Attenuating sound in large ductwork using reactive and dissipative silencers

It is common to use a combination of reactive and dissipative silencers to attenuate noise emitted by internal combustion engines. The combination of different silencers enables both tonal and broadband noise to be reduced, and the acoustic performance of each silencer component is now reasonably well understood for typical applications in the exhausts of cars and smaller trucks. This understanding is based on theoretical models that focus on optimising the sound power reduction, or transmission loss, of each component. These models normally assume that planar sound waves are present in the exhaust pipe, even if higher order acoustic modes are accommodated inside the silencer. However, such an approach will be successful only if the diameter of exhaust pipe is relatively small, so that sound propagation inside this pipe remains planar over the frequency range of interest, which normally extends up to about 800 Hz to 2 kHz at room temperature. For larger internal combustion engines, the assumption of plane wave propagation at higher frequencies is not always appropriate, and this presents a more complex design challenge. Accordingly, this article examines the noise attenuation in larger internal combustion engines, with capacities extending up to about 60 litres. This involves the use exhaust pipes with diameters of the order of 250 mm and above, so that the limit of plane wave propagation lies within the frequency range of interest. Higher order propagating modes in the exhaust pipe must therefore be included, and this is addressed here using finite element based models that combine both reactive and dissipative silencers. Therefore, the interaction between different silencer components is included, and the effect of the higher order modes on their performance is examined. This delivers a more comprehensive analysis of sound attenuation in an exhaust pipe, which is applicable to larger internal combustion engines. Predictions are generated for straight-through reactive and dissipative silencers, and current design practice is reviewed for larger internal combustion engines over a wide frequency range.