Diffuse Sound Field Research Articles

Mixing materials in false ceilings to increase sound diffusion in education spaces

According to the International standards on education spaces, in situ acoustic tests should achieve specific reverberation time targets. Since the match between predictive formulas and measurements increase with the sound field diffuseness, it is extremely important to pursue such condition through proper design choices. For example, false ceilings can play a key role in controlling room acoustic features; however, the use of such elements is generally considered only for absorption purposes. For this reason, the present work concerns mixing materials in false ceilings within a group of teaching spaces here taken as case studies. The alternation of materials with different acoustic impedances has been investigated through experimental acoustic measurements and numerical models in order to assess the related sound diffusion increase. The ceiling treatment here proposed proved to be an efficient and smart method to exploit diffraction effects along material discontinuities junctions. The match between early design predictive formulas and the results of the acoustic measurements enhanced the reliability of the acoustic design process and set up a potential new guideline for indoor acoustic treatments.

The imaginary part of the diffuse field forced normalized radiation impedance of a rectangular panel

This paper modifies previous approximate formulae for calculating the imaginary part of the azimuthally averaged forced normalized radiation impedance experienced on one side of a rectangular panel mounted in an infinite rigid plane baffle by a forced travelling wave on the rectangular panel which is excited by an infinite plane sound wave incident at an angle to the normal to the rectangular panel on one side of the rectangular panel. This modification is made so that the formulae can be analytically integrated over all possible solid angles of incidence on one side to obtain formulae for the imaginary part of the diffuse sound field forced normalized radiation impedance of the rectangular panel. The paper also gives existing formulae for the real part of this diffuse sound field forced normalized radiation impedance. The reason for developing approximate formulae for the imaginary part of this diffuse sound field forced normalized radiation impedance is so that they can be used when approximating the equation for the diffuse field sound transmission coefficient of a limp single leaf wall.

Proposal of a simplified methodology for reverberation time prediction in standard medium size rooms with non-uniformly distributed sound absorption

Sabine, Eyring and Millington formulas are commonly used for reverberation time prediction, mainly, as a calculation tool in building acoustics design. These classical theories are valid only for rooms with diffuse sound fields, in which the energy density is constant throughout the enclosure, an acoustic condition that is achieved only when using surfaces with low sound absorption. Despite these limitations, Sabine’s formula is still the most widely used in the prediction of the reverberation time, when spaces such as classrooms or offices are addressed. However, for these rooms, after the construction works are completed, it is quite often verified that the implemented sound-absorbent surface area is manifestly insufficient to fulfill the reverberation time requirement. In this technical note a simplified approach for reverberation time prediction, based on the use of Sabine’s formula, is proposed, that can be useful in acoustic design of classrooms or offices, due to its simplicity. A previous correction to the sound absorption coefficient of the lining materials declared by the manufacturer is here proposed, making use of an empirical correction that was achieved fromin situexperimental results and through geometrical room acoustic modelling. The empirical correction can be employed for room conditions where diffuse sound field is not met, composed of small or medium volumes (volume below 300 m3), with regular geometry, approaching parallelepipedal shapes, where the average height is below 4.0 m.

Measurement of the diffuse field sound absorption using a sound field synthesis method

A method for measuring the diffuse field sound absorption coefficient of a material using sound field synthesis is proposed. A planar loudspeaker array is first used to generate acoustic plane waves with variable incidence angle on the surface of a material under test. Using a two-microphone probe positioned closely to the sample’s surface, the angle-dependent sound absorption coefficients are then estimated. Finally, the diffuse field absorption coefficient is computed following Paris formula. Numerical simulations are used to evaluate the respective effects of the maximum incidence angle value and the number of individual incidence angles that are required for a robust calculation of the diffuse sound field absorption. Measurements are conducted on three different materials and compared with either simulation results obtained using the Johnson-Champoux-Allard theory, or with measurement results obtained using the standard reverberation chamber method. For all considered materials and over a wide frequency range, the proposed method leads to results that are in better agreement with theoretical predictions than those obtained using standardized methods.

