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Insertion loss in one-third or octave bands is widely used in industry to assess the performance of large silencers and mufflers. However, there is no standard procedure for determining the transmission loss in one-third or octave bands using measured data or simulation. In this paper, assuming that the source is broadband, three different approaches to convert the narrowband transmission loss data into one-third and octave bands are investigated. Each method is described in detail. To validate the three different approaches, narrowband transmission loss data of a simple expansion chamber and a large bar silencer is converted into one-third and octave bands, and results obtained from the three approaches are demonstrated to agree well with one another.

Prior research on assessing multiple inlet and outlet mufflers is limited, and only recently have researchers begun to consider suitable metrics for multiple inlet and outlet mufflers. In this paper, transmission loss and insertion loss are defined for multiple inlet and outlet mufflers using a superposition method that can be extended to any m-inlet n-outlet muffler. Transmission loss is determined assuming that the sources and terminations are anechoic. On the other hand, insertion loss considers reflections. For both metrics, the amplitude and phase relationship between the sources should be known a priori. This paper explains both metrics, and measurement of transmission and insertion loss are demonstrated for a 2-inlet 2-outlet muffler with good agreement.