Duct Acoustics

High end splitter-type silencers represent the object of the present paper, that summarizes the results obtained in an experimental campaign conducted in the framework of FlowAirS Project ITN FP7, Marie Curie Actions. Sound measurements on several fully functional splitters scaled 1:3 with Mach and Reynolds number similarity to the typical conditions in gas turbine exhaust stack have been performed. Different trailing edge shapes including tapered and serrated extensions have been tested. Only moderate changes in the measured noise have been found, despite a reduction of the total pressure loss that can be achieved by appropriate design of the splitter trailing edges. Considerable effects due to the covering perforated plate have been found. The results are analyzed applying Nelson and Morfey’s theory for obstacles in ducts, the scaling provides a good data collapsing, in particular for the frequencies above the cut-on.

A finite-difference time-domain (FDTD) approach is proposed to model extended-reacting liners under a grazing mean flow. It is based on an equivalent fluid model of the material together with the auxiliary differential equation method (ADE). The methodology is applicable to any liner material amenable to an equivalent fluid description, such as rigid-frame porous materials and metamaterials. It has been validated against a semi-analytical solution in a 1D test case and against experimental measurements in a duct.

Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications such as in the development of mufflers for IC-engines. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. Techniques to improve the acoustic two-port determination have been tested in this paper. A number of possible configurations for connecting loudspeakers to the flow duct have been investigated. It was found that using a perforate pipe section with about 50% porosity between the loudspeaker side branch and the duct gave the best signal-to-noise ratio out of the studied configurations. Different signal processing techniques have been tested for reducing the adverse effects of flow noise at the microphones. The most successful techniques require a reference signal which can be either the electric signal being input to the loudspeakers or one of the microphone signals. As a reference technique stepped sine excitation with cross-spectrum based frequency domain averaging was used. This technique could give good results for most cases. Using a periodic signal (saw-tooth) and synchronised time domain averaging good results could be obtained if a sufficient number of averages was used. At flow velocities higher than M=0.2 about 10000 averages were needed. Random excitation together with cross-spectrum based frequency domain averaging also gave good result if the same number of averages was used. Ordinary frequency domain averaging is not sufficient at high flow velocities. It was also shown that using cross-spectrum based frequency domain averaging an improvement could be obtained if the microphone with the highest signal-to-noise ratio at each frequency was used as the reference microphone rather than a fixed microphone.

Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications. It is an important tool for instance in the development of mufflers for IC-engines. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. The plane wave acoustic two-port is a 2x2 matrix containing 4 complex quantities at each frequency. To experimentally determine these unknowns the acoustic state variables on the inlet and outlet side must be measured for two independent test cases. The two independent test cases can be created by: changing the acoustic load on the outlet side leading to the so-called two-load technique or by using one acoustic source on the inlet side and one acoustic source on the outlet side leading to the so-called two-source technique. In the latter case the independent test cases are created by first using the source on the inlet side and then the source on the outlet side. As pointed out by Åbom it is also possible to run both sources simultaneously to create more than two independent test cases. This over-determination could be used to improve the measurement results for instance if the data is contaminated by flow-noise. In this paper over-determination is tested by applying up to 5 different test cases. This procedure has been applied to a single orifice test object.

A numerical technique based on the finite-element method has been developed for analyzing the performance of systems of acoustic elements including expansion chambers. The theories developed are, first, the variational formulation of the acoustic field existing in the system and, second, the finite-element approximate solutions of the variational problems. The predictions of the transmission loss then are made by forming the equivalent acoustic four-terminal transmission network in which the acoustic four-pole constants are calculated from the finite-element method. As used, the finite-element approach is perfectly general and may be applied to any system component with arbitrary boundary geometry, provided this may be realized by an assembly of rectangular elements. The method is applied to simple expansion chamber models because the results are tractable and theoretical results from acoustic filter theory are available for comparison purposes. The results from the various models b...

A finite-difference time-domain (FDTD) approach is proposed to model extended-reacting liners under a grazing mean flow. It is based on an equivalent fluid model of the material together with the auxiliary differential equation method (ADE). The methodology is applicable to any liner material amenable to an equivalent fluid description, such as rigid-frame porous materials and metamaterials. It has been validated against a semi-analytical solution in a 1D test case and against experimental measurements in a duct.

