Used to construct ESS-type acoustic environments, aA structure comprising a number of wells of different, carefully-chosen depths. As a ray of sound strikes the irregular surface, instead of bounding off it like a mirror, it bounces out of each well at a slightly different time, resulting in many small reflections, spread out in both time and space. The operating range of a single diffuser is limited to about four octaves because, if the deepest well is deeper than about fifteen times its width, it begins to behave as a diaphragmatic absorber. The well depths are most commonly given by: where d is the depth of the diffuser, h is the well number, N is the prime number on which the sequence is based, and L is the wavelength of the lowest operating frequency. This is called the quadratic residue sequence.