Ysbrand Wijnant - Academia.edu (original) (raw)
Papers by Ysbrand Wijnant
Former studies have shown that the Local Plane Wave (LPW) method can be used to measure the (effe... more Former studies have shown that the Local Plane Wave (LPW) method can be used to measure the (effective) in-situ sound absorption coefficient of any surface. The LPW-method is based on a local plane wave assumption, in which the normal component of the sound field in each spatial coordinate is approximated by an incident and a reflected plane wave. This is contrary to conventional methods that rely on a known global sound field, like the impedance tube (plane wave) and reverberation room (diffuse field) methods. The LPW-method can therefore be applied in sound fields for which a model is not available. This has been numerically and experimentally validated using a probe consisting of 8 MEMS-microphones in an open cubical structure. In this paper, the LPW-method will be applied to a rigid spherical microphone array. The wellknown plane wave expansion in spherical harmonics will be used to formulate a local model of the sound field in the presence of a rigid sphere. The resulting model will be used to infer the sound field for the case the rigid sphere would not be present. The (effective) in-situ sound absorption coefficient is then computed from the parameters of this inferred sound field. Using numerical and experimental results, the validity and usefulness of the LPW-method to measure sound absorption using a spherical microphone array will then be shown.
The acoustic absorption coefficient is a number that indicates which fraction of the incident aco... more The acoustic absorption coefficient is a number that indicates which fraction of the incident acoustic power impinging on a surface is being absorbed. The incident acoustic power is obtained by spatial integration of the incident intensity, which is (classically) defined as the time-averaged intensity associated with the incident sound field. The measurement of the effective, in situ, sound absorption coefficient is problematic as the determination thus requires a decomposition of the sound field in an incident and reflected field which, generally, is virtually impossible to do. This paper introduces an alternative coefficient with which the effective acoustic absorption can be expressed. This coefficient is based on an alternative definition of the incident intensity; the time average of the positive values of the instantaneous intensity. The alternative coefficient is much easier to use in a sense that it follows directly from an in situ, instantaneous intensity measurement. The coefficient does not rely on any assumptions other than the assumption that the linearized wave equation is satisfied (and thus the acoustic energy corollary). As a result, one does not need to decompose the sound field in incident and reflected waves. Hence, one does not need to have prior information about the incident sound field. Accordingly, one does not need to have prior information about the source. The coefficient can be determined in any sound field, either transient or stationary, free field and diffuse/(semi-)reverberant sound fields. The alternative coefficient is illustrated by means of several numerical examples.
Solving the Helmholtz equation for high wavenumbers is a major challenge. Since one needs at leas... more Solving the Helmholtz equation for high wavenumbers is a major challenge. Since one needs at least some elements per wavelength, the computational effort in finite element or boundary element calculations increases drastically with increasing wavenumbers. However, for exterior problems in unbounded domains, the amplitude and phase are smooth and non-oscillatory functions (at least some distance away from the radiating object). Therefore, we propose to solve the Helmholtz equation in terms of amplitude and phase instead of pressure. The paramount advantage of this approach is that any common discretization remains accurate for high wavenumbers. A drawback of the method is that the equations for amplitude and phase are non-linear and hence need to be solved iteratively. Substitution of p = Ae iφ in the Helmholtz equation yields two, non-linear, coupled, real-valued, differential equations. These equations have been reported some decades ago but have only been used to trace 'rays' for given solutions of the Helmholtz equation. To eventually be able to include inhomogeneous regions and fluid-structure interaction efficiently, a finite element discretization is preferred over a boundary element discretization. Therefore, finite elements were used to discretize the governing equations. We present several solutions and, despite of its non-linearity, show the efficiency of the method for high wavenumbers.
Three efficient alternatives to the model in COMSOL's thermoacoustics interface are presented. Th... more Three efficient alternatives to the model in COMSOL's thermoacoustics interface are presented. The higher efficiency of these models are explained from theory and are demonstrated by means of two examples.
Acta Acustica United With Acustica, Mar 1, 2014
Using alocal plane wave assumption, one can determine the normal incidence sound absorption coeff... more Using alocal plane wave assumption, one can determine the normal incidence sound absorption coefficient of a surface by measuring the acoustic pressure and the particle velocity normal to that surface. As the measurement surface lies in front of the material surface, the measured active and incident acoustic power will generally deviate from those at the material surface, leading to ap ossibly inaccurate sound absorption coefficient. This phenomenon is particularly pronounced for poorly absorbing surfaces if sound is not normally incident overthe whole material surface. Based on an analytical model, it is shown that the accuracycan be improvedbyextending the measurement surface upon which the active acoustic power is measured. Experimental results demonstrate the usefulness of this approach, in particular for poorly absorbing surfaces.