An experimental study on the sound transmission loss of dissimilar fuselage sandwich panels of turbojet aircraft

The analysis of acoustic performance of sandwich panels, designed explicitly for fuselage sidewalls of turbojet aircraft, has been revisited numerous times. However, the layouts and the sound transmission loss of the airborne fuselage sandwich panels have not been made available for public release. Against this idiosyncratic practice, this study envisioned disseminating the know-how on the sound attenuation ability of dissimilar sandwich panels implemented in the fuselage of turbojet aircraft. To efficiently absorb and dampen the acoustic energy, the sandwich panels were systematically configured using a honeycomb core, felts and a closed cell aluminum-foam in the material lay-up. The face sheets of the sandwich panels were composed of either glass fiber-reinforced epoxy or carbon fiber-reinforced epoxy composites. The sandwich panels were tested as acoustic barriers to a diffuse sound field in a sound transmission suite consisting of two adjacent sound reverberation rooms. As found, a rubber damping layer in the material lay-up of a sandwich panel reinforced the sound transmission loss at frequencies beyond 500 Hz. Multiple felts augmented the sound transmission loss in the frequency regime beyond 315 Hz. The panel edges needed to be sealed properly to reproduce the sound transmission loss of a sandwich panel in successive experiments. This study leveraged many-faceted insights into the configuration (constituent materials, number of material layers, thickness, and stacking sequence) of commercially feasible sandwich panels. Besides, this study proposed optimized sandwich configurations based on the sound transmission loss, which will help aircraft manufacturers to apply these sandwich panels without further modifications.

Sound absorption characteristics of thin parallel microperforated panel (MPP) for random incidence field

In this study, the absorption coefficient of a thin parallel MPP for random incidence field is investigated. A coiled backing air cavity realizes the thin structure, and the parallel configuration is applied to have a wider sound absorption bandwidth than a single MPP. The effects of the oblique sound incidence on the thin parallel MPP are investigated by numerical studies for both single oblique incidence and random incidence. These effects are significant from a practical standpoint, whereas the result of a coiled backing air cavity must be carefully evaluated. This paper focuses on developing, modeling, and evaluating parallel MPP with a coiled backing cavity. It is found that the oblique incidence can result in lower sound absorptions and shift in resonance absorptions compared to the normal incidence angle, especially for high incidence angles (>70°). In addition, the performance of the thin MPP at a normal incidence angle can be maintained up to the incident angle of 70°, where the sound absorption coefficients of 0.8 and above are present. Compared with a straight backing cavity, the use of coiled one does not introduce deviation in sound absorption characteristics except for the cavity modes slightly higher resonance frequency under the same axial length. Moreover, the proposed structure behaves as a locally reacting material as long as the cavity’s longest dimension is smaller than half an operating wavelength for the targeted frequency range. This allows the sound absorption characteristics of normal incidence and diffuse sound field excitation is insignificantly different. Hence, this characteristic can deal with two folded problems in terms of bulky absorber issue and sensitive performance on oblique incidence.

Research on the Robustness of Active Headrest with Virtual Microphones to Human Head Rotation

The movement and rotation of the human head can significantly degrade the control performance of active headrests with virtual microphones. This paper investigates the performance robustness of an active headrest with virtual microphones against the head rotation in a pure tone diffracted diffuse sound field. A physical model of a headrest system with a diffractive sphere is firstly developed, based on which the influence of acoustic transfer responses on the performance robustness against head rotation is analyzed. Then, the multi-objective optimization with constraints is developed to improve the performance robustness of the system, by designing optimized plant responses. Simulation results show that the method is effective in improving the robustness of single and dual channel systems. For a dual-channel headrest system with the proposed arrangements of secondary sources and microphones, the promotion of the minimum noise reduction is more than 7 dB after optimization when the head rotates from −90 degrees to 90 degrees at 125 Hz, 250 Hz and 500 Hz. In addition, the influence of different parameters on the optimization method is discussed for this dual-channel system. Finally, experiments carried out in an ordinary room validate the effectiveness of the proposed method.

