Normal incidence sound transmission loss in impedance tube – Measurement and prediction methods using perforated plates

For the determination of the transmission loss of samples in an impedance tube, two different approaches is found in the literature, one based on determining the full transfer matrix (TM method) of the acoustic element, the other based on the wavefield decomposition theory (WD method). In this paper both methods are implemented and measured results are compared using samples which includes different types of perforated plates, also combined with porous material. Measurements are conducted in a tube of square cross section with dimensions 200 × 200 mm, thereby limiting the workable frequency range upwards to approximately 850 Hz. The main purpose of the paper is, however, to compare measured results with predictions using the transfer matrix method. For a bare plate with cylindrical apertures two models are compared as well; a “classical” one and another based on modeling the perforated plate as a porous material having a rigid frame. As for these transmission loss measurements, the two measurement approaches turn out to give identical results within the numerical accuracy. The fit between measured and predicted results are reasonably good with a maximum deviation mostly within 2 dB.