Prediction of surface admittance impulse responses from frequency-dependent sound absorption coefficients

In numerical room acoustic modeling certain material properties are required as inputs for setting the boundary conditions for the calculations. In finite element or finite difference models, locally reactive boundaries are often represented as FIR or IIR filters. However, most building material manufacturers provide sound absorption coefficient or equivalent absorption area data only, in accordance with the relevant standards, so a method to convert sound absorption to admittance or impedance impulse responses is required. Since there is a loss of information when admittance is converted to sound absorption coefficients, the conversion in the opposite direction has multiple solutions. We derive an iterative method of determining either the impedance or admittance impulse response from arbitrary, frequency-dependent diffuse sound absorption coefficients and validate it with the transfer matrix method. We present the implementation of the method in the soundy.ai application.