The well known wave equation describes isentropic wave propagation. In this equation, non-isentro... more The well known wave equation describes isentropic wave propagation. In this equation, non-isentropic boundary layer effects are neglected. This is allowed if the characteristic dimensions of the acoustic domain are large with respect to the thickness of the boundary layers. However, in small acoustic devices such as hearing aid loudspeakers, the boundary layer effects are significant and can not be neglected. A model that describes viscothermal wave propagation is needed to model such devices. For viscothermal wave propagation, the compressibility of air depends on the thermal behavior that can range from adiabatic to isothermal. Moreover, the propagation behavior can range from propagation with negligible viscosity to propagation with negligible inertia (Stokes flow). This complete range is accurately described by the low reduced frequency model. This model's major drawback is that it is only defined for simple geometries such as thin layers and narrow tubes. It is not valid for arbitrary geometries. To overcome this drawback, a three dimensional viscothermal finite element has been developed. Like the LRF model, it covers the complete range from isothermal Stokes flow to isentropic acoustics. As opposed to the LRF model, the viscothermal finite element can be used to analyze complicated geometries. This paper presents the weak formulation of the finite element. Furthermore, two examples are presented in which the results of the finite element models are compared to measurements.
The EU project FACE (Friendly Aircraft Cabin Environment) aims to improve the environmental comfo... more The EU project FACE (Friendly Aircraft Cabin Environment) aims to improve the environmental comfort in aircraft cabins. As part of this project, this paper focuses on the reduction of noise in aircraft cabins. For modern aircraft flying at cruise conditions, this cabin noise is known to be dominated by turbulent boundary layer noise. The purpose of this work is to reduce the resulting sound pressure levels in the cabin by means of optimised sound absorbing trim panels with quarter-wave resonators. Sound absorption with quarter-wave resonators is mainly realised by dissipation of sound energy as a result of viscous and thermal losses. The viscothermal wave propagation of the air inside the resonators is efficiently and accurately described by the so-called low reduced frequency model. By optimisation of the dimensions of the resonators, desired sound absorption characteristics can be obtained for different specified frequency ranges. This means that the panels can be tailored to different positions in the aircraft cabin with different prevailing sound pressure levels. Results of optimisations for various frequency ranges show that a very good agreement is obtained between the desired and the calculated absorption curves. With the same optimisation procedure, panels have also been tuned for the dominant frequency range of a sound spectrum measured in a modern aircraft. Experimental validation of the numerically predicted optimal configurations, by means of impedance tube measurements, shows that a fairly good agreement is obtained between the numerical and experimental results.
Recently a new acoustic measurement device, the SonoCat, has become available. The probe consists... more Recently a new acoustic measurement device, the SonoCat, has become available. The probe consists of 8 MEMSmicrophones, embedded in a small rigid sphere. By accurately calibrating the amplitude and phase mismatch betweenthe microphones, the probe is able to measure the sound intensity accurately. In this paper, we will explain the methodthat has been used to derive the sound intensity from the measured pressure signals and how the effect of the presenceof the sphere has been accounted for. In addition, we will show experimental results, obtained in the anechoic room ofthe University of Twente, comparing the sound intensity, as measured by the probe, with a benchmark sound intensityprobe. The accuracy and precision of the new probe will be discussed.
![Research paper thumbnail of Experimental Validation of an Optimised Resonator Configuration for Sound Absorption [Poster]](https://attachments.academia-assets.com/106592819/thumbnails/1.jpg)
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A rolling tyre radiates noise in all directions. Conventional measurement techniques for the soun... more A rolling tyre radiates noise in all directions. Conventional measurement techniques for the sound absorption of road surfaces, however, only give the absorption coefficient for normal incidence. The absorption coefficient for oblique incidence is often computed assuming a locally reacting surface. In this paper, a measurement technique is described with which it is possible to perform in situ sound absorption measurements for oblique incidence. The measurements are performed with a small 3D microphone array. The theory behind the measurement technique is based on the local plane wave assumption. In this paper, an approach is proposed to determine whether a surface behaves as a locally reacting surface or as a non-locally reacting surface, which is an important characteristic for optimising the noise absorption properties of asphalt pavements and for modelling techniques. Preliminary measurements at various angles of incidence are performed to demonstrate this approach as well as measurements of the absorption coefficient at normal incidence to validate the microphone array technique with impedance tube measurements.
