Qualification of a Reverberation Chamber Equipped With Lightweight Diffusers

This paper describes how reverberation chambers are used for acoustic measurements, such as sound absorption and sound power. A reverberation chamber is intended to provide a diffuse sound environment, where the sound field is sufficiently independent of the position of a source or receiver, i.e. the total energy density is uniform throughout the chamber, and this is achieved by designing a chamber with hard walls, acoustically isolated from outside noise. The design of the chamber dimensions is unequal in separating its eigenmodes, and often, the walls are non-parallel to reduce the severity of standing waves. Almost invariably, reflecting panels are placed in the chamber to further diffuse sound. With panels orientated at random angles, the sound is reflected repeatedly, resulting in directional blending of sound. These reflecting panels, referred to as diffusers, can be made of various materials, but it is recommended that the material be relatively dense, at least 5 kilograms per square meter of surface area. Reverberation chamber qualification is a standardized process that must be conducted and documented in order for a chamber to be used for acoustic measurements according to national standards. ASTM C423 is a standard that details the measurement of a sample’s sound absorption using a reverberation chamber; its appendices direct the configuration and qualification of reverberation chambers. At The Georgia Institute of Technology, there is a new reverberation chamber that must be qualified for sound absorption measurements. This chamber has steel panel construction for its walls and ceiling. The floor is a concrete slab, isolated from the host space by a 3 Hz isolation system. The room dimensions are 8 x 6.3 x 5 m. To promote diffusion within the chamber, stationary fiberglass diffuser panels will be suspended from the ceiling at random positions throughout the volume. These panels are made of 2.1 x 2.1 m sheets of fiberglass, weighing 1.3 kg/m2. These panels are much lighter than those recommended in ASTM C423, but are desirable since they afford easy, safe chamber reconfiguration. The fiberglass is corrugated to promote further randomness of the sound field. The work at hand is intended to determine if such diffusers are suitable to the required task.