4.1 Phase diffusers (Schroeder's diffusers)

They are usually built as a sequence of thin linear apertures with different depths. The sound waves penetrate the material, experiment many reflections into the apertures, and emerge from them with a different phase, that is, in a different interference pattern. As a consequence, a good acoustic energy scattering is achieved.

The so-called geometric Schroeder's diffusers or RPG diffusers (reflection phase grating) can be modified in order to be used as acoustic absorbers (Schroeder's absorbers) [12]. Caution is needed to avoid obtaining an undesired behavior (sound absorption instead of sound diffusion).

The most frequent design methods are presented below.

#### 1. MLS diffusers (maximum length sequence)

They are designed using a periodic number sequence that decides the position of the apertures on the surface of the material. If the width of the irregularities is reduced, the design frequency of the diffuser will be higher, while if the depth will be higher, the frequencies to be corrected will be lower. The width of the openings is λ/2 and the depth is λ/4. This kind of diffusers has a good performance only for one frequency octave.