Journal of the Acoustical Society of America, Sep 1, 2005
Optical drives inside PCs typically operate at high speed, which may result in significant noise.... more Optical drives inside PCs typically operate at high speed, which may result in significant noise. These drives function both as airborne and structural vibration sources. Three main paths can be distinguished through which noise is emitted to the surroundings: (1) the vibrations of the front of the optical drive emit noise directly into the far field, (2) the sound waves induced by the vibrations of the faces of the optical drives inside the enclosure excites the enclosure, and (3) the structural path from the mounting points of the optical drive to the enclosure. The techniques used to determine the contribution of each path are described and the results of an experimental setup are presented in this paper. The contribution of the structural path is determined by comparing the results of a normal setup with the result of a setup for which the structural path is eliminated by mounting the CD drive on a support that is structurally uncoupled from the PC. Direct measurements of the pressure with a scanning microphone were made. Also, a reconstruction of the pressure field using the boundary element method based on the measured surface velocities of the main radiating surfaces was made.
One of the main noise sources in computers are the cooling fans. Tonal noise at the rotational fr... more One of the main noise sources in computers are the cooling fans. Tonal noise at the rotational frequency of the fan, the blade passing frequency (BPF) and its higher harmonics are important in fan noise. So called 'side-resonators' integrated in the duct of an in-duct axial fan cause an impedance change in the duct and, having proper dimensions and correct position, act as an acoustic mirror reflecting the noise back to the fan. As a result, noise emitted from the computer can be reduced. In this paper, a model describing viscothermal wave propagation in the duct and side-resonator is outlined. The model can be used to determine the resonator dimension and position to optimally reflect noise near the BPF. The developed model is compared with the models for prismatic tube and cylindrical resonators and the physical differences between these three resonators are explained. Preliminary results of a parameter study are presented. The results point out that the range and magnitude of the effective frequency band of a resonator setup can be successfully manipulated by changing the different dimensions of the setup.
Journal of the Acoustical Society of America, Aug 21, 2012
The accuracy of a method [Wijnant et al., Proc. of ISMA 31, Leuven, Belgium (2010), Vol. 31] for ... more The accuracy of a method [Wijnant et al., Proc. of ISMA 31, Leuven, Belgium (2010), Vol. 31] for measurement of the effective areaaveraged in situ sound absorption coefficient is investigated. Based on a local plane wave assumption, this method can be applied to sound fields for which a model is not available. Investigations were carried out by means of finite element simulations for a typical case. The results show that the method is a promising method for determining the effective areaaveraged in situ sound absorption coefficient in complex sound fields.
Former studies have shown that the Local Plane Wave (LPW) method can be used to measure the (effe... more Former studies have shown that the Local Plane Wave (LPW) method can be used to measure the (effective) in-situ sound absorption coefficient of any surface. The LPW-method is based on a local plane wave assumption, in which the normal component of the sound field in each spatial coordinate is approximated by an incident and a reflected plane wave. This is contrary to conventional methods that rely on a known global sound field, like the impedance tube (plane wave) and reverberation room (diffuse field) methods. The LPW-method can therefore be applied in sound fields for which a model is not available. This has been numerically and experimentally validated using a probe consisting of 8 MEMS-microphones in an open cubical structure. In this paper, the LPW-method will be applied to a rigid spherical microphone array. The wellknown plane wave expansion in spherical harmonics will be used to formulate a local model of the sound field in the presence of a rigid sphere. The resulting model will be used to infer the sound field for the case the rigid sphere would not be present. The (effective) in-situ sound absorption coefficient is then computed from the parameters of this inferred sound field. Using numerical and experimental results, the validity and usefulness of the LPW-method to measure sound absorption using a spherical microphone array will then be shown.
The acoustic absorption coefficient is a number that indicates which fraction of the incident aco... more The acoustic absorption coefficient is a number that indicates which fraction of the incident acoustic power impinging on a surface is being absorbed. The incident acoustic power is obtained by spatial integration of the incident intensity, which is (classically) defined as the time-averaged intensity associated with the incident sound field. The measurement of the effective, in situ, sound absorption coefficient is problematic as the determination thus requires a decomposition of the sound field in an incident and reflected field which, generally, is virtually impossible to do. This paper introduces an alternative coefficient with which the effective acoustic absorption can be expressed. This coefficient is based on an alternative definition of the incident intensity; the time average of the positive values of the instantaneous intensity. The alternative coefficient is much easier to use in a sense that it follows directly from an in situ, instantaneous intensity measurement. The coefficient does not rely on any assumptions other than the assumption that the linearized wave equation is satisfied (and thus the acoustic energy corollary). As a result, one does not need to decompose the sound field in incident and reflected waves. Hence, one does not need to have prior information about the incident sound field. Accordingly, one does not need to have prior information about the source. The coefficient can be determined in any sound field, either transient or stationary, free field and diffuse/(semi-)reverberant sound fields. The alternative coefficient is illustrated by means of several numerical examples.
