Wavenumber domain analysis of isotropy in reverberant spaces

This is a recording of an Acoustics Lunchtime Research Seminar given by Dr Mélanie Nolan of the Technical University of Denmark (DTU) to the Salford Acoustics Group on the 16th June 2021. Abstract: Standardized reverberation-room measurements of sound absorption coefficients are based on the simple reverberation formulae, for which a diffuse sound field must be established both before and during the sound decay. The absorption properties of the measuring sample are described by a random incidence absorption coefficient, which corresponds to the average fraction of energy absorbed by the sample when sound is arriving on it equally from all directions. As evidenced by many reports, the sound field in a reverberation chamber containing a single concentrated sample of absorbing material is often far from diffuse, and the sample is unlikely to be exposed to isotropic sound incidence. Consequently, results obtained in one reverberation chamber may differ considerably from those obtained in another, even under the conditions specified by the standard. In this seminar, I will present an experimental method for characterizing sound field isotropy in reverberant sound fields. The symmetry of the sound field is analyzed by considering the wavenumber spectrum, which results from expanding a measured pressure distribution (with an array of sensors) into a plane-wave basis. Measurements in standardized reverberation chambers show that the sound field is clearly anisotropic, both in steady state and during the decay process. Further, by separating the incident from the reflected components in the wavenumber domain, a method is introduced to characterize the distribution of sound incidence on the sample, and to infer angle-dependent absorption properties. Biography: Mélanie Nolan received the M.Sc. degree in general engineering from the Ecole Centrale de Lyon, Lyon, France, in 2014, and the M.Sc. and Ph.D. degrees in acoustics from the Technical University of Denmark, Kgs. Lyngby, Denmark, in 2014 and 2019, respectively. From 2019 to 2021, she held a postdoctoral researcher position at the R&D and Innovation department, Saint-Gobain Ecophon, Hyllinge, Sweden. In January 2021, she received a DFF-International Postdoctoral Grant by the Danish Research Council and currently holds a postdoctoral researcher position between the Acoustic Technology group, Department of Electrical Engineering, Technical University of Denmark, Kgs. Lyngby, Denmark, and the Laboratoire d’Acoustique de l’Université du Mans, Le Mans Université, Le Mans, France. She was recently awarded the 2021-22 F. V. Hunt Postdoctoral Research Fellowship in Acoustics by the Acoustical Society of America and will start a new postdoctoral research associate position at the School of Architecture, Rensselaer Polytechnic Institute, Troy NY, USA, in September 2021. Her research interests include sound field analysis, acoustic transducer array processing, acoustic materials and measurement of sound absorption.