High end splitter-type silencers represent the object of the present paper, that summarizes the results obtained in an experimental campaign conducted in the framework of FlowAirS Project ITN FP7, Marie Curie Actions. Sound measurements on several fully functional splitters scaled 1:3 with Mach and Reynolds number similarity to the typical conditions in gas turbine exhaust stack have been performed. Different trailing edge shapes including tapered and serrated extensions have been tested. Only moderate changes in the measured noise have been found, despite a reduction of the total pressure loss that can be achieved by appropriate design of the splitter trailing edges. Considerable effects due to the covering perforated plate have been found. The results are analyzed applying Nelson and Morfey’s theory for obstacles in ducts, the scaling provides a good data collapsing, in particular for the frequencies above the cut-on.

A numerical method for aeroacoustic source characterization of induct components at frequencies beyond the cut-on frequencies of several acoustic modes is presented. Assuming linearity and time invariance, any ducted component can be fully characterized using a network (multi-port) model including source strength and scattering. A two-step multi-source approach is applied to numerical data in order to educe the multi-port characteristics. First, a scale resolving compressible flow simulation, here the Improved Delayed Detached Eddy Simulation (IDDES), is run to compute the channel flow that also contains the acoustic sources. Second, a linear acoustic computation, here the Linearized Navier Stokes Equations (LNSE), around a mean flow is solved for different acoustic loads to determine the component's scattering. The work uncovers the high potential of twostep numerical multi-port eduction methods. Particularly, it is shown that the acoustic source power spectra can be accurately extracted from IDDES data and the total acoustic power prediction is very good. Furthermore, a good result in the scattering data obtained from a second computationally inexpensive LNSE computation is achieved. The approach is interesting when describing mid-size duct systems, for example ventilation systems in aircraft and buildings, with a moderate number of higher order modes propagating in the considered frequency range. Therefore, the increasing availability of compressible flow data opens a wide field of applications.

Investigations on noise mitigation strategies for duct systems with a low number of propagating modes, as for instance found in middle size HVAC systems, are sparse. Here, the design of a silencer is presented based on cascades of modal filters and a Cremer silencer. The modal filter aims at mitigating spinning duct modes. The Cremer silencer is a silencer that realizes impedance for optimal attenuation for the plane wave at one frequency (Cremer impedance). The combination of both can create high damping both for planar and nonplanar waves. The modal filter consists of micro-perforated plates parallel to the duct axis in mean flow direction. As the concept of modal filters is unprecedented, general investigations of its acoustic behaviour are carried out by solving the Helmholtz equation numerically to investigate the effect of different plate positions, the number of plates and the plate impedance on the acoustic absorption. Finally, the combination of the Cremer silencer and the modal filter is investigated by performing measurements on prototypes. It is shown, that even though the damping of the modal filter is low compared to the Cremer silencer, improved damping and dissipation of fields consisting of plane waves and spinning modes can be achieved by combining the two types.

Sound transmission in a 180 ø bend containing a curved partition positioned on the axial center line is investigated theoretically and experimentally using equations for sound propagation in straight and curved ducts. Good agreement is obtained and small discrepancies are discussed. The partition is found to significantly alter the sound propagation through the bend and reasons for the different acoustic behavior are given.

This paper presents a general sub-structuring approach to predict the acoustic performance of ducted ventilation systems. The modeling principle is to determine the subsystem characteristics by calculating the transfer functions at their coupling interfaces, and the assembly is enabled by using a patch-based interface matching technique. For a particular example of a bended ventilation duct connecting an inlet and an outlet acoustic domain, a numerical model is developed to predict its sound attenuation performance. The prediction accuracy is thoroughly validated against finite element models. Through numerical examples, the rigid-walled duct is shown to provide relatively weak transmission loss (TL) across the frequency range of interest, and exhibit only the reactive behavior for sound reflection. By integrating sound absorbing treatment such as micro-perforated absorbers into the system, the TL can be significantly improved, and the system is seen to exhibit hybrid mechanisms for sound attenuation. The dissipative effect dominates at frequencies where the absorber is designed to be effective, and the reactive effect provides compensations at the absorption valleys attributed to the resonant behavior of the absorber. This ultimately maintains the system TL at a relatively high level across the entire frequency of interest. The TL of the system can be tuned or optimized in a very efficient way using the proposed approach due to its modular nature. It is shown that a balance of the hybrid mechanism is important to achieve an overall broadband attenuation performance in the design frequency range.

