Splitter Silencers Research Articles

Pressure Loss in Ducts by Dissipative Splitter Silencers: Comparative Study of Standardized, Numerical and Experimental Results

Dissipative splitter silencers are widely used in industry for the reduction of propagated sound waves in ducts. Even though these systems are effective from the acoustics point of view when they are properly designed, they also introduce a pressure loss in the system, due to the modification of the properties of the flow circulating inside the duct. This effect is not desired in some industrial applications, so it is necessary to be able to predict the pressure loss as precisely as possible to design silencers according to the needs. Nevertheless, the prediction made by standards are usually limited to given geometries or flow speed. In this work, we present a comparative study on the results obtained for the pressure loss by means of the standards ISO 14163 and VDI 1801-1, numerical simulations with the finite element method, and experimental measurements. Additionally, two different profile shapes and four input face velocities are tested in order to know the influence of these parameters in the variations of the flow and the accuracy of the prediction of the different methods.

A three-dimensional analytical approach for large rectangular splitter silencers in the presence of mean flow

The dissipative splitter silencer is a common means for broadband noise attenuation in flow ducts such as ventilation and gas turbine systems where many higher order modes are cut-on. To predict the effect of a splitter silencer, analytical approaches have obvious advantages over the numerical methods on computation efficiency, but the development of analytical approaches are often based on many simplifications like the infinite length assumption. This paper presents a three-dimensional analytical method to solve the sound propagation in a duct with a finite length splitter silencer in the presence of mean flow. In the developed method, the convergence problem, which is often encountered by using the existing mode-based methods for high-frequency computations of soft-wall eigenmodes, is overcome by applying the equivalent surface source concept. With very low computational cost, the present method is validated to give consistent transmission loss predictions with the numerical results and experimental measurements available in the literature. Further, the performance of a splitter silencer is studied with different higher order incident modes, and reflection versus dissipation analysis is conducted by using the developed method. It is found that the sound attenuation is dependent on the sound energy spatial distributions of the incident sound modes, and the dissipation dominates the transmission loss behavior of the splitter silencer.

A hybrid approach to active and passive noise control for open windows

This research focuses on the use of an open window with an active noise control (ANC) system and a splitter silencer to improve the attenuation level of broadband noise. We previously introduced a similar idea with a splitter silencer and a loudspeaker inside a duct. However, it is challenging to achieve noise reduction with a splitter silencer in a short duct. In our previous research, we achieved noise reduction at an open window with an array of ANC units, in which a reference microphone with a loudspeaker is collocated. However, this system had limited high-frequency noise reduction due to the distance between each loudspeaker. Here, we improved noise reduction by integrating a collocated ANC system and a splitter silencer, which has an open ratio of 50%. Each unit has a reference microphone at the front and a loudspeaker at the back, which generate anti-noise. Four units are combined in an array with a splitter silencer. In an experiment, we implemented two splitter silencers in an eight-channel ANC system at an open window (24 × 48 × 12 cm). The results show that the splitter silencers attenuated high-frequency noise above 2 kHz. Moreover, the ANC system achieved an attenuation of 2 to 10 dB for noise in the range of 200 Hz to 2 kHz. Overall, we were able to achieve a total attenuation of 2 to 17 dB above 200 Hz.

대형 스플리터 소음기 성능에 대한 3차원 수치해석적 연구

Acoustic performance of splitter silencers was investigated by using 3-dimensional commercial software and experiments. Flow resistivity of sound absorbing material was indirectly estimated by using an impedance tube setup and a curve fitting method. In addition the acoustic impedance of perforated plate was determined by an empirical formulation. Such properties have been used as input parameters in the commercial software. The prediction for a splitter silencer with 1000 mm length was compared with the experimental result. The numerical method is then applied to identify the effects of number of splitters, length of splitters, absorptive material density, and porosity of a perforated plate on the performance of the splitter silencers. As the number and length of splitter increases, the acoustic performance significantly increases. Although the increase of density of absorptive material also increase the acoustic performance, a change in the density over a certain level hardly affect it. The increase of porosity will enhance the performance especially at higher frequencies.

