Three-dimensional characterization of acoustic diffusers from near-field measurements

Characterizing acoustic diffusers is difficult due to the complexity of the diffracted sound field they generate. The standardized method to measure the three-dimensional directivity of diffusers (ISO 17497-2) requires a large anechoic room and a hemispherical microphone array located far from the sample under test. This study proposes using plane-wave expansion from near-field measurement data to measure real-sized rigid diffusers in free-field. The sound field is sampled by a sequential microphone array located in the vicinity of the diffuser surface. The formulated inverse problem enables the separation of incident and diffracted sound fields directly in the wave-number domain, which encapsulates the 3D directivity of the diffuser. The results obtained are compared against reference simulations using the Boundary Element Method. This research also explores the influence of the array discretization, aperture, and number of layers, which serve as initial measurement guidelines. The investigation demonstrates that the proposed method is a robust tool for characterizing acoustic diffusers for research and industrial purposes while reducing the complexity of directivity measurements.