In-situ measurement of the impedance of acoustic materials is of general interest for many applications. One might think that the specific impedance measured near the material under test would be a good approximation to its impedance. However, this is not the case; a model of the sound field is necessary. Moreover, in practice one cannot generate the sound field by a distant source, and thus the model of the sound field involves the spherical reflection factor, which depends on the unknown acoustic impedance in a complicated way. This paper examines an iterative method for determining the surface impedance. An initial model of the unknown impedance is used to simulate a measurement which is then compared with a real measurement. In the next step the impedance is adjusted and the sound field recalculated; and this is repeated until the impedance that minimizes the difference between the real and the simulated measurement is found. Very good agreement is found in simulations, but experiments give acceptable results only above 200 Hz.
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