Numerical study of the accuracy and reproducibility of sound absorption measurements in reverberation rooms at low frequencies

Standardised absorption measurements in reverberation rooms suffer from relatively low reproducibility, especially at low frequencies and for highly absorptive samples. In these conditions, complete sound field diffusivity cannot be achieved. However, recent research has demonstrated that the theoretical diffuse absorption emerges as the ensemble average across a wide range of rooms with different geometries, even at very low frequencies. The present study aims to investigate the influence of the room geometry, sound source positioning, and presence of panel diffusers on the sound absorption values obtained in a specific reverberation room, as well as on the difference between those values and the theoretical diffuse values. The focus is on the lowest frequency bands.A numerical simulation approach is proposed, in which the room without sample is modelled in full detail using the finite element method and coupled to the sample with a Rayleigh–Ritz approach. The measurement of diffuse sound absorption is simulated in an efficient way using a stationary power balance approach. This approach is validated against measured data and against a detailed simulation of the impulse response. From the parametric study conducted with the model, it can be concluded that, for highly absorptive samples, good agreement with the theoretical diffuse absorption values can be obtained for certain room designs. In these rooms, the measured absorption is also less sensitive to the number and positioning of sources and diffusing elements.

Evaluating the reverberation chamber as a small room: How room dimensions affect the generalization of testing results

Testing acoustic properties is performed in specially designed reverberation chambers according to relevant ASTM, ISO, and other international standards. Performing this testing is complicated by the fact that practical considerations limit the size of the reverberation rooms. Where the dimensions of the room are similar in size to the wavelength of the frequency of interest, modal behavior becomes dominant, and the statistical analysis of room acoustics based on the diffuse sound field theory is not sufficient to characterize the sound field. However, standard methods implement formulas which assume a diffuse sound field to compare results from different labs. Using the Maxon Acoustics Lab, a purpose-built floor ceiling test lab comprised of two stacked, 300 m3 reverberation chambers, we will examine the various physical criteria for evaluating a theoretically diffuse sound field. Discussion will include historical debates concerning the validity of comparing measurements in different labs and an analysis of the Maxxon Lab through the lens of various methods for evaluating the acoustic environment in comparison to the statistical analysis for room acoustics. Methods will include analysis of modal density and distribution, sampling of the sound field per ASTM standards E90, E492, and C423, the Schroeder frequency, and other statistical analyses.

RESEARCH OF THE INFLUENCE OF GEOMETRIC PARAMETERS OF ROOMS FOR SOUND INSULATION OF LIGHTWEIGHT PARTITIONS

Statement of the problem. Sound insulation of lightweight partitions depends significantly on the place of installation in the building. It is necessary to study the structure of the sound field in dis- proportionate rooms and analyze its effect on the sound insulation of light enclosures.Results. Natural and laboratory experimental studies of the sound insulation of the frame partitions installed in the corridor were carried out. For the case of a commensurate room, good convergence of the results was obtained. The structure of the sound field with mirror reflection of sound has been theoretically investigated using the method of tracing of sound rays. Calculations were performed to obtain the distributions of the proportions of axial sound rays, tangential sound rays, oblique sound rays in the premises, and the calculations of the intensity levels of sound waves incident on the light-weight partition and other enclosures of the corridor. The position of the noise source relative to the lightweight partition and other enclosures of the corridor was taken into account.Conclusions. The proportions of the rooms affect the sound insulation of lightweight enclosures. The structure of the sound field of a disproportionate room with a noise source is non-uniform in the range below the boundary frequency of diffuse sound field. Sound rays fall on the lightweight partition unevenly from different directions. This leads to a decrease in the coincidence of wave modes in the air and wave modes in the lightweight partition. The frequency characteristic of the sound insulation of the lightweight partition has a peak-like appearance.

A High‐Performance Sound Insulation Component for Filter Capacitors Based on Coiled‐up Acoustic Metamaterials

In the light of the spectrum characteristics of filter capacitor noise, a high-performance sound insulation component (HSIC) based on coiled-up acoustic metamaterials is developed. By efficiently absorbing harmonic components whose acoustic energy is predominant, HSIC can insulate the middle- and low-frequency noise radiated by filter capacitors significantly. Simulated results show that under the excitation of a normal incident sound wave, the sound transmission loss of HSIC is 8.0, 7.0, 6.9, and 4.3 dB higher than that of a homogeneous panel with same area density (HPSAD) at the frequencies of 400, 500, 600, and 700 Hz, respectively. The transmission loss of HSIC for filter capacitor noise is 6.4 dB(A) higher than that of HPSAD. Experimental results show that under the excitation of a diffuse sound field, HSIC still has an excellent sound insulation performance and the transmission loss for filter capacitor noise can increase by 4.5 dB(A). Furthermore, an impedance analysis and a complex frequency plane analysis are conducted to elucidate the sound insulation and absorption mechanism of HSIC.

