Reducing low frequency tonal noise in large ducts using a hybrid reactive-dissipative silencer

Paul Williams,Ray Kirby,James Hill,Mats Åbom,Colin Malecki

doi:10.1016/j.apacoust.2017.10.018

Paul Williams, Ray Kirby + Show 3 more

Open Access

https://doi.org/10.1016/j.apacoust.2017.10.018

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Abstract

Noise generated by fans or turbines normally consists of a combination of narrow and broadband noise. To lower transmitted noise levels, it is attractive to use a combination of reactive and dissipative elements. However, this approach presents a number of challenges for larger systems. This is because reactive elements are commonly placed around the duct circumference where they are normally only effective up to the frequency at which the first higher order mode cuts on in the duct. For larger systems, this means that reactive elements work only in the low, and often very low, frequency range, whereas dissipative elements, which are distributed across the duct cross-section, generally work well in the medium to high frequency range. This can cause noise problems in the low to medium frequency range in larger systems. This article presents an alternative approach for delivering noise attenuation over the low to medium frequency range that is suitable for application in larger duct systems. This approach takes advantage of those splitter silencer designs commonly used in larger systems to integrate a reactive element into the splitter design. This delivers a hybrid splitter that uses a combination of dissipative and reactive elements so that the reactive element partitions the main airway. This has the advantage of introducing a quasi-planar transverse sound pressure field for each resonator in the low to medium frequency range, including frequencies above the first cut-on. It is demonstrated using predictions and measurements taken for a number of example silencers, that this approach enables reactive elements to work over an extended low to medium frequency range, including at frequencies above the first cut-on mode in the main duct. Accordingly, it is shown that a hybrid dissipative-reactive splitter design is capable of delivering improved levels of attenuation in the crucial low to medium frequency range.

Highlights

The control of noise emissions from power generating equipment is normally addressed using passive noise control techniques such as reactive and dissipative sound attenuators
The reactive elements normally address low frequency noise, especially tonal noise that often arises in power generation, and the dissipative elements address the medium to high frequency range
There is likely to be some change in the incident sound power, and this will be seen as local oscillations in the insertion loss (IL) curves; this effect is seen to be negligible in the measured data that follows, especially once this data has been averaged into one-third octave bands

Summary

Introduction

The control of noise emissions from power generating equipment is normally addressed using passive noise control techniques such as reactive and dissipative sound attenuators. An alternative approach is required and this article presents a new method for a method for deploying reactive silencer elements in larger ductwork This aims to that delivers higher levels of sound attenuation in the crucial region between the first cut-on frequency of the main duct, and the frequency at which a typical dissipative splitter silencer begin to perform well. This will enable the targeting of nodal pressure lines in higher order modes and for the method to continue to be applicable when larger ductwork is encountered This further adds to the complexity of a hybrid silencer design and it is clearly possible to design many different configurations for the baffles and dissipative/reactive elements, and to use many different parameters for the porous materials and the perforated sheets.

Theory

Perforate

Experiment

Results and Discussion

Conclusions

20. European Standard EN ISO 29053