Research On The Pneumatic Through-hole Silencer

Pneumatic systems usually produce large exhaust noise in typicalapplications. In order to control the noise pollution to the working environment,it is necessary to attach silencers to all the exhaust pipes in these systems.However, the widely-used close-chamber pneumatic silencers may result inexcessive pressure drop which will influence the working performance of theupstream systems and accumulation of filth with long time usage which willblock the air pass in some specific applications. Hence, research on thepneumatic through-hole silencer is conducted in order to overcome theshortcomings of these applications.The relationship of characteristic parameters of porous absorbing material inthe through-hole silencers with sound frequency is calculated with Johnson-Allard equivalent fluid model. The relationship of characteristic impedance ofperforated plate with sound frequency is calculated with empirical equations.Combined with these results, two dimensional analytical model of the through-hole silencer is established. The solution techniques of the characteristic wavenumber equations are discussed and the transmission loss (TL) is calculated bythe mode matching method. It is concluded that two acoustic modes in the inletand outlet pipe and at least three acoustic modes should be preserved in thechamber to obtain precise enough prediction of TL by comparison results of TLwith different mode orders.The influences of parameters of porous absorbing material includingporosity, resistivity, tortuosity, viscous and thermal characteristic length on theTL of the through-hole silencer are discussed by acoustic finite element analysis.Results indicate that material with larger resistivity, smaller tortuosity, viscousand thermal characteristic length tends to improve the TL while the influence ofporosity is negligible. The influence of perforated plate including the porosity ofthe plate, hole diameter and wall thickness are also discussed. Results indicatedthat the perforated plate structure obviously shorten the effective frequencydomain of the through hole silencer while only bring limited improvement to TLin the low frequency, and the influence of wall thickness is minimal. With largerperforation porosity and hole diameter, these disadvantages also become moreevident. Thus it is recommended to choose perforated plates with largerperforation porosity and smaller hole diameter as the protection of porous material. The influences of chamber length and porosity material lining thicknessare also discussed. It is concluded that hrough-hole silencer with larger chamberlength and lining thickness have better performance over the whole frequencydomain.The effects of improved silencer structures including extended tube,conical airway, air gap, and partitions inside the chamber on the TL of thethrough-hole silencer are discussed through acoustic finite element analyses.Results indicate that extended tubes with proper length, conical airway withgradually increasing cross section, partitions inside the silencer chamber couldimprove the TL of the through-hole silencer while the air gap tends to impair thesilencing performance and the conical airway with gradually decreasing crosssection show little influence. By further analyses, it is pointed out that theimprovements of the extended tubes and partitions on the through-hole silencers’TL are realized by preventing specific sound modes excitement and propagationunder characteristic frequencies and thus the optimal configurations of thesestructures are near the modal nodes of the through-hole silencer chamber.The relationship of total sound pressure level of the noise of the through-hole silencer attached to a vacuum generator with the vacuum flux and inletpressure is obtained by the sound level meter at the measurement point1m infront of the exhaust of the silencer. Noise signals are recorded by the microphoneand the frequency spectrum of the through-hole silencers are obtained by signalprocessing. Experiment results show that the most promising materials amongtest specimens are cotton and rock wool. The conclusions that material, withlarger resistivity, longer silencer chambers, thicker linings will improve thethrough-hole silencer’s performance are verified by the experiment. Experimentresults also show that the conical airway with gradually increasing cross sectionand partitions are effective in improving the performance of the through-holesilencer. However, the inlet extended tube didn’t improve the performance of thesilencer as expected while the conical airway with decreasing cross sectionwhich show medium effect in the simulation unexpectedly improve theperformance of the silencer.