Introduction

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. Hence, one of the central topics in acoustics is how to manipulate these reflections that affect the way the sound propagates, and 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 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, there are hybrid surfaces, which can both absorb and diffuse to varying degrees. Over the past 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. In contrast, significant scientific knowledge about the role of scattering (diffusely reflecting) surfaces has only been developed much more recently. Over the past 20-30 years, significant research on methods to design, predict, measure and quantify diffusing surfaces has resulted in a growing 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, the right 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. An example of a sound production room is the performing arts facility such as

concert halls for classical music or a theatre for speech. The room acoustic contributes greatly to the perceived sound of the music or speech. The arrival time, direction and temporal density, and level of the early reflections, coupled with the balance of the early to late energy, decay time, temporal and spatial density of the late reflections,

define the quality of sound that is heard. In large sound production rooms, reflection and diffuse reflection are the primary acoustic tools. This is schematically illustrated in Figure 2. Absorption may be used to control reverberance, but the unavoidable absorption due to paying customers must also be considered. In contrast, the acoustics of sound reproduction rooms, like recording studios and

home theatres, should be neutral. All of the spectral, timbre and spatial information is pre-recorded on the playback media, and the reproduction room is only there to allow a listener to hear what has been recorded, as is was recorded. In a sound reproduction room, absorption and diffuse reflection play a key role, and specular reflection is a minor contributor. This is illustrated in Figure 3. Absorption and diffusion are used to control the colouration that would otherwise occur in the space from early arriving reflections and low frequency modes. In noise control situations, like gymnasiums, swimming pools and factories, the

objective is simply to reduce the reverberance and sound level. This might be done to reduce sound levels to prevent hearing damage or to improve the intelligibility of speech. Uniform distribution of absorption is the primary acoustic tool, and specular reflection and diffuse reflection have more minor roles. This is illustrated in Figure 4.

(Although it has been suggested that diffusers can play a useful role in disproportionate spaces, but then Figures 2-4 are all generalizations of the true situation.) Surface acoustic treatment also plays an important role outdoors. For instance, the

absorption of the ground can have a significant impact on sound levels from ambient noise sources such as roads and industrial premises. The treatment of noise levels might involve the use of noise barriers, and these might be treated with absorption, or less commonly, diffusers to reduce the noise levels. This introductory description has sketched out a few of the issues concerning where

and why absorbers and diffusers are applied. More detailed descriptions can be found in Chapter 1 for absorbers and Chapter 2 for diffusers. The following section, however, tries to give an overview of the relative merits of absorption and diffuse reflections.