroeder diffusers. Left—original design, right—fractal design. The diffusers are 0.6m wide, 0.6m high and about 0.2m deep.
Architectural trends
Wallace Sabine was the first person to apply “the scien- tific method” to room acoustic designs a little over a century ago. However, there are halls built before Sabine’s work that are held to be great halls. An example would be the archetyp- al “best” concert hall, the Grosser Musikvereinssaal in Vienna. With these older halls, ornamentation and relief work appeared in a hall because it was the architectural style of the day. Walls were naturally diffusing.
In the twentieth century architectural trends changed and large flat areas appeared in many concert halls. The style of the day was to produce clean lines following a modernist style, and these surfaces then had little or no diffusing capa- bility. While it is possible to design successful halls with flat surfaces (Symphony Hall Birmingham, UK), expanses of flat surfaces can lead to distortion due to comb filtering, echoes and other mechanisms.
Against this architectural backdrop, Schroeder devel- oped his diffusers in the 1970s. An early motivation was the need to generate binaural dissimilarity at the listener, by pro- moting laterally propagating sound in concert halls. In the 1960s and 70s, various studies showed how binaural dissimi- larity leads to a sense of envelopment, a greater sense of being involved in the music, and therefore, a “better” sound. The evidence for the beneficial effects of lateral reflections come from laboratory and field measurements on human percep- tion, and these followed techniques pioneered in experimen- tal psychology.
An example of Schroeder’s original design can be seen in Fig. 4 (left). These surfaces offered acoustical consultants the designs for which they were looking—defined acoustic per- formance based on very simple design equations. While it is known that old-fashioned ornamentation produces diffu- sion, it does this in an ill-defined and haphazard fashion and most architects were no longer interested in such out-of-date styling.
One of the pioneering applications of Schroeder dif-
fusers was by Marshall and Hyde in the Michael Fowler
4,5
Centre, New Zealand . Figure 5 illustrates the application.
Large overhead surfaces were used to provide early reflec- tions to the audience in the balconies. This was a design where a hall could have good clarity, and yet maintain a large volume for reverberation. Much of the volume is in the space behind the surfaces. Not many years before the design of the
Fig 3. Schroeder diffusers (QRDs®) applied to the rear wall of Carnegie Hall to pre- vent echoes. (After D’Antonio and Cox17)
A Brief History of Room Acoustic Diffusers 19