Development of a small-scale reverberation chamber and validation of the diffuse field

Abstract

Noise absorbers are in a growing demand in the automotive, aerospace and building construction industries. Testing acoustic properties of materials is mandatory for developing an efficient acoustic controlling product design. Two main methods available to measure acoustic properties of materials are impedance tube method and reverberation room method. Impedance tube method uses small test specimens (usually less than 10 cm in diameter) and measures only normal incidence sound absorption, whereas reverberation room method is a relatively expensive setup which usually requires a large space (100-200 m3 ) and allows large samples (10-12 m2 ). To overcome the drawbacks of the current test methods and to make a comparative analysis of the test samples, a small scale reverberation room was designed. A chamber with 2.06 m3 volume was constructed. All the inner surfaces including the door were lined with highly reflective ceramic tiles to obtain maximum reflectivity. The randomness of the incident angles was achieved by an asymmetric shaped room with an inclined roof to obtain all walls non parallel. A modal analysis was performed to validate the small scale reverberation chamber for acoustic measurements. The pressure variations inside the enclosure within resonance frequencies are too small. Even though the cut-off frequency of a purely rectangular chamber with the same volume is 270 Hz, the new design of the reverberation room allows taking measurements below 270 Hz.