Diffuse Acoustic Field Excitation Research Articles

Coiled quarter wavelength resonators for low-frequency sound absorption under plane wave and diffuse acoustic field excitations

To reduce their space requirement, quarter wavelength resonators are coiled up into a spiral to have the shape of a flat cylinder. The practical implementation of these resonators is tested using 3D printing, laser cutting and machining. Numerical simulations and impedance tube measurements are first compared for a normally incident plane wave excitation. This step allows to evaluate the effect of the resonator’s inlet diameter, internal wall thickness, overall height as well as its constitutive material. A series of samples are then 3D printed following the identified optimal geometrical configuration. An array of resonators is tested alone and in combination with a glass wool layer under diffuse acoustic field excitation in a small reverberation chamber. The obtained results show that coiled quarter wavelength resonators can provide large sound absorption at their lowest resonance frequency and under both considered excitations, even if the largest external dimension of the resonators is only 1/18th of the targeted acoustic wavelength.

Laboratory and in situ sound absorption measurement under a synthetized diffuse acoustic field

This article reports numerical and experimental results concerning the estimation of the diffuse field sound absorption coefficient of several different materials under a synthetized diffuse acoustic field excitation in laboratory and in situ conditions. The proposed measurement method is based on a sound field reproduction approach and a synthetic array of acoustic monopoles facing the material to be tested. Numerical simulations are first conducted to optimize the geometrical parameters of the method and to compute theoretical sound absorption coefficients of the considered materials. Measurements on a set of six typical acoustic materials are then conducted following the standardized reverberant room method as well as the proposed approach in a hemi-anechoic room and in two realistic rooms. Albeit showing limitations in the low-frequency domain, the proposed method enables a significant reduction of the tested specimen dimensions compared with the reverberant room method and allows performing tests in non-ideal acoustic environments.

Simulating the sound transmission loss of complex curved panels with attached noise control materials using periodic cell wavemodes

The sound transmission loss of complex curved aircraft panels under diffuse acoustic field excitation is experimentally and numerically studied. Two different aircraft sidewall panels are considered: a thick composite sandwich panel and a thin aluminium panel with stiffening elements (stringers and frames). Both bare configuration and with attached soundproofing material are tested in laboratory conditions in coupled rooms. The numerical approach relies on a wave finite element method including modal order reduction at cell scale and an extension based on the transfer matrix method, for the inclusion of poroelastic treatments. The results obtained show that the proposed numerical scheme is efficient for predicting the sound transmission loss of such complex structures.

Schemes for the sound transmission of flat, curved and axisymmetric structures excited by aerodynamic and acoustic sources

The paper describes a methodology for computing the sound transmission loss of any flat, curved and cylindrical, homogeneous and periodic structure, under any type of acoustic and/or aerodynamic load. An approximate excitation model is introduced to reproduce uncorrelated and spatially-correlated loads using a wavenumber integration of surface waves. Then, a wave finite element formulation is developed and interfaced with the excitation models in order to cover industrially-relevant case studies. Analytical, numerical and experimental transmission losses are presented for validation purposes. Finite size effects are also taken into account using a spatial windowing and a cylindrical analogy, for curved structures. Different periodic-cell designs are also compared and investigated under turbulent boundary layer and diffuse acoustic field excitations.

Time-space identification of mechanical impacts and distributed random excitations on plates and membranes

The identification of dynamic loads acting on structures is a key aspect of several engineering domains involving structure-borne sound and vibration problems, stress analysis, or even the study of fatigue-induced structural damages. This work is concerned with the reconstruction of localized transient and distributed random excitations on plates and membranes from their measured vibration response. In previous investigations by the authors, the virtual fields method, an identification approach based on the principle of virtual work, was employed to identify mechanical and acoustic loadings applied to a bending panel. However, vibration data were obtained using scanning laser Doppler vibrometry, which limits the application of the virtual fields method to stationary excitations in both space and frequency. In contrast, the deflectometry technique used here is an optical method that directly provides a full-field measurement of local slopes. With the addition of a high-speed camera, the measurements are resolved in both space and time, enabling the study of nonstationary excitations. Moreover, since the acquisition time is independent of the number of measurement points, high spatial density measurements can be performed in seconds. This paper reviews the principles of the virtual fields method for nonstationary excitations on plates and membranes. The deflectometry technique is then demonstrated and experimental reconstruction results on an aluminum panel are presented for two different load cases: impacting metal marbles (multiple unknown transient excitations) and a diffuse acoustic field excitation. Finally, the identification of acoustic and aerodynamic (turbulent boundary layer) excitations is considered using a membrane as the receiving structure.

