Design of Large Reactive Silencers for Automotive Applications

It is common for large internal combustion engines to generate high levels of noise in harmonic tones below 100 Hz. The attenuation of low frequency tonal noise is challenging because if one wishes to rely on passive techniques then it is normally necessary to use large reactive chambers, as dissipative elements are ineffective at such low frequencies. However, it is common for the physical space available for the silencer to be limited and so one is often faced with the challenge of delivering high levels of sound attenuation in a relatively confined space. This paper reviews some different approaches to addressing this design challenge, and examines the trade-off between levels of attenuation and the frequency bandwidth over which the silencers are effective. This investigation includes the analysis of elliptical as well as circular expansion chambers, and the use of airways that are offset from the geometrical centre of the chamber cross-section. Predictions are generated using an optimised finite element approach, and it is shown that through careful design one can achieve relatively high levels of attenuation in the low frequency range.