Measurement of acoustic impedance and prediction of transmission loss of the porous woven hose in engine intake systems

Abstract For the prediction of the acoustic performance of an engine intake system with a porous woven hose, the acoustic wall impedance of the hose should be known. In this paper, a measurement technique is proposed that is valid over the low frequency range in the absence of mean flow. The impedance measurement is performed in two stages. The first stage of impedance measurement is performed in a cylindrical apparatus, within which a short specimen of given ‘porous frequency’ is laid. Because the predicted transmission loss (TL) does not agree with the measured one, due to error in the measured resistance, the second stage of impedance measurement is attempted to estimate the resistance from the measured reactance in the first stage and the measured TL data. Using such new impedance data measured in the second stage for the prediction of TL for samples of different lengths, the agreement between measured and predicted TL is generally improved, as compared to the initial comparison in the first stage. When the curve-fitted impedance for an arbitrary porous frequency is used, it is shown that the measured TL for samples with both arbitrary length and porous frequency generally agree reasonably well with the predicted ones. Exceptions are samples which require the impedance to be extrapolated from measured results, or samples with highly inhomogeneous weaving and coating conditions, and in particular samples with low porous frequency.