Room Acoustic Applications Research Articles

Time-domain simulation of sound propagation through anisotropic porous media for room acoustic applications

Wave-based room acoustic simulations require accurate modeling of sound propagation in the presence of extended (bulk) reacting porous absorbers. The sound absorption in porous media is typically addressed by means of equivalent fluid models that provide frequency-dependent effective material parameters, such as density and bulk modulus. In the time domain, these frequency dependent properties result in the computation of convolution integrals, which is effectively handled via an auxiliary differential equations (ADE) method. In this work, we apply the time-explicit discontinuous Galerkin approach to modelling of sound propagation in anisotropic porous media (with an anisotropic equivalent fluid model) together with the ADE method. We demonstrate the importance of using the extended approach for porous media, where the sound absorption depends not only on the frequency and incident angle, but also on the travelling wave direction.

Diffusion equation modeling in room-acoustic applications

The diffusion equation application in room acoustic simulations can be traced back to 1969. However, much attention from room-acoustic community to the diffusion equation was given in the late 1990s and the early 2000s. In the literature, the acoustic diffusion equation may be considered as an approximation of the acoustic transport equation. The diffusion equation modeling has been considered suitable for applications in enclosed spaces with proportionate dimensions. The most intriguing features of the diffusion equation are its computational efficiency due to a sparse meshing condition and its capability of energy flux solutions due to Fick’s law. Since the late 2000s, the diffusion equation modeling has evolved with boundary conditions formulated by the rigorous physical–mathematical theory [Jing and Xiang, JASA 123, 145–153 (2008]] and its validity has also been systematically investigated [Xiang et al., JASA 133, 3975–3985 (2013)]. This paper reviews the vibrant development in the room-acoustics during this era and discusses wide range of applications from coupled-volume systems, historically significant worship spaces to recent investigations on sound absorption measurements in reverberation chambers. This paper also discusses its link to the acoustic transport equation and compares the modeling results of both. Different numerical approaches to solving the diffusion equations will also be reviewed.

Tunable Helmholtz Resonators Using Multiple Necks.

One of the uses of Helmholtz resonators is as sound absorbers for room acoustic applications, especially for the low frequency range. Their efficiency is centered around their resonance frequency which mainly depends on elements of their geometry such as the resonator volume and neck dimensions. Incorporating additional necks on the body of a Helmholtz resonator (depending on whether they are open or closed) has been found to alter the resulting resonance frequency. For this study, tunable Helmholtz resonators to multiple resonance frequencies, are proposed and investigated utilizing additional necks. The resonance frequencies of various multi-neck Helmholtz resonators are first modeled with the use of the finite element method (FEM), then calculated with the use of an analytical approach and the results of the two approaches are finally compared. The results of this study show that Helmholtz resonators with multiple resonances at desired frequencies are achievable with the use of additional necks, while FEM and analytical methods can be used for the estimation of the resonance frequencies. Analytical and FEM approach results show a good agreement in cases of small number of additional necks, while the increasing differences in cases of higher neck additions, were attributed to the change in effective length of the necks as demonstrated by FEM. The proposed approach can be useful for tunable sound absorbers for room acoustics applications according to the needs of a space. Also, this approach can be applied in cases of additional tunable air resonances of acoustic instruments (e.g., string instruments).

Modeling sound transmission through apertures with diffraction

Sound transmission through an aperture in a thin wall is a classical scattering problem with various applications in building and room acoustics. In particular, the use of active noise control for open windows can be viewed as an aperture scattering problem. Diffraction-based modeling of scattering is very efficient and accurate for convex scattering objects but has been shown to be less accurate for the transmission through circular apertures, at low frequencies. In this study we investigate the accuracy of edge-diffraction based modeling of sound transmission through rectangular apertures. Reference solutions are computed with a boundary element formulation for this case. Results confirm that the diffraction modeling gives accurate results for mid-to-high frequencies. For low frequencies and skewed transmission angles, the diffraction-based method gives larger errors. For aspect ratios between 1:1 and 10:1, the sound power transmission ratio predicted by the edge diffraction approach is maximally ±1.5 dB in error for very low frequencies.