Improving sound transmission loss at ring frequency of a curved panel using tunable 3D-printed small-scale resonators.

An important dip in the sound transmission loss of curved panels occurs at the ring frequency. The relevance of using small-scale resonators to solve this issue is experimentally demonstrated on an aircraft sidewall panel. The effect of varying the spatial distribution of single frequency resonators (including combination with a broadband soundproofing treatment), as well as using multi-frequency resonators with a fixed spatial distribution is studied. Large improvement of the measured sound transmission loss under a diffuse acoustic field excitation is obtained around the ring frequency with limited added mass and very small alteration of the overall sound insulation performance.

Optimization of Sound Transmission Loss Characteristics of Orthogonally Stiffened Plate

An analytical model is developed to investigate the sound transmission loss from orthogonally rib-stiffened plate structure under diffuse acoustic field excitation. The validity and feasibility of the model are verified by comparing the present theoretical predictions with the numerical results published previously. The influences of structure geometrical parameters on sound transmission loss are subsequently presented. The optimization algorithm is used to search for the optimal structural parameters with the objective to maximize the sound transmission loss over a frequency band. Furthermore, the sensitivity of structural parameters on the overall vibration and acoustic performance of the stiffened plates structure is also analyzed.

Estimating the sound transmission loss of a single partition using vibration measurements

The sound transmission loss of a homogeneous, isotropic, thin panel under a diffuse acoustic field excitation is derived from a measurement of its airborne induced vibration field. Using this single dataset, the virtual fields method allows identifying the wall pressure field exciting the panel and estimating the corresponding incident acoustic power, provided that the differential operator governing the plate dynamics and its material properties are known a priori. Using the same dataset, the radiated acoustic power is calculated using the radiation resistance matrix method. For an aluminium plate, a comparison of transmission loss values obtained using this approach and a standardized measurement shows good agreement off resonance but large discrepancies on resonance and close to resonance, due to ill conditioning of the virtual fields method. A simple correction is proposed on resonance.

An analytical study of the vibroacoustic response of a ribbed plate

An analytical model is developed to investigate the sound transmission loss from orthogonally rib-stiffened plates structure under diffuse acoustic field excitation. The validity and feasibility of the model are verified by comparing the present theoretical predictions with numerical and experimental results published previously. The influences of modal coupling terms, boundary condition and stiffener spacing on sound power and sound transmission loss are subsequently presented. Numerical discussion of the model demonstrates the significant influence of both boundary conditions and stiffener spacing upon the mode shape, sound power and sound transmission loss for stiffened plate, wherein sound power decreases and sound transmission loss increases as the amount of constraint increases.

Optimization of Sound Transmission Loss Characteristics of Orthogonally Stiffened Plate

An analytical model is developed to investigate the sound transmission loss from orthogonally rib-stiffened plate structure under diffuse acoustic field excitation. The validity and feasibility of the model are verified by comparing the present theoretical predictions with the numerical results published previously. The influences of structure geometrical parameters on sound transmission loss are subsequently presented. The optimization algorithm is used to search for the optimal structural parameters with the objective to maximize the sound transmission loss over a frequency band. Furthermore, the sensitivity of structural parameters on the overall vibration and acoustic performance of the stiffened plates structure is also analyzed.

Vibroacoustic response of panels under diffuse acoustic field excitation from sensitivity functions and reciprocity principles

This paper aims at developing an experimental method to characterize the vibroacoustic response of a panel to a diffuse acoustic field (DAF) excitation with a different laboratory setup than those used in standards (i.e., coupled rooms). The proposed methodology is based on a theoretical model of the DAF and on the measurement of the panel's sensitivity functions, which characterize its vibroacoustic response to wall plane waves. These functions can be estimated experimentally using variations of the reciprocity principle, which are described in the present paper. These principles can either be applied for characterizing the structural response by exciting the panel with a normal force at the point of interest or for characterizing the acoustic response (radiated pressure, acoustic intensity) by exciting the panel with a monopole and a dipole source. For both applications, the validity of the proposed approach is numerically and experimentally verified on a test case composed of a baffled simply supported plate. An implementation for estimating the sound transmission loss of the plate is finally proposed. The results are discussed and compared with measurements performed in a coupled anechoic-reverberant room facility following standards.