Absorption measurements—A never ending story?!

A brief historical review on measurement techniques of sound absorption or the acoustic surface impedance will be presented, starting from procedures in the 1930s. Many in situ and laboratory methods have been described over the years. Most of these methods are based on the assumption of plane wave propagation or diffuse sound field. Measurement examples will be shown for some of the methods presented and compared to each other. A commonly used measurement procedure of so-called diffuse sound absorption coefficients is described in the international standard ISO 354. The present revision of the standard only focuses on the conditioning of the sound field and further applying Sabine equation. A diffuse sound field is atheoretical approach. Geometrical acoustics as the basis of sound field simulation has been applied to measurements in a virtual reverberation chamber on absorber baffle set-ups [see Probst (2015)] and showed good agreement between measured and calculated values. This approach is used to investigate the effect of sample size and sample position. Correspondingly, non-standard measurements are compared with simulation results.

Numerical realization of diffuse sound pressure fields using prolate spheroidal wave functions.

A diffuse sound field is conventionally defined as a zero-mean circularly symmetric complex Gaussian random field. A more recent, generalized definition is that of a sound field having mode shapes that are diffuse in the conventional sense, and eigenfrequencies that conform to the Gaussian orthogonal ensemble. Such a generalized diffuse sound field can represent a random ensemble of sound fields that share gross features, such as modal density and total absorption, but otherwise have any possible arrangement of local wave scattering features. The problem of generating realizations or Monte Carlo samples of a conventional diffuse sound field or, equivalently, of the mode shapes of a generalized diffuse sound field, is addressed here. Such realizations can be obtained from an eigenvalue decomposition of the spatial correlation function. A discrete decomposition is numerically expensive when the sound pressures at many locations are of interest, so a fast analytical decomposition based on prolate spheroidal wave functions is developed. The approach is numerically validated by comparison with a detailed room model, where random wave scatterers are explicitly modeled as acoustic point masses with random positions, and good correspondence is observed. Furthermore, applications involving correlated sound sources and sound-structure interaction are presented.

Consideration and understanding of various acoustic phenomena and acoustic rules.

In the field of acoustic engineering, there are phenomena and concepts that are unfamiliar to applied mechanics, such as mass law, sound absorption, and diffuse sound fields. In addition, there are some expressions that are useful in engineering, such as acoustic radiation impedance and acoustic radiation efficiency of plate vibration, but their mechanical explanations are insufficient. I accepted it vaguely when I was working at a company, but since I moved to university, I have deepened my consideration and understanding from the viewpoint of applied mechanics. It is outlined so that various acoustic phenomena and acoustic rules can be understood as applied mechanics.

A greedy reduced basis algorithm for structural acoustic systems with parameter and implicit frequency dependence

AbstractIn this article, a greedy reduced basis algorithm is proposed for the solution of structural acoustic systems with parameter and implicit frequency dependence. The underlying equations of linear time‐harmonic elastodynamics and acoustics are discretized using the finite element and boundary element method, respectively. The solution within the parameter domain is determined by a linear combination of reduced basis vectors. This basis is generated iteratively and given by the responses of the structural acoustic system at certain parameter samples. A greedy approach is followed by evaluating the next basis vector at the parameter sample which is currently approximated worst. The algorithm runs on a small training set which bounds the memory requirements and allows applications to large‐scale problems with high‐dimensional parameter domains. The computational efficiency of the proposed scheme is illustrated based on two numerical examples: a point‐excited spherical shell submerged in water and a satellite structure subject to a diffuse sound pressure field excitation.

Tracking the evolution of structural resonances during the build phase of a wire-arc additively manufactured part

Acoustic resonance tracking is investigated for a wire arc additive manufacturing process as a possible real time in situ diagnostic during the build. A small 40kHz acoustic emission sensor was attached to one end of a 5 × 1.5 in. cylindrical 316 SS rod. A series of layers (weld beads) were programmed into the control software of the robot to build a thin bar upward off the end of the cylinder. A high-speed long-duration digitizer-recorder was used to capture the transient acoustic signals (100 Hz–70 kHz) during the build process. Typical build durations were on the order of 1–2 h. The transient data were analyzed using short time window FFTs, as a function of build parameters. The model and experiment share similar features, however there are some notable differences. In particular, the number of actual modes recorded in the experiment is less than predicted due to the sensors low in-plane sensitivity. The current results indicate that the evolving spectral response of a part during the build process has the potential to provide real time process monitoring information. The time-frequency analysis of the diffuse sound field in the part, reveals a unique spatial and temporal perspective of temperature, stress and geometric features.

