In situ estimation of acoustic impedance on the surfaces of realistic interiors: an inverse approach

In situ measurement of acoustic impedance is traditionally performed using pairs of microphones located close to the test surface. However, this method becomes troublesome if inaccessible complex-shaped surfaces, such as those in a real room, are considered. To overcome this problem a method to estimate the normal acoustic impedance on the interior surfaces of a room is proposed. As input data, the algorithm takes: 1) the 3D shape of the room, 2) the strength of the sound source, and 3) a set of sound pressures measured at random locations in the interior sound field. The estimation of the acoustic impedance at each surface is achieved via the solution of an inverse problem which arises from the boundary element method applied to the discretized interior boundaries of the room. Unfortunately, the solutions of this kind of problems are known to be unstable and sensitive to noise due a rank-deficient linear system. Dealing with such a system is avoided in the proposed method by formulating an iterative optimization approach which is shown to be more robust to noise. Previous work has reported examples with numerical simulations. This paper goes further and presents results obtained with real data from experiments