Conference_programme: 19.1 - Duct Acoustics
Lecture: Measurement of dissipative silencer performance under a non-planar excitation
Author(s): Williams Paul, Kirby Ray, Thomson Jamie, Hill James
Summary:
Measurements of silencer insertion loss are commonly taken with the assumption that the incident sound field is a plane wave. However this is unlikely to be the case across the entire frequency range of interest when using loudspeakers as a sound source. Instead it is likely that non-planar modes will be excited with a significant ratio of the total sound power. These non-planar duct modes are known to affect silencer performance; however this has not been studied in detail. This presents a significant commercial risk when designing silencers for large ducts. Standard test methods for the measurement of silencer performance do not identify non-planar excitations. Laboratory measured insertion loss may therefore be different from the performance under the expected plane wave source. The complex acoustic field within real-world sources, such as gas turbine exhausts or fans, further complicates design as the distribution of sound power across the modes is unknown. Silencers in situ may therefore perform differently to silencers measured under laboratory conditions. The effect of non-planar modes is investigated here using measurements and predictions of silencer transmission loss. A method is presented whereby the sound power incident upon and transmitted by a silencer is determined from induct sound pressure measurements. Decomposing this data allows for the transmission loss to be calculated using the sound power in higher order modes. The equivalence of insertion loss and transmission loss metrics are demonstrated beyond the plane wave region when changes to the incident sound power are accounted for. Finite element methods are used to create numerical models of parallel baffle silencers to investigate the effect of non-planar sources. Predictions make use of the source characteristics measured during the experiment to demonstrate the importance of having knowledge of the source when designing silencers.
Corresponding author
Name: Dr Paul Williams
e-mail: This email address is being protected from spambots. You need JavaScript enabled to view it.
Country: United Kingdom