© Proceedings of ISMA 2018 - International Conference on Noise and Vibration Engineering and USD 2018 - International Conference on Uncertainty in Structural Dynamics. All rights reserved. This paper presents an experimental study of the aeroacoustic characteristics of Helmholtz resonators. The two-port characterization technique is applied to compute the scattering properties of the resonators from measured pressure spectra. The onset of aeroacoustic instabilities is investigated using an acoustic power balance based on the linear scattering matrix formulation. The influence of the grazing flow velocity on the aeroacoustic instability onset is discussed. Additionally, the effect of the orifice dimensions is analyzed with two different configurations. The results offer a valuable database for the validation of computational techniques for the prediction of whistling onset in duct systems.status: Published onlin

Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications. It is an important tool for instance in the development of mufflers for IC-engines. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. The plane wave acoustic two-port is a 2x2 matrix containing 4 complex quantities at each frequency. To experimentally determine these unknowns the acoustic state variables on the inlet and outlet side must be measured for two independent test cases. The two independent test cases can be created by: changing the acoustic load on the outlet side leading to the so-called two-load technique or by using one acoustic source on the inlet side and one acoustic source on the outlet side leading to the so-called two-source technique. In the latter case the independent test cases are created by first using the source on the inlet side and then the source on the outlet side. As pointed out by Åbom it is also possible to run both sources simultaneously to create more than two independent test cases. This over-determination could be used to improve the measurement results for instance if the data is contaminated by flow-noise. In this paper over-determination is tested by applying up to 5 different test cases. This procedure has been applied to a single orifice test object.

The purpose of this work is to optimize the acoustic performance of low cost, simple geometry mufflers by using microperforated panels (MPP) in their expansion chambers. The Transmission Loss (TL) given by a computed model is compared with laboratory measurements, both for the mufflers containing the microperforated panels and without them. The optimization calculation is based on the easy computing transfer matrix approach. Then, the Boundary Element Method (BEM) is used in order to compare the evaluation of the TL. Different configurations have been tested so as to detect the real effect of resonator absorbers based on microperforated panels in the expansion chambers. It is shown that their presence increases the TL at certain frequencies if their parameters are well chosen, but their dissipative effect is negligible when occurs at a reactive effect resonance. Thus, the MPPs can be an alternative to improve the TL for low frequencies when the reactive effect of the mufflers decreases.

In combustion devices, thermo-acoustic instabilities are often encountered and are formed by the complex interaction between acoustics, hydrodynamics and combustion. A common geometrical feature found in these devices is the sudden area expansion. Downstream of the area expansion a recirculation zone is formed together with an unstable shear layer when flow is present. This shear layer creates a pathway for interactions between the acoustic and the hydrodynamic field and this interaction could be a precursor for thermoacoustic instabilities. The work presented here takes a step to gain more insight into these interactions by experimentally investigating the aero-acoustic properties of a sudden area expansion with mean flow. The aero-acoustic properties are characterized by a linear two port model and the scattering matrix representation is used to relate the state variables upand downstream of the area expansion. The scattering coefficients of the area expansion have been determined for frequencies up to the first cut on frequency of the duct system and for a range of subsonic flow speeds. The measurements have been performed by applying a stepped sine excitation as sound excitation and by using the multi-microphone method.

In an earlier work the authors have presented a 1-D acoustic model for diesel particulate filters (DPFs). One shortcoming of this first model is the approximate treatment of the viscous and thermal losses along the narrow channels. In the present paper this issue is analyzed in more detail, by solving the convective acoustic wave equations for two neighboring channels simplified in the manner of the Zwikker and Kosten theory. From the solution the acoustic two-port has been calculated to predict the sound transmission losses for an entire DPF unit. The theoretical results are compared with experimental data for clean filter units at room temperature and the agreement is very good and better, in particular for very small Mach numbers, than for the earlier presented 1-D model. A modified 1-D model using the classical (exact) Kirchhoff solution for a plane wave in a narrow tube is also presented. This modified 1-D model is in close agreement with the predictions of the new model. Furthermore, the earlier proposed 1-D model, which assumes isothermal sound propagation, works satisfactorily up to 800-1000 Hz for a typical filter at operating (hot) conditions.

