Zhang, Y. and Herrin, D., "A Comparison of the Transfer and Scattering Matrix Approaches for Determining Muffler Transmission Loss using the Two Load Method," SAE Technical Paper 2015-01-2308, 2015, doi:10.4271/2015-01-2308.
The two-load method is commonly applied to determine the transmission loss for a muffler especially if an impedance tube rig is used. Although one procedure and algorithm is detailed in ASTM E2611, the quality of the transmission loss curve is dependent on several factors that are not discussed in detail in the standard. In this paper, several practical concerns are investigated including (1) the number of channels used in the measurement, (2) the selection of the reference channel, and (3) the choice of data processing algorithm (transfer or scattering matrix). Results are compared for a simple expansion chamber first, then for mufflers of other types. Recommendations are made for obtaining smoother transmission loss curves for various measurement methods.