Simulation of sound field in a reverberation chamber using the diffusion equation model

The acoustic diffusion equation model has been applied within a wide range of enclosure types to simulate diffuse sound fields. Recently, focus has been drawn to its applicability in reverberation chambers to simulate sound fields among a heterogeneous distribution of absorbing materials, particularly those that are highly absorptive. Where quasi-diffuse or non-diffuse sound fields are generated in the presence of highly absorptive material, the acoustic diffusion equation model has convenient capabilities to simulate sound energy flow. Furthermore, it has continued to show promise in characterizing the absorption coefficients of sound absorbing material. Challenges for reproducibility in reverberation room measurements have been well-documented and where traditional chamber-based measurement have shown to be inconsistent, this work will evaluate the acoustic diffusion equation model’s suitability as an alternative and accurate method to characterize absorption coefficients.

Design and verification of ultrathin Schroeder diffuser based on membrane-type resonators

We present the design and experimental verification of a membrane-type resonator-based Schroeder diffuser (MRSD). The membrane-type resonator consists of a decorated membrane and a rigid back cavity. By combining multiple resonators with the phase shift profile based on the quadratic residue sequence, the MRSD can produce a high efficiency diffuse sound field. Furthermore, the designed MRSD has a thickness with deep-subwavelength scale, which is 1 order of magnitude thinner than the conventional diffuser constructed with periodical wells. The proposed MRSD has advantages in low frequency applications and offers versatility for novel manipulations of sound.

Vibro-acoustic response of spacecraft instrument subjected to diffuse sound field: Numerical simulations and experimental verification

The vibro-acoustic environment generated during a rocket launch induces dynamic loads which may damage critical components of spacecraft and scientific instruments. The research is focused on the vibro-acoustic response of the Wide Field Imager (WFI) instrument subjected to an intense diffuse sound field. Noise-induced structural vibrations in low- to mid-frequency can be computed by means of coupled FEM-BEM numerical models. Nevertheless, this approach is unsuited to calculations of cavity-structure interactions of light-weight and thin structures. In this paper the application and assessment of the elasto-acoustic FEM-FEM coupling approach for the cavity - optical blocking thin foil filter interactions is investigated. The diffuse sound field excitation is reconstructed by matching values of the sound pressure level to these obtained from experimental measurements. The viscosity effects on the wave propagation in thin layers are included by using the visco-thermal fluid model. The accuracy and efficiency of this approach are investigated and results obtained are compared to measured data from a reverberant test chamber. The results show that a very good agreement is obtained in terms of the acoustic pressure in the vicinity of the optical blocking filter. The behaviour of mechanical vibrations is correctly reproduced, although some considerable discrepancies in the levels of the vibrations are observed. The case of strong coupling between the cavity acoustic field and the filter membrane is finally discussed.

SOPRANOISE - update and analysis of noise barrier database including new current results

In the frame of the SOPRANOISE project (funded by CEDR in the Transnational Road Research Programme 2018) work package 2 focused first on providing theoretical and practical background information on measurement of the acoustic performance of noise barriers due to a state-of-the-art regarding correlations and possible trends between diffuse (EN 1793-1, EN 1793-2) and direct sound field methods (EN 1793-5, EN 1793-6). After that, the objective of this research was to extend and update the database of the European noise barrier market developed during the QUIESST project, including more detailed analyses on single-number ratings as well as third-octave band measurement results. The data collected and the analysis performed show relevant facts and figures about acoustic performances of noise barriers measured under diffuse and direct sound field conditions, together with a better understanding of the respective significance, similarities and differences of these standardized methods, improving data analysis and correlations between these methods. This paper gives a general overview of the data collected, summarising the main results of the statistical analyses performed. Overall results and comparisons between results of measurements performed under diffuse and under direct sound field conditions are shown. Finally, conclusions and possible outlook of the research are presented.