In an earlier work the authors have presented a 1-D acoustic model for diesel particulate filters (DPFs). One shortcoming of this first model is the approximate treatment of the viscous and thermal losses along the narrow channels. In the present paper this issue is analyzed in more detail, by solving the convective acoustic wave equations for two neighboring channels simplified in the manner of the Zwikker and Kosten theory. From the solution the acoustic two-port has been calculated to predict the sound transmission losses for an entire DPF unit. The theoretical results are compared with experimental data for clean filter units at room temperature and the agreement is very good and better, in particular for very small Mach numbers, than for the earlier presented 1-D model. A modified 1-D model using the classical (exact) Kirchhoff solution for a plane wave in a narrow tube is also presented. This modified 1-D model is in close agreement with the predictions of the new model. Furthermore, the earlier proposed 1-D model, which assumes isothermal sound propagation, works satisfactorily up to 800-1000 Hz for a typical filter at operating (hot) conditions.

In an earlier work the authors have presented a 1-D acoustic model for diesel particulate filters (DPFs). One shortcoming of this first model is the approximate treatment of the viscous and thermal losses along the narrow channels. In the present paper this issue is analyzed in more detail, by solving the convective acoustic wave equations for two neighboring channels simplified in the manner of the Zwikker and Kosten theory. From the solution the acoustic two-port has been calculated to predict the sound transmission losses for an entire DPF unit. The theoretical results are compared with experimental data for clean filter units at room temperature and the agreement is very good and better, in particular for very small Mach numbers, than for the earlier presented 1-D model. A modified 1-D model using the classical (exact) Kirchhoff solution for a plane wave in a narrow tube is also presented. This modified 1-D model is in close agreement with the predictions of the new model. Furthermore, the earlier proposed 1-D model, which assumes isothermal sound propagation, works satisfactorily up to 800-1000 Hz for a typical filter at operating (hot) conditions.

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Background: Exhaust noise is known to be a major pollutant in the environment and workplaces due to the development of industry and transportation. Exhaust noise can be reduced to normal levels by mufflers or silencers. A reactive muffler efficiently dampens noise at low frequencies by reflecting sound waves. Therefore, muffler design is of great importance in exhaust noise reduction. Transmission loss (TL) is an essential characteristic of mufflers, demonstrating their acoustical properties. Any acoustical appliance is selected based on its damping performance and reliability. Predicting TL through experimentation is different from theoretical calculations. Methods: In the present study, a double-expansion chamber muffler was designed as a reflective muffler on a laboratory scale by equations. Next, TL was evaluated by an impedance tube applying a 4-microphone technique to determine the acoustical performance of the designed muffler. Results: Findings revealed that the TL of the mu...

In combustion devices, thermo-acoustic instabilities are often encountered and are formed by the complex interaction between acoustics, hydrodynamics and combustion. A common geometrical feature found in these devices is the sudden area expansion. Downstream of the area expansion a recirculation zone is formed together with an unstable shear layer when flow is present. This shear layer creates a pathway for interactions between the acoustic and the hydrodynamic field and this interaction could be a precursor for thermoacoustic instabilities. The work presented here takes a step to gain more insight into these interactions by experimentally investigating the aero-acoustic properties of a sudden area expansion with mean flow. The aero-acoustic properties are characterized by a linear two port model and the scattering matrix representation is used to relate the state variables upand downstream of the area expansion. The scattering coefficients of the area expansion have been determined for frequencies up to the first cut on frequency of the duct system and for a range of subsonic flow speeds. The measurements have been performed by applying a stepped sine excitation as sound excitation and by using the multi-microphone method.

Several acoustic elements are used in internal combustion engine to tune engine intake/exhaust manifold systems. Components in intake and exhaust systems that create flow separation can for certain conditions and frequencies amplify incident sound waves. This vortex-sound phenomena is the origin for whistling, i.e., the production of tonal sound at frequencies close to the resonances of a duct system. One way of predicting whistling potential is to compute the acoustic power balance, i.e., the difference between incident and scattered sound power. This can readily obtained if the scattering matrix is known for the object. For the low frequency plane wave case this implies knowledge of the two-port data, which can be obtained by numerical and experimental methods. In this paper the development of multi-port models to describe linear acoustic problems in ducts with flow is presented. From an engineering point of view this field covers many important applications ranging from ventilati...

