Analysis of the error sources of the two-microphone transfer function method for measuring absorption coefficient in the free field using numerical modeling

Abstract

Extension of the two-microphone transfer function absorption coefficient measurement technique from impedance tubes to the free field introduces several error sources. The three-dimensional nature of the sound field necessitates consideration of factors that are not relevant in an enclosed tube below the cutoff frequency. The impedance tube technique has been modified to account for non-planar wave propagation due to an acoustic point source. The sound field contamination from sample edge diffraction has generally restricted use of the technique to frequencies such that wavelengths are small relative to sample dimensions. This requires the use of very large test panels for low frequencies. Numerical models of the two-microphone free field technique have been created to quantify these effects. Each effect was isolated to better understand its independent impact on the accuracy of the technique. Finally, a series of experimental tests were conducted to validate the numerical modeling results.