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The sound that is heard in most environments is a combination of the direct sound straight from the source or sources and the indirect reflections from surfaces and other objects. For instance, in room acoustics, both the direct sound and the reflections from the walls, ceiling, and floor are key in determining the quality of the acoustic. To take another example, outdoors, the reflection from the ground can significantly reduce noise at certain low frequencies. Hence, one of the central topics in acoustics is how to manipulate these reflections that alter the way the sound behaves, and is ultimately perceived. Sound striking a surface is transmitted, absorbed, or reflected; the amount of energy going into transmission, absorption, or reflection depends on the surface’s acoustic properties. The reflected sound can either be redirected by large flat surfaces (specularly reflected) or scattered by a diffusing surface. When a significant portion of the reflected sound is spatially and temporally dispersed, this is a diffuse reflection, and the surface involved is often termed a diffuser. Figure 1.1 illustrates temporal and spatial characteristics of absorbing, specularly reflecting, and diffusing surfaces, which form the acoustical palette. In addition to the surface types shown in Figure 1.1, there are also hybrid surfaces, which can both absorb and diffuse to varying degrees. For more than 100 years, since the founding of architectural acoustics by Sabine, there has been considerable effort devoted to studying surface absorption. Over this time, a considerable library of absorption coefficients has been tabulated based on accepted standards of measurement, and a reasonable understanding of how absorbers should be designed and applied has been achieved. This development continues, and in recent decades, many innovative absorber designs have been developed, and new ways to predict and measure absorptive materials have been found. For noise control, the focus is on removing energy, whereas in architectural acoustics, both absorbers and diffusers have a role to play in creating a good acoustic. However, significant scientific knowledge about the role of scattering (diffusely reflecting) surfaces has only been developed much more recently. Over the past four decades, significant research on methods to design, optimize, predict, measure, and quantify diffusing surfaces has resulted in a body of scientific knowledge and understanding. All these issues, and many more, are covered in this book. Good architectural acoustic design requires the right room volume, an appropriate room shape, and surface treatments, utilizing an appropriate combination and placement of absorbers, diffusers, and flat surfaces. Architectural acoustic spaces can be loosely divided into sound production, sound reproduction, and noise control environments.
Acoustic Absorbers and Diffusers Theory, Design and Application