Solving the Helmholtz equation for high wavenumbers is a major challenge. Since one needs at leas... more Solving the Helmholtz equation for high wavenumbers is a major challenge. Since one needs at least some elements per wavelength, the computational effort in finite element or boundary element calculations increases drastically with increasing wavenumbers. However, for exterior problems in unbounded domains, the amplitude and phase are smooth and non-oscillatory functions (at least some distance away from the radiating object). Therefore, we propose to solve the Helmholtz equation in terms of amplitude and phase instead of pressure. The paramount advantage of this approach is that any common discretization remains accurate for high wavenumbers. A drawback of the method is that the equations for amplitude and phase are non-linear and hence need to be solved iteratively. Substitution of p = Ae iφ in the Helmholtz equation yields two, non-linear, coupled, real-valued, differential equations. These equations have been reported some decades ago but have only been used to trace 'rays' for given solutions of the Helmholtz equation. To eventually be able to include inhomogeneous regions and fluid-structure interaction efficiently, a finite element discretization is preferred over a boundary element discretization. Therefore, finite elements were used to discretize the governing equations. We present several solutions and, despite of its non-linearity, show the efficiency of the method for high wavenumbers.
Three efficient alternatives to the model in COMSOL's thermoacoustics interface are presented. Th... more Three efficient alternatives to the model in COMSOL's thermoacoustics interface are presented. The higher efficiency of these models are explained from theory and are demonstrated by means of two examples.
Acta Acustica United With Acustica, Mar 1, 2014
Using alocal plane wave assumption, one can determine the normal incidence sound absorption coeff... more Using alocal plane wave assumption, one can determine the normal incidence sound absorption coefficient of a surface by measuring the acoustic pressure and the particle velocity normal to that surface. As the measurement surface lies in front of the material surface, the measured active and incident acoustic power will generally deviate from those at the material surface, leading to ap ossibly inaccurate sound absorption coefficient. This phenomenon is particularly pronounced for poorly absorbing surfaces if sound is not normally incident overthe whole material surface. Based on an analytical model, it is shown that the accuracycan be improvedbyextending the measurement surface upon which the active acoustic power is measured. Experimental results demonstrate the usefulness of this approach, in particular for poorly absorbing surfaces.
The well known wave equation describes isentropic wave propagation. In this equation, non-isentro... more The well known wave equation describes isentropic wave propagation. In this equation, non-isentropic boundary layer effects are neglected. This is allowed if the characteristic dimensions of the acoustic domain are large with respect to the thickness of the boundary layers. However, in small acoustic devices such as hearing aid loudspeakers, the boundary layer effects are significant and can not be neglected. A model that describes viscothermal wave propagation is needed to model such devices. For viscothermal wave propagation, the compressibility of air depends on the thermal behavior that can range from adiabatic to isothermal. Moreover, the propagation behavior can range from propagation with negligible viscosity to propagation with negligible inertia (Stokes flow). This complete range is accurately described by the low reduced frequency model. This model's major drawback is that it is only defined for simple geometries such as thin layers and narrow tubes. It is not valid for arbitrary geometries. To overcome this drawback, a three dimensional viscothermal finite element has been developed. Like the LRF model, it covers the complete range from isothermal Stokes flow to isentropic acoustics. As opposed to the LRF model, the viscothermal finite element can be used to analyze complicated geometries. This paper presents the weak formulation of the finite element. Furthermore, two examples are presented in which the results of the finite element models are compared to measurements.
The EU project FACE (Friendly Aircraft Cabin Environment) aims to improve the environmental comfo... more The EU project FACE (Friendly Aircraft Cabin Environment) aims to improve the environmental comfort in aircraft cabins. As part of this project, this paper focuses on the reduction of noise in aircraft cabins. For modern aircraft flying at cruise conditions, this cabin noise is known to be dominated by turbulent boundary layer noise. The purpose of this work is to reduce the resulting sound pressure levels in the cabin by means of optimised sound absorbing trim panels with quarter-wave resonators. Sound absorption with quarter-wave resonators is mainly realised by dissipation of sound energy as a result of viscous and thermal losses. The viscothermal wave propagation of the air inside the resonators is efficiently and accurately described by the so-called low reduced frequency model. By optimisation of the dimensions of the resonators, desired sound absorption characteristics can be obtained for different specified frequency ranges. This means that the panels can be tailored to different positions in the aircraft cabin with different prevailing sound pressure levels. Results of optimisations for various frequency ranges show that a very good agreement is obtained between the desired and the calculated absorption curves. With the same optimisation procedure, panels have also been tuned for the dominant frequency range of a sound spectrum measured in a modern aircraft. Experimental validation of the numerically predicted optimal configurations, by means of impedance tube measurements, shows that a fairly good agreement is obtained between the numerical and experimental results.