Propagation of Acoustic Waves in Two Waveguides Coupled by Perforations. II. Analysis of Periodic Lattices of Finite Length

The paper deals with the generic problem of two waveguides coupled by perforations, which can be perforated tube mufflers without or with partitions, possibly with absorbing materials. Other examples are ducts with branched resonators of honeycomb cavities , which can be coupled or not, and splitter silencers. Assuming low frequencies, only one mode is considered in each guide. The propagation in the two waveguides can be very different, thanks e.g. to the presence of constrictions. The model is a discrete, periodic one, based upon 4th-order impedance matrices and their diagonalization. All the calculation is analytical, thanks to the partition of the matrices in 2nd-order matrices, and allows the treatment of a very wide types of problems. Several aspects are investigated: the local or non-local character of the reaction of one guide to the other; the definition of a coupling coefficient; the effect of finite size when a lattice with n cells in inserted into an infinite guide; the relationship between the Insertion Loss and the dispersion. The assumptions are as follows: linear acoustics, no mean flow, rigid wall. However the effect of the series impedance of the perforations, which is generally ignored, is taken into account, and is dis- cussed. When there are no losses, it is shown that, for symmetry reasons, the cutoff frequencies depend on either the series impedance or the shunt admittance , and are the eigenfrequencies of the cells of the lattice, with zero-pressure or zero-velocity at the ends of the cells.

On the acoustic performance of rectangular splitter silencers in the presence of mean flow

Dissipative splitter silencers are often used to reduce the noise emitted in ventilation and gas turbine systems. It is well known that the acoustic performance of a splitter silencer changes under the influence of the convective effects of a mean gas flow and so in this article a theoretical model is developed to include the effects of mean flow. The theoretical model is based on a hybrid finite element method which enables the inclusion of bull nose fairings and a perforated screen separating the mean gas flow from a bulk reacting porous material. Predictions are compared against experimental measurements obtained both with and without mean flow. Good agreement between prediction and measurement is generally observed in the absence of mean flow, although it is seen that for silencers with a low percentage open area the silencer insertion loss is over predicted at higher frequencies. When mean flow is present, problems with the experimental methodology are observed at relatively modest mean flow velocities, and so comparison between prediction and experiment is limited to relatively low face velocities. However, experiment and theory both show that the insertion loss reduces at low frequencies when mean flow is in the direction of sound propagation, and at high frequencies the influence of mean flow is generally much smaller. Following additional theoretical investigations it is concluded that the influence of mean flow on splitter silencer performance should be accounted for at low frequencies when silencer airway velocities are greater than about 20m/s; however, at higher frequencies one may generally neglect the effect of mean flow, even at higher velocities. Predictions obtained using the hybrid method are also compared to a simplified point collocation approach and it is demonstrated that the computationally efficient point collocation method may be used to investigate the effects of mean flow in a splitter silencer without loss of accuracy.

A three dimensional investigation into the acoustic performance of dissipative splitter silencers.

Splitter silencers are found in ventilation and gas turbine systems and consist of parallel baffles of porous material placed within a duct so that they split the mean gas flow. Theoretical investigations into dissipative splitter silencers have generally been limited to two dimensions and this limits the analysis to finding the silencer eigenmodes or, for a finite length silencer, to rectangular baffles only. In this article a numerical point collocation approach is used to extend theoretical predictions to three dimensions. This facilitates the analysis of more complex silencer designs such as "bar" silencers and theoretical predictions are validated by comparison with experimental measurements. The insertion loss of different silencer designs is evaluated and the performance of a bar silencer is compared to traditional designs for rectangular and circular ducts. It is shown that a bar silencer with a volume of material identical to an equivalent parallel baffle design delivers a significant improvement in insertion loss at higher frequencies, although this is at the expense of a small reduction in performance at low frequencies. It is also shown that under most circumstances it is possible to get good agreement between prediction and experiment even for relatively large Helmholtz numbers.

Fan Noise Control Using Microperforated Splitter Silencers

Splitter or baffle silencers are commonly used, for example, in heating ventilation and air conditioning (HVAC) systems and as inlet/outlet silencers on gas turbines. Another application is to reduce noise from the cooling fan inlet for large IC-engines. A splitter silencer can be seen as a periodic arrangement of parallel rectangular absorbers, which can be placed in a rectangular duct. The noise reduction afforded by parallel splitters depends not only on the physical properties of the lining but also upon the angle of incidence of the impinging sound waves, and the splitter and duct dimensions. In this paper, the potential of using splitters made of microperforated plates (MPPs) is investigated, with a particular focus on cooling fan inlet/outlet applications.

Noise control for cooling fans on heavy vehicles

In this paper two different objects for fan passive noise control have been examined: heat exchangers and inlet/outlet parallel splitter silencers based on micro-perforated panels. The first object is theoretically and experimentally examined while the second is only examined experimentally. Throughout this paper two measurement methods were used. The ISO 15186-1:2000 to test the acoustic transmission for a diffuse field and plane wave testing in a duct of a sample cut from each heat exchanger type. Based on an anisotropic equivalent fluid model a theoretical model for the heat exchanger acoustic transmission is presented. A new type of splitter silencers based on micro-perforated plates, which can add damping up 10-20 dB in the frequency range of interest (<5 kHz), are also presented.