In a duct at high frequencies there is a high modal density and sound propagation can be modelled using ray acoustics. The acoustic performance of a duct network can also be simulated using simple power-based models. The application of such models is common practice for HVAC systems and large silencers for power generation. Several standards are available for the analysis of HVAC systems using power-based models, such as ASHRAE and VDI. For each element, the flow-generated sound power inside the element is estimated and added to the input sound power, and the output sound power level is calculated by subtracting the attenuation of the element. However, the existing standards neglect the reflected sound power and only accounts for the transmitted power. In this paper this assumption is investigated using two approaches that can include reflections. First, by using power based two-ports and secondly by combining this with a ray-tracing approach. Several common HVAC elements are considered, calculating the amount of power dissipated inside the element and the amount reflected. Two network cases are considered: purely reflective networks, and mixed reflective and dissipative networks. The paper also discusses using both simple mathematical models and ray-tracing based models in the same network.

Measurement of plane wave acoustic transmission properties, so called two-port data, of flow duct components is important in many applications. It is an important tool for instance in the development of mufflers for IC-engines. Measurement of two-port data is difficult when the flow velocity in the measurement duct is high because of the flow noise contamination of the measured pressure signals. The plane wave acoustic two-port is a 2x2 matrix containing 4 complex quantities at each frequency. To experimentally determine these unknowns the acoustic state variables on the inlet and outlet side must be measured for two independent test cases. The two independent test cases can be created by: changing the acoustic load on the outlet side leading to the so-called two-load technique or by using one acoustic source on the inlet side and one acoustic source on the outlet side leading to the so-called two-source technique. In the latter case the independent test cases are created by first u...

A MPP can be made of plastic or metal but for machinery or vehicle noise control plates of metal are more useful, since they are more robust and can simply be integrated in an existing design. This is of special interest in vehicle noise control where solutions which do not add weight are essential. For instance MPP:s have the potential to be used instead of porous materials in dissipative mufflers, which not only could save weight but also offer a non-fibrous alternative. Furthermore, since MPP:s have a large steady flow resistance they can be used as acoustically absorbing guide vanes at duct bends or in fans. For many of these new possible applications the MPP:s will both be subjected to flow, high temperatures as well as high sound levels. It was a Chinese scientist Maa [1] that first presented the idea of absorbing sound by a so called micro-perforated panel and presented a model for it. The definition provided by Maa basically defines a MPP as a perforated plate where the hole...

The reduction of the exhaust noise from internal combustion engine (IC-engine) is mainly managed by proper silencer design, while less attention is paid to the acoustic performance of the after treatment devices (ATD). It is known from the earlier studies, that the transmission loss of a typical ATD unit can be quite significant. An ATD unit for diesel engines is classically assembled from several specific parts such as selective catalytic reducers (SCR), diesel oxidation catalysts (DOC) and diesel particulate filters (DPF). One new alternative to the conventional DPF is the particle oxidation catalyst (POC®). The substrate used in the POC-X type filter consists of fine, corrugated metallic wire mesh screens piled askew and rolled into a cylindrical shape. In this paper an acoustic two-port simulation model for POC-X is sought starting from the classical Kirchhoff solution for prediction of the acoustic wave attenuation in narrow channels. According to experimental studies, correcti...

A numerical study of the aeroacoustic properties of a pair of orifices placed in a duct is presented. Two configurations with two different distances in between the orifices have been analysed to consider conditions with or without tonal noise sources. Large Eddy Simulations have been performed and validated with respect to experimental measurements for both aerodynamic and acoustic predictions. Data series from an acoustically excited LES have been post-processed through System Identification techniques to concurrently model noise sources and acoustic scattering. The model of the noise obtained has been used as to confirm the good prediction of the acoustic scattering.

The present paper deals with the identification of noise sources in internal ducted flows. Large Eddy Simulation (LES) and System Identification (SI) are combined in order to characterize simultaneously both the acoustic passive scattering and the active noise generation of an orifice placed in a duct. This system is studied within the plane wave linear acoustic regime, through the so-called multi-ports representation. The acoustic response to a given acoustic perturbation is modeled by the system acoustic scattering matrix, whereas the noise source is represented by a noise vector. In this work, the scattering matrix and the active noise generation are determined introducing a parametric system identification analysis based on a Box-Jenkins model structure. This model structure is an extension of the common correlation analysis approach and includes a dynamic model of the noise sources. The identified passive acoustic scattering is in good agreement with experimental results. The identified noise source shows a nonlinear behavior for different levels of excitation and it is compared with the noise extracted from a non-excited LES.

