ABSTRACT
A diffuser needs to break up the reflected wavefront. While this can be achieved by shaping a surface, it can also be achieved by changing the impedance of the surface. Indeed, the Schroeder diffuser is often interpreted as a variable impedance surface. In this chapter, variable impedance is achieved by patches of absorption and reflection. Unlike the Schroeder diffuser, these cannot be designed for minimum absorption. These surfaces are hybrids somewhere between pure absorbers and non-absorbing diffusers. Partial absorption is inherent in the design, and any reflected sound is dispersed. Using patches of absorption to generate dispersion is not particularly new. In studio
spaces people have been arranging absorption in patches rather than solid blocks for many years. In recent times, however, a new breed of surface has been produced, where the absorbent patches are much smaller, and the arrangement of these patches is determined by a pseudorandom sequence to maximize the dispersion generated. This chapter will start by discussing some implementations of these surfaces to give a sense of how they can be constructed. Then a more detailed theoretical basis for their design will follow.