Predicting the Acoustic Performance of HVAC Splitter Silencers

Mathematically modelling sound propagation in splitter silencers has the potential to provide a fast and effective method for optimising silencer design, and for alleviating the need to undertake repetitive and expensive experimental measurements. Moreover, understanding the physics behind sound attenuation in silencers will ultimately help to improve in situ silencer performance and to avoid current problems whereby commissioned silencers do not provide the attenuation expected from laboratory measurements. Accordingly, a mathematical model is discussed here that is capable of predicting silencer insertion loss and predictions are compared with experimental measurements. Results demonstrate the potential of this model for predicting silencer performance under laboratory conditions, but also illustrate that multi-modal sound fields significantly affect silencer performance so that laboratory tests are unlikely to represent accurately in situ silencer performance.

The influence of baffle fairings on the acoustic performance of rectangular splitter silencers

A numerical model based on the finite element method is developed for a finite length, HVAC splitter silencer. The model includes an arbitrary number of bulk-reacting splitters separated from the airway by a thin perforated metal sheet and accommodates higher-order modes in the incident sound field. Each perforated sheet is joined to rigid, impervious, metallic fairing situated at either end of a splitter. The transmission loss for the silencer is quantified by application of the point collocation technique, and predictions are compared to experimental measurements reported in the literature. The splitter fairing is shown to significantly affect silencer performance, especially when higher-order incident modes are present. It is concluded that laboratory measurements, and theoretical predictions, that are based on a predominantly plane wave sound source are unlikely to reflect accurately the true performance of an HVAC silencer in a real ducting system.

A point collocation approach to modelling large dissipative silencers

A numerical matching technique known as point collocation is used to model mathematically large dissipative splitter silencers of a type commonly found in HVAC ducts. Transmission loss predictions obtained using point collocation are compared with exact analytic mode matching predictions in the absence of mean flow. Over the frequency range in which analytic mode matching predictions are available, excellent agreement with point collocation transmission loss predictions is observed for a range of large splitter silencers. The validity of using point collocation to tackle large dissipative silencers is established, as is the computational efficiency of the method and its suitability for tackling dissipative silencers of arbitrary, but axially uniform, cross-sections.

THE CALCULATION OF ACTIVELY ABSORBING SILENCERS IN RECTANGULAR DUCTS

Active silencers can provide effective solutions especially for the control of low-frequency noise in ducts. To evaluate the performance of this technology in the early design stages it is necessary to predict the insertion loss and adjust the silencer sufficiently precisely to the specific requirements of an application. This paper describes different models for the calculation of actively absorbing wall linings with proportional feedback control applied in splitter silencers as used in rectangular air-conditioning ducts. On the basis of well-established theories for the calculation of passive splitter silencers and a network model of electro-acoustic lumped elements for the wall impedance of each active cassette, it is conceivable to determine their insertion loss. Starting with a rather basic approach, the computational model is refined to increase its modelling accuracy. It is shown that a combination of active wall linings with passive linings yields a high attenuation for a wide frequency band. The theoretical findings compare well with experimental results from a laboratory set-up.

Scattering of stable and unstable waves in a flow duct

The transmission and reflection properties of sound at sharp edges are the key elements when calculating the performance of absorptive and reactive splitter silencers using the Building Block method. These acoustic properties are here determined for a two-dimensional duct, having an air flow in its lower part only. In one, half-infinite, section an acoustically hard wall separates the moving and still media, while in the other there is an infinitely shear layer, that is, a vortex sheet is present. Leading and trailing edges are dealt with, while scattering problems, expressed through Fourier methods as Wiener-Hopf equations, solved under causality and edge behaviour constraints, provide a unique solution. Explicit expressions for scattering matrices are constructed and numerical examples are presented and discussed. A vital part of the theory is an analysis of the modal structure in the part of the duct having the infinitely thin shear layer. To this end Green's function is determined, and by applying causality the unique solution is found as an inverse Fourier integral, which is expressed as an infinite sum of modes. Therefore the modal system is complete. These modes, the properties of which are essential in establishing causality, have been studied by analytical techniques and the results are reported. The existence of ordinary acoustic modes as well as hydrodynamic ones is verified and the latter propagate only downstream. One of the hydrodynamic modes is found to be unstable for all frequencies, a consequence of causality: its amplitude grows rather than decays with the downstream distance. An example is given of an unusual acoustic mode that gets cut-on at a certain frequency and stays cut-on for higher frequencies except for a frequency interval where it is cut-off.

