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Micro-perforated (MPP) panels are acoustic absorbers that are non-combustible, acoustically tunable, lightweight, and environmentally friendly. In most cases, they are spaced from a wall, and that spacing determines the frequency range where the absorber performs well. The absorption is maximized when the particle velocity in the perforations is high. Accordingly, the absorber performs best when positioned approximately a quarter acoustic wavelength from the wall, and larger cavity depths improve the low frequency absorption. At multiples of one half acoustic wavelength, the absorption is minimal. Additionally, the absorption is minimal at low frequencies due to the limited cavity depth behind the MPP. By partitioning the backing cavity, the cavity depth can be strategically increased and varied. This will improve the absorption at low frequencies and can provide absorption over a wide frequency range.

This paper documents a finite element approach to predict the attenuation of muffler and silencer systems that incorporate diesel particulate filters (DPF). Two finite element models were developed. The first is a micro FEM model, where a subset of channels is modeled and transmission matrices are determined in a manner consistent with prior published work by Allam and Åbom. Flow effects are considered at the inlet and outlet to the DPF as well as viscous effects in the channels themselves. The results are then used in a macro FEM model of the exhaust system where the transmission relationship from the micro-model is used to simulate the DPF. The modeling approach was validated experimentally on an example in which the plane wave cutoff frequency was exceeded in the chambers upstream and downstream to the DPF.

The two-load method is commonly used to determine the transmission loss of a muffler or silencer. Several practical measurement considerations are examined in this paper. First of all, conical adapters are sometimes used to transition between impedance tubes and the muffler. It is demonstrated that the effect of adding the adapter can be quite significant at low frequencies especially if the adapter is short in length. The effect of changing the length of the adapter was examined via measurement and plane wave theory. Secondly, the effect of selecting the reference microphone was examined experimentally. It was found that measurements are improved by selecting a downstream reference. Finally, the effect of using different frequency response function estimation algorithms (H1, H2 and Hv) was compared sans flow. This had little effect on the measurement.