Recently a new acoustic measurement device, the SonoCat, has become available. The probe consists... more Recently a new acoustic measurement device, the SonoCat, has become available. The probe consists of 8 MEMSmicrophones, embedded in a small rigid sphere. By accurately calibrating the amplitude and phase mismatch betweenthe microphones, the probe is able to measure the sound intensity accurately. In this paper, we will explain the methodthat has been used to derive the sound intensity from the measured pressure signals and how the effect of the presenceof the sphere has been accounted for. In addition, we will show experimental results, obtained in the anechoic room ofthe University of Twente, comparing the sound intensity, as measured by the probe, with a benchmark sound intensityprobe. The accuracy and precision of the new probe will be discussed.
A rolling tyre radiates noise in all directions. Conventional measurement techniques for the soun... more A rolling tyre radiates noise in all directions. Conventional measurement techniques for the sound absorption of road surfaces, however, only give the absorption coefficient for normal incidence. The absorption coefficient for oblique incidence is often computed assuming a locally reacting surface. In this paper, a measurement technique is described with which it is possible to perform in situ sound absorption measurements for oblique incidence. The measurements are performed with a small 3D microphone array. The theory behind the measurement technique is based on the local plane wave assumption. In this paper, an approach is proposed to determine whether a surface behaves as a locally reacting surface or as a non-locally reacting surface, which is an important characteristic for optimising the noise absorption properties of asphalt pavements and for modelling techniques. Preliminary measurements at various angles of incidence are performed to demonstrate this approach as well as measurements of the absorption coefficient at normal incidence to validate the microphone array technique with impedance tube measurements.
Journal of the Acoustical Society of America, Sep 1, 2005
Optical drives inside PCs typically operate at high speed, which may result in significant noise.... more Optical drives inside PCs typically operate at high speed, which may result in significant noise. These drives function both as airborne and structural vibration sources. Three main paths can be distinguished through which noise is emitted to the surroundings: (1) the vibrations of the front of the optical drive emit noise directly into the far field, (2) the sound waves induced by the vibrations of the faces of the optical drives inside the enclosure excites the enclosure, and (3) the structural path from the mounting points of the optical drive to the enclosure. The techniques used to determine the contribution of each path are described and the results of an experimental setup are presented in this paper. The contribution of the structural path is determined by comparing the results of a normal setup with the result of a setup for which the structural path is eliminated by mounting the CD drive on a support that is structurally uncoupled from the PC. Direct measurements of the pressure with a scanning microphone were made. Also, a reconstruction of the pressure field using the boundary element method based on the measured surface velocities of the main radiating surfaces was made.
One of the main noise sources in computers are the cooling fans. Tonal noise at the rotational fr... more One of the main noise sources in computers are the cooling fans. Tonal noise at the rotational frequency of the fan, the blade passing frequency (BPF) and its higher harmonics are important in fan noise. So called 'side-resonators' integrated in the duct of an in-duct axial fan cause an impedance change in the duct and, having proper dimensions and correct position, act as an acoustic mirror reflecting the noise back to the fan. As a result, noise emitted from the computer can be reduced. In this paper, a model describing viscothermal wave propagation in the duct and side-resonator is outlined. The model can be used to determine the resonator dimension and position to optimally reflect noise near the BPF. The developed model is compared with the models for prismatic tube and cylindrical resonators and the physical differences between these three resonators are explained. Preliminary results of a parameter study are presented. The results point out that the range and magnitude of the effective frequency band of a resonator setup can be successfully manipulated by changing the different dimensions of the setup.
Journal of the Acoustical Society of America, Aug 21, 2012
The accuracy of a method [Wijnant et al., Proc. of ISMA 31, Leuven, Belgium (2010), Vol. 31] for ... more The accuracy of a method [Wijnant et al., Proc. of ISMA 31, Leuven, Belgium (2010), Vol. 31] for measurement of the effective areaaveraged in situ sound absorption coefficient is investigated. Based on a local plane wave assumption, this method can be applied to sound fields for which a model is not available. Investigations were carried out by means of finite element simulations for a typical case. The results show that the method is a promising method for determining the effective areaaveraged in situ sound absorption coefficient in complex sound fields.