The experiment involves creating a sound wave that propagates down a pipe with 8 transducers attached at equally spaced intervals of 0.01016 m. The numerical method-the Cross Correlation Method, used to solve for the phase component, creates a high correlation value, but the speed of sound varies immensely. The method involves a Fast Fourier Transform (FFT) of the collected data, which is used to find the phase of the sound wave, and the slope of the position versus time graph, which is used to calculate the speed of sound. This high correlation value shows that the data are correct, but the numerical method for analyzing the data is incorrect.

Sound reflection at a duct open end induces resonant amplification of acoustic energy in the duct and causes high intensity sound radiation. This phenomenon sometime causes low frequency noise pollution radiation from pipe and some self-sustained acoustic oscillation in machines. In order to reduce such kind of acoustic resonance, acoustic damping should be added to the duct. Sound reflection coefficient at a duct open end is determined by its outlet shape and wave length. Sometimes a non-reflecting horn with sound absorbing materials is used at the open end in order to realize non-reflecting condition. But, as for low frequency sound, it becomes too huge to be set on real application. In this paper two different outlet shapes; unflanged straight duct and horn that are made of Micro-Perforated Panel (MPP), which can provide wide-band absorption without fibrous and porous materials will be presented. Analytical and 3D Finite element solutions for both cases are presented and used throughout this analysis. The numerical and analytical solutions are compared with measurements and it has been shown that sound reflection and radiation at that open end can be remarkably reduced. The duct side wall next to open end that is made of appropriate MPP plate can reduce the radiated sound up to 5 dB(A) when L/D =10 (pipe length to its diameter). It can be used as an efficient noise control element in automotive intake and exhaust systems.

This paper describes the application of an acoustic 2-port model to describe flow generated noise in ducts. An experimental procedure that enables determination of both the passive (the scattering matrix) as well as the active (source) 2-port data is described. The method is applied to investigate the aero-acoustics of an orifice plate in a duct. The passive data is compared with a simple quasi-stationary model and the active part is analyzed using a scaling law procedure, based on the assumption of a compact dipole source.

Modal or power based methods • In the low frequency (plane wave) range (f < f cut-on) a source is strongly coupled to a system and the acoustic output (power) can vary strongly. • In the mid frequency range up to (2-3)x f cut-on , plane + non-plane waves exist. Also in this range strong coupling between source and system is possible. • In the high frequency range f > 3xf cut-on , sound propagates as rays, there is no coupling between a source and a system and the acoustic power equals the free field value.

Modal or power based methods • In the low frequency (plane wave) range (f < f cut-on) a source is strongly coupled to a system and the acoustic output (power) can vary strongly. • In the mid frequency range up to (2-3)x f cut-on , plane + non-plane waves exist. Also in this range strong coupling between source and system is possible. • In the high frequency range f > 3xf cut-on , sound propagates as rays, there is no coupling between a source and a system and the acoustic power equals the free field value.

This paper describes the application of an acoustic 2-port model to describe flow generated noise in ducts. An experimental procedure that enables determination of both the passive (the scattering matrix) as well as the active (source) 2-port data is described. The method is applied to investigate the aero-acoustics of an orifice plate in a duct. The passive data is compared with a simple quasi-stationary model and the active part is analyzed using a scaling law procedure, based on the assumption of a compact dipole source.

The aim of the work reported in this paper was to study the characterization of fluid machines (e.g., fans, pumps) as sources of fluid-borne sound in connected pipe or duct systems. Of special interest is the possibility of extending earlier published methods beyond the plane wave region: i.e., to include higher order modes effects. For machines which can be modelled as an acoustic multi-port source, a method which makes it possible to measure the source data is suggested together with a modal decomposition technique. In order to verify and test the measurement method and modal decomposition technique, measurements were first carried out on a small test-rig without flow, with an ordinary loudspeaker as a source. Then the method was tested on an axial fan mounted to the end of a duct. In both cases ten microphone positions, divided equally between two duct cross-sections, were used to separate the waves (five modes) into left-and right-going components. It was verified that the measured source data can be used to predict the sound field created by the fan at a given acoustic load.

