Comparison of errors in the three- and four-microphone methods used in the measurement of the acoustic properties of porous materials

The three-microphone and four-microphone methods have been shown useful for measuring the characteristic impedance and propagating wavenumber of porous materials. In this paper the effects of uncertainties in the apparatus and pressure transfer function measurements are analyzed using Monte Carlo methods. It is shown that for materials with low flow resistivities the four-microphone method is appreciably more accurate than the three-microphone method at low frequencies. The dominant contributors to the overall error are uncertainty in the magnitude and phase of the transfer function measurement.