A method to measure in situ the surface acoustic impedance of a material was recently introduced by Allard and Champoux [Noise Control Eng. J. 32 (1), 15–23 (1988)]. It requires the utilization of a numerical method based on the well-known fixed-point method. This numerical method is shown to yield sufficiently precise results regardless of the value of the surface impedance. Using an alternative expression for the complex integral that is evaluated numerically, values for the upper integration and summation limits are determined to make the in situ method viable in practice.