A general scheme to predict the sound pressure radiated from an automotive exhaust system is presented. The scheme is demonstrated with a step-wise computational procedure on a model system comprised of an electroacoustic driver source, an expansion chamber muffler, and a tailpipe radiating to free space. The predicted sound pressure level spectra agree well with the corresponding measured spectra for various test eases with zero mean flow. Also, studies are made of the theoretical effects of variations of the source impedance on the sound pressure radiated from the model system.

Investigations on noise mitigation strategies for duct systems with a low number of propagating modes, as found in middle size HVAC systems, are sparse. Here, the design of a silencer is presented based on cascades of modal filters and a Cremer silencer. The modal filter aims at mitigating spinning duct modes. The Cremer silencer is a silencer that realizes impedance for optimal attenuation for the plane wave at one frequency (Cremer impedance). The combination of both can create high damping both for planar and non-planar waves. The modal filter consists of micro-perforated plates parallel to the duct axis in mean flow direction. As the concept of modal filters is unprecedented, general investigations of its acoustic behaviour are carried out by solving the Helmholtz equation numerically to investigate the effect of different plate positions, the number of plates and the plate impedance on the acoustic absorption. Finally, the combination of the Cremer silencer and the modal filter is investigated by performing measurements on prototypes. It is shown, that even though the damping of the modal filter is low compared to the Cremer silencer, improved damping and dissipation of fields consisting of plane waves and spinning modes can be achieved by combining the two types.

Methods to design test-procedures for acoustic multi-ports in ducts with a focus on pressure sampling positions for accurate modal decomposition are demonstrated. Acoustic fields up-and downstream of an induct acoustic element are excited by external sources and decomposed into transmitted and reflected aeroacoustic modal pressure amplitudes in order to first determine the acoustic scattering of the element. Secondly, the determination of the element source strength requires tests with no external sources, but with known terminations and scattering data. Unfavourable source and sensor positions lead to mode coupling and to ill-conditioned or even singular decomposition matrices, which results in high amplifications of uncertainties within the wave decomposition. An unoptimised but over-determined assembly is compared with a setup containing a minimum of sensors but with optimised positions. Lower uncertainty amplification, despite the usage of fewer sensors, is achieved for most frequencies, especially after the cut-on of t he higher order acoustic modes. A genetic algorithm (GA) is used to achieve this optimised setup by minimising the condition number of the decomposition matrix, which is a multi-dimensional optimisation problem with numerous local minima. To estimate the stability of the optimised configuration, a Monte-Carlo Method (MCM) is deployed to introduce normal distributed complex pressure uncertainties into the decomposition. In order to estimate the wave number, different approaches are compared-namely the classical non-dissipative wav e number estimate, an extended Kirchhoff method for viscous-thermal damping and an eigenvalue solution of the Linearised Navier Stokes Equations by Dokumaci. The presented decomposition method is not only applicable to measurement data but is equally useful to post-process results from numerical computations.

The effects of discontinuities on higher order modes is important in the design of passive and active noise control systems in ducts which operate near or above the cut-off frequency of the duct. Accurate acoustic monitoring of mechanical systems in ducts at frequencies near and above the cut-off frequency of the first mode must include the effects of discontinuities.

In this study, the effect of perforation edge profile in micro-perforated plates (MPPs) is investigated and compared with the sharp edge case. MPPs are metal plates with small holes (perforations) that absorb incident sound waves using the Stokes boundary layers developed inside and in the vicinity of the perforations. The perforation diameter should be optimized so that the Stokes layers cover almost the entire perforation thus maximizing the absorbed sound energy due to viscosity. Hence, designing MPPs is a tuning problem. The existing analytical model for evaluating the transfer impedance of the MPPs is based on the assumption that the edge profiles of the perforations are sharp. In other words, the angle between the plate surface and the inner surface of the perforation is 90 • ;. This case is not very practical to achieve in mass production of MPPs, since the perforations are formed by drilling and / or punching in manufacturing. To investigate the effect of the edge profile on...

This paper describes the application of an acoustic 2-port model to describe flow generated noise in ducts. An experimental procedure that enables determination of both the passive (the scattering matrix) as well as the active (source) 2-port data is described. The method is applied to investigate the aero-acoustics of an orifice plate in a duct. The passive data is compared with a simple quasi-stationary model and the active part is analyzed using a scaling law procedure, based on the assumption of a compact dipole source.