Identification of surface acoustic impedances in a reverberant room using the FDTD method

In numerical room acoustic simulations the acoustic impedance of walls and boundaries is of fundamental importance for the correct physical and perceptual synthesis of a sound field. Acoustic impedances are usually measured in reverberation chambers as absorption coefficients. In this type of measurements the sound field is assumed to be diffuse, which is an ideal condition usually not met in real rooms. This paper proposes a different approach to obtain the acoustic impedances, where a numerical model of the room is tuned with measurements using nonlinear optimization. The finite difference time domain (FDTD) method is used to simulate a sound field of a room and an optimization problem is solved to recover the acoustic impedances per frequency octave band. Results show that the use of a time-domain numerical method is advantageous as compared to a frequency-domain method, by making the measurement set-up more practical and improving the optimization convergence.