Author: Cédric Van hoorickx

Abstract ID: 1444

This contribution presents a numerical approach to quantify the response of an absorber in a diffuse reverberation room. Conventionally, this is done by considering an infinite absorber coupled to an acoustic halfspace. It is, however, well known that the diffuse absorption coefficient for a finite absorber can be quite different due to what is referred to in literature as the edge effect. A finite size correction has been developed previously, but it is only applicable to homogeneous absorbers and is based on a computationally costly quintuple integration. This contribution presents an alternative approach in which a deterministic model, e.g. using the finite element or modal transfer matrix method, is coupled with a statistical model of the room using a hybrid deterministic-statistical energy analysis framework. With this framework, also the theoretical uncertainty on this diffuse sound absorption that is inherent in the diffuse field assumption can be quantified, i.e. the variance of sound absorption results that can be theoretically expected across an ensemble of reverberation rooms of the same volume. The methodology is numerically and experimentally validated for several absorber types.

Author: Wei Zhao

Abstract ID: 1706

Tire acoustic cavity resonance noise (TACRN) is a typical annoying lower-frequency interior noise of a passenger car. The widely used attenuating method of attaching the porous sound absorption material in tire cavity can reduce TACRN effectively, but causes the increase of tire-wheel assembly weight and cost, also the poor durability. Additionally, the Helmholtz resonator (HR) is also used in the wheel of some cars although having only narrow effective band. The existing investigation shows that the frequency of TACRN varies with the car speed and load and also has the split characteristics. The change of TACRN frequency causes a certain difficulty to suppress TACRN effectively. Aiming at this problem, in this paper, TACRN frequency range of a specific tire cavity under different operating conditions is first calculated and analyzed. Then, for a specific aluminum alloy wheel, a HR assembly including several HRs is designed to make the natural frequencies of HR assembly cover the TACRN frequencies. Finally, the reduction effect of TACRN is simulated and evaluated by comparing the sound fields in tire cavity with/without HR assembly under same volume velocity sound source. This work is helpful for attenuating TACRN effectively under the changing operating conditions.

Author: Björn Knöfel

Abstract ID: 1972

The wall listens different? Variable acoustics in rehearsal rooms using small volume structures in an acoustic panel.

Author: Diana Maria Garza-Agudelo

Abstract ID: 2056

It has been shown in several recent publications that acoustic materials consisting of a combination of resonators tuned to different frequencies can render high absorption coefficient values over an extended frequency range while maintaining compactness. This makes them attractive solutions for applications in which low frequency sound control is needed, and/or when there are significant space constraints. Nevertheless, the acoustic performance of these surfaces varies with the angle at which a wave impinges on the surface. The changes in the absorption characteristics with the incidence angle occur both on the maximum absorption coefficient, and on the effective frequency bandwidth. Numerical optimization is a tool that can help realize designs with a large degree of geometrical freedom, and using this framework we have demonstrated an array of coupled 2D Helmholtz resonators that is less sensitive to changes in the incidence angle.

Author: Jonathan Dessi-Olive

Abstract ID: 2074

Architectural acoustics has not traditionally had unified design methods that specify acoustical performance, visual appearance, and sustainable material selection, leading to underperforming products that contribute to a waste stream of petro-chemical foam and fiberglass materials. The evolution of design, materials, and manufacturing techniques in recent years has created new opportunities to reimagine acoustic diffusers and absorbers. Previous work by the authors have demonstrated a unifying framework for design and collaboration in architectural acoustics. The framework uses visually-driven computational design method inspired by shape grammars that generate a wide range of acoustic phase grating diffuser arrays that display unique visual and performative qualities. Simulation and evaluation metrics to assess the complexity of each design are rated in terms of their diffusion and absorption coefficients and a visual aesthetic coefficient. This paper extends the framework to include digital fabrication protocols and sustainable material specifications – including the use of fungi-based materials. Built prototypes demonstrate an expanded acoustic design space that gives acousticians the potential to create custom diffuser shapes with precise acoustical response. The innovative combination of computational design methods and sustainable fabrication protocols will be discussed, and the acoustic properties of arrays will be evaluated and compared to simulations of corresponding designs.

This work proposes an acoustic metamaterial-based muffler that effectively blocks a transmission noise for a target frequency range. Since the acoustic metamaterial-based muffler consists of arrayed unit cells, its noise attenuation performance is strongly affected by the internal layout of the unit cell. The wave transmission characteristics of an acoustic metamaterial is explained by the effective bulk modulus and dispersion curve of an unit cell. Therefore, the internal layout of the unit cell should be optimally designed so that its band gap should include the target frequency range of a muffler. To the end, an acoustical size optimization problem is formulated to design a unit cell of the muffler and is solved for a given design requirement. The noise blocking frequency range of the unit cell is characterized by the bandgap in its dispersion curve during the optimization process. The wave transmission characteristics of the metamaterial muffler is validated experimentally.

Author: Bunghun An

Abstract ID: 2465

This work proposes an acoustic metamaterial-based muffler that effectively blocks a transmission noise for a target frequency range. Since the acoustic metamaterial-based muffler consists of arrayed unit cells, its noise attenuation performance is strongly affected by the internal layout of the unit cell. The wave transmission characteristics of an acoustic metamaterial is explained by the effective bulk modulus and dispersion curve of an unit cell. Therefore, the internal layout of the unit cell should be optimally designed so that its band gap should include the target frequency range of a muffler. To the end, an acoustical size optimization problem is formulated to design a unit cell of the muffler and is solved for a given design requirement. The noise blocking frequency range of the unit cell is characterized by the bandgap in its dispersion curve during the optimization process. The wave transmission characteristics of the metamaterial muffler is validated experimentally.