外部から拡散音場加振される隙間を有するカバー内音場のアクティブ制御

To increase the noise proof performance of a cover with aperture excited by diffuse acoustic fields from the outside, it is a problem to prevent the occurrence of the inner acoustic mode at the lower frequencies. Authors had suggested the feed forward active noise control (ANC) for the acoustic field inside the cover. Diffuse acoustic field excitation experiment using reverberant room has been examined to comprehend acoustic characteristics inside the cover. The results show that the inner cover field is in the state of locally coherent at the frequencies of inner acoustic mode, so that it is good environment for the active noise control. Then, a calculation with two-dimensional finite difference time domain (FDTD) method has been done to examine the feasibility of feed forward ANC. The results show that it is possible to realize ANC for the acoustic field inside the cover by proper design of control filter. Finally, experimental measurement of diffuse acoustic field excitation in the reverberant room has been done again to examine the ANC with the design method mentioned above. The results show that feed forward ANC can reduce noise level by 5dB at the peak frequency caused by the inner acoustic mode. Furthermore, the inner acoustic pressure distribution is also reduced uniformly at the frequency. Therefore, it is concluded that it is possible to realize feed forward active noise control for the acoustic field inside the cover with aperture excited by diffuse acoustic fields from the outside, even if the excitation is random or the causality of I/O is not satisfied.

Identification of spatially correlated excitations on a bending plate using the Virtual Fields Method

This paper aims at identifying the autospectral density and spatial correlation functions of random excitations acting on the surface of a thin plate, from its measured vibration response. The general framework is the Virtual Fields Method (VFM), which was previously applied by the authors to the identification of deterministic excitations on plates. In the present paper, the VFM framework is extended to the case of spatially correlated excitations. It is shown that extraction of the loading power spectral density requires measuring power spectral density functions of transverse displacements and bending curvatures, which can be typically derived from contactless Laser Doppler Vibrometry measurements. The paper details the implementation of the VFM for random excitations, presents numerical simulations and experimental results for diffuse acoustic field excitation of a plate.

Vibroacoustic Tailoring of a Rod-Stiffened Composite Fuselage Panel with Multidisciplinary Considerations

An efficient multi-objective design tailoring procedure seeking to improve the vibroacoustic performance of a fuselage panel while maintaining or reducing weight is presented. The structure considered is the pultruded rod stitched efficient unitized structure, a highly integrated composite structure concept designed for a noncylindrical, next-generation flight vehicle fuselage. Modifications to a baseline design are evaluated within a six-parameter design space including spacing, flange width, and web height for both frame and stringer substructure components. The change in sound power radiation attributed to a design change is predicted using finite-element models sized and meshed for analyses in the 500 Hz, 1 kHz, and 2 kHz octave bands. Three design studies are carried out in parallel while considering a diffuse acoustic field excitation and two types of turbulent boundary-layer excitation. Kriging surrogate models are used to reduce the computational costs of resolving the vibroacoustic and weight objective Pareto fronts. The resulting Pareto optimal designs are then evaluated under a static pressurization ultimate load to assess structural strength and stability. Results suggest that choosing alternative configurations within the considered design space can reduce weight and improve vibroacoustic performance without compromising strength and stability of the structure under the static load condition considered, but the tradeoffs are significantly influenced by the spatial characteristics of the assumed excitation field.

Acoustic and vibration response of a structure with added noise control treatment under various excitations.

The evaluation of the acoustic performance of noise control treatments is of great importance in many engineering applications, e.g., aircraft, automotive, and building acoustics applications. Numerical methods such as finite- and boundary elements allow for the study of complex structures with added noise control treatment. However, these methods are computationally expensive when used for complex structures. At an early stage of the acoustic trim design process, many industries look for simple and easy to use tools that provide sufficient physical insight that can help to formulate design criteria. The paper presents a simple and tractable approach for the acoustic design of noise control treatments. It presents and compares two transfer matrix-based methods to investigate the vibroacoustic behavior of noise control treatments. The first is based on a modal approach, while the second is based on wave-number space decomposition. In addition to the classical rain-on-the-roof and diffuse acoustic field excitations, the paper also addresses turbulent boundary layer and point source (monopole) excitations. Various examples are presented and compared to a finite element calculation to validate the methodology and to confirm its relevance along with its limitations.

Transmission loss of curved sandwich-composite panels with attached noise control materials

This paper discusses the modeling of the transmission loss of curved sandwich-composite panels with attached noise control treatments using both analytical and numerical methods. Special attention is devoted to the modeling using the Transfer Matrix Method (TMM) and Statistical Energy Analysis (SEA) of these structures in various mounting conditions (single wall and double wall) under diffuse acoustic field excitation with a systematic comparison with an efficient FEM/VBEM formulation of the problem. Classically, in SEA models the sound package is unwrapped and the TMM is used to calculate its effects in terms of added damping, absorption, and insertion loss. Examples are presented to examine the validity of this practice and demonstrate its range of applicability and usefulness.