FINITE ELEMENT COMPUTATION OF ATTENUATION IN BAR-SILENCERS AND COMPARISON WITH MEASURED DATA

A finite element formulation for sound attenuation in “bar-silencers”, consisting of rectangular prisms of sound-absorbing material placed in a rectangular lattice arrangement within a rigid-walled duct, is described. The presence of a uniform mean gas flow in the “airway” of the silencer is taken into account, as is anisotropy of the absorbent. Comparisons are made between experimental insertion less data and numerical predictions, and generally reasonable agreement is noted. Comparison is also made between the acoustic performance of bar-silencers and the equivalent splitter silencers, and it is shown that bar-silencers tend to have superior attenuation characteristics at low frequencies. The present results are discussed in the context of previously published work on bar-silencers.

Program for the computation of absorptive silencers in straight ducts

The design of silencers is frequently based on results from standardised measurements which are assembled in the form of a data-base. Some silencer manufacturers offer their clients programs on diskette (free of charge) which employ such data-bases, together with interpolation between available data points, to aid silencer selection for specified applications. However, such programs cannot be applied if the required operating parameters are outside the data-base range. Similar comments apply when using the so-called trapezoid diagrams from the relevant literature. These present in simplified form the results of analytical procedures, but in a manner simple enough for calculations using personal computers. In this paper a computer program is presented which allows the calculation of the insertion loss of splitter silencers in such a way that the results are comparable to results measured according to ISO 7235. Different methods were used for the evaluation of the components that contribute to the insertion loss. The propagation loss in the air passage due to absorption by the splitter material is obtained using analytical calculation procedures. The amount of reflected sound at the splitters is determined from a large pool of insertion loss data previously measured according to the standards at the Institut für Bauphysik (IBP). The data pool is analysed using regression methods in order to obtain statistically the components of propagation loss and reflection loss. From this analysis, approximate formulae are derived for the reflection loss and a correction is obtained which accounts for the bypass transmission along the splitter frame. This correction is used to modify the propagation loss for the calculation of the insertion loss. The calculated insertion loss is then obtained by the sum of the propagation loss, the reflection loss and the bypass correction. Comparison of calculated and measured insertion loss is presented, showing good agreement. A user programme for the calculation of silencers on personal computers was developed from these procedures and has been applied already in a bigger project ( Eckoldt, D. et al., IBP-Bericht B-TA 3/1993, 1993).

Design curves for rectangular splitter silencers

Passive silencers with acoustic fill such as glass fiber, rock wool or foam are commonly used in conventional heating, ventilation and air conditioning systems. Acoustic performance can be estimated for a few basic silencers through the use of design curves available in the literature. Recently, a large microcomputer data base using design curves generated to cover the entire range of manufactured rectangular silencers was made available. Details of the design curves and the mathematical model are presented. Insertion loss of the silencers is estimated from attenuation rates calculated from a finite element method. Unlike existing models, the present method considers multimodal acoustic propagation and can be extended to account for the effects of shearing airflows in the airway. The sound-absorbing material is considered to be bulk reacting. Wave propagation in the material is thus included. Design curves are grouped by using three nondimensional parameters, thereby covering the entire line of conventional rectangular duct silencers. Results from the model are compared to least attenuated mode predictions and to actual test data. The results show that the present model gives better predictions than least attenuated mode models. Good comparison between the current model and the test data was also observed. Development of a computer program for a quick estimation of the insertion loss is also described.

An experimental study of swinbanks' method of active attenuation of sound in ducts

The concept of attenuating sound propagating down a duct by means of an antiphase copy of the sound was first put forward forty years ago, but to date no practical system has been produced. The principal problem is to introduce the antiphase signal in such a way that it propagates only in the direction of propagation of the original sound and to ensure that this property can be maintained over a useful frequency range. A suitable arrangement of secondary sound sources has recently been proposed by Swinbanks and this paper describes experiments carried out in an attempt to realise a practical system. A unidirectional array of secondary sources has been successfully constructed around a rectangular duct, using loudspeaker drive units and electronic delays. Sound propagating in the direction of these sources was sampled and a control signal applied to the sources which in turn acted to significantly reduce the amplitude of the sound. Pure tones at frequencies around 150 Hz have been attenuated by more than 50 dB but results with band-limited noise have been less successful. Further work is suggested which should result in a device having significant advantages over conventional splitter silencers, at low frequencies.