Author: Ho Yong Kim

Abstract ID: 2470

Back by a rigid cavity filled with a layer of porous layer, the sound absorption performance of a micro-perforated panel (MPP) can be enhanced in comparison with other resonance based sound absorbers. In this paper, a theoretical model of a finite flexible MPP back by a rigid air cavity filled with a fibrous porous material is developed to predict normal sound absorption coefficients. Displacements of MPP and sound pressure field in fibrous porous material and acoustic cavity are expressed using a series of modal functions, and the sound absorption coefficients of MPP system are obtained. Additionally, comparison of energy dissipation by MPP and fibrous material is performed to identify effects of a fibrous material on the sound absorption of a MPP. As expected, at anti-resonance frequency of an MPP, the fibrous material provide an alternative energy dissipation mechanism.

Author: Takamasa Sato

Abstract ID: 2497

In this study, we conducted theoretical analyses and experiments related to the acoustic characteristics of the situation where sound waves are incident upon the side surfaces of a group of cylinders forming a pin-holder structure. The sound-absorption coefficient, entering its clearance between cylinders through the geometrical dimension of the clearance or the physical property of gas, was calculated. In the analytical model, the gap part of the pin-holder structure was divided into elements and approximated as a gap surrounded by two parallel planes. The characteristic impedance and propagation constant of the approximate gap were obtained and treated as one-dimensional transfer matrices; the sound-absorption coefficient was then calculated using the transfer-matrix method. The calculated value was compared to that obtained in an experiment with a sample prepared using a 3D printer; the sound-absorption coefficient was measured using a 2-microphone impedance-measuring tube. We attempted to make a simple yet accurate estimation of sound-absorption coefficient using these procedures. Our theoretical values displayed a similar tendency to that obtained by experiment.

Author: Takumi Yoshida

Abstract ID: 2731

The preset paper proposes a novel acoustic diffuser which we call cross rib diffuser (CRD) and investigates its acoustical performance in rooms experimentally. CRD consists of overlapping two one-dimensional periodic rib diffuser (OPRD) with different structural configurations. CRD can achieve high scattering coefficient with wider frequency band than OPRD. Moreover, unlike other diffusers with high scattering property such as metadiffuser and two-dimensional quadratic diffuser, CRD keeps simple and familiar design of OPRD suitable for use in various architectural spaces. In the paper, we firstly evaluated random-incidence scattering coefficient of CRD using 1/5 scaled reverberation room. Then, random-incidence absorption coefficient was measured in 1/1 reverberation room. Finally, an implementation experiment was conducted to examine applicability of CRD in improving acoustics in small meeting room with small absorption treatments. The results indicated that CRD reduced EDT and reverberation time, and increased D50 more than JND values. Additionally, CRD improved reverberation and speech intelligibility more significantly than OPRD with same installation area.

Author: Harshavardhan Ronge

Abstract ID: 2863

In convective air-cooled heat sink applications with space constraints, corrugated geometries can be used as in-duct sound absorbing structures offering lower duct-flow resistance than other geometries such as block-shape, wedge-shape geometries. Sound wave propagation through this geometry is presented using a simple 1-D acoustic model. Using the model, acoustic performance of corrugated sample is evaluated in terms of its transmission loss in dB. Thermal resistance and pressure drop values are also reported and compared with acoustic performance as function of number of corrugations and length of corrugated sample. A rectangular corrugated geometry has alternate inlet and outlet channels separated by porous walls. Sound propagation across this arrangement is modelled by extending prior model from literature with similar geometries. Prior model by Allam and Åbom (2005) is highly symmetric about the channels and porous walls are modelled by simple steady flow resistance equation. In current work, appropriate considerations are taken into account for the configuration of corrugated geometries suitable to general heat sink applications and sound wave propagation through porous walls is predicted by using Johnson-Champoux-Allard (jca) model. The porous walls at ends of the geometry are modelled as in acoustically series-parallel network combinations. Further, effect of heat sink temperature on sound wave propagation is also explored using the model.

Author: Caoyang Li

Abstract ID: 2871

The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) Handbook provides tabulated information that can be used to determine the insertion loss of a variety of lined and unlined ducts.  However, the tabulated cross-sections are primarily for square ducts.  Not surprisingly, there is a need for information on the insertion loss of ducts having different aspect ratio cross-sections.  Hence, the insertion loss for large aspect ratio cross-sections are investigated using a previously validated finite element simulation approach.  A coupled structural-acoustic finite element analysis is performed, and data is compared to measurement results from the literature for a few configurations.  An analysis campaign is then performed which better explains the effect of aspect ratio on duct insertion loss.

Author: Thomas Geyer

Abstract ID: 2308

Dissipative mufflers are often used for the reduction of broadband noise transmitted in ducts. Many common calculation procedures for the transmission loss of such mufflers require conventional shapes like rectangular or circular cross-sectional areas. In an effort to analyze the effect of the cross-sectional area of dissipative mufflers on the resulting noise reduction, the transmission loss of axially uniform mufflers with polygonal cross-sectional areas was investigated using the finite element method. The mufflers are designed to have the same open area, and hence in a practical application would lead to a similar pressure drop. The results were compared to those obtained with the well known approximative method of Piening. Good agreement between simulation and estimation was found regarding basic trends at low frequencies, while notable differences were revealed regarding the maximum transmission loss.