Equivalent Circuit Method Based Double Layer Micro-perforated Panel (MPP) Design to Widen the Sound Absorption Bandwidth

Acoustical solutions that occupy minimum space and simultaneously allow for flexibility in interior spaces are not feasible by using thick absorbers like conventional porous and fibrous materials. Micro-perforated panels are an excellent alternative to those classical materials due to their environmental friendliness, design flexibility on various materials, and easy installation. However, they come with the handicap of having a very narrow absorption frequency bandwidth. This study explores two inhomogeneous microperforated layers arranged in a cascade with different back cavities in order to widen the sound absorption bandwidth. This serial-parallel architected MPP system is investigated by mathematical models using Maa's model for a single MPP and the Equivalent Circuit Method (ECM). The numerical results were validated through experiments for normal sound incidence in the impedance tube using the transfer function method. Increasing the back cavity length of the MPP having a higher perforation ratio and larger holes results in a higher absorption in the high-frequency range. In contrast, combinations of larger cavity lengths and smaller perforation ratios are more effective in the low-frequency spectrum. By tuning the design parameters of those structures, it is possible to achieve wideband absorption by fibreless materials for room acoustic applications.

Dimensionality Reduction of Room Acoustic Impulse Responses and Applications to System Identification

Dimensionality Reduction of Room Acoustic Impulse Responses and Applications to System Identification

Investigation on the sound absorption of cross-stitched fabric by airflow resistance and sound-induced vibration effects

The main aim of this work is to investigate the acoustical properties of cross-stitched fabric, which exhibits a similar structure with micro-perforated plate. The sound absorption coefficients of cross-stitched fabric with different specifications were measured, where the backing air cavity distance is 1 cm, 2 cm, and 3 cm, respectively. These distances are suitable to the practical interior decoration applications of cross-stitched fabric painting. Cross-stitched fabric can be made into diversity of artistic patterns through manual or mechanized embroidery technology. This work has also studied the effects of multifarious embroidered patterns on the sound absorption coefficients of fabrics with different backing air cavity distance. Classical micro-perforated model, vibrational micro-perforated model, and airflow resistance based model were used to predict the sound absorption of cross-stitched fabric. In summary, cross-stitched fabric with exquisite patterns is expected to be a promising sound absorber for potential room acoustics applications.

Carrier frequency influence on the audible and ultrasonic fields generated by an omnidirectional parametric loudspeaker excited with exponential sine sweeps

While the parametric acoustic array (PAA) phenomenon has been traditionally exploited for generating focused sound spots, an omnidirectional parametric loudspeaker (OPL) is a sound source that uses the PAA for producing omnidirectional sound fields. An OPL is made of hundreds of ultrasound transducers set on a sphere, each one emitting an ultrasonic carrier wave modulated by an audio signal. Thanks to non-linear propagation effects in air, the air itself demodulates the emitted ultrasonic signal bringing it back to the audible range. The carrier frequency fc is a critical parameter to the efficiency of the OPL and it is investigated in this work. To that goal, the performance of an OPL prototype consisting of 750 ultrasound transducers set on a sphere is evaluated for a large set of fc values, which are selected to be close to the resonance frequency of the transducers. Measurements are carried out in free-field conditions using (modulated) exponential sine sweeps as the excitation signal, which have recently proved useful to obtain a good frequency response, sound pressure level and directivity of the OPL in the audible range. That methodology is extended in this work to not only analyse the audible field but also the ultrasonic one. The measurements reveal that the OPL behaviour is very sensitive to fc, yielding to large variations in the ultrasonic and audible range. Therefore, when selecting an fc value, a balance must be found between maximizing the sound pressure levels in the low frequency range and preserving the sound field omnidirectionality, which is important for room acoustic applications.

Broadband acoustic absorbing metamaterial via deep learning approach

Sound absorption is important for room acoustics and remediation of noise. Acoustic metamaterials have recently emerged as one of the most promising platforms for sound absorption. However, the working bandwidth is severely limited because of the strong dispersion in the spectrum caused by local resonance. Utilizing the coupling effect among resonators can improve the absorbers' performance, but the requirement of collecting coupling effects among all resonators, not only the nearest-neighbor coupling, makes the system too complex to explore analytically. This Letter describes deep learning based acoustic metamaterials for achieving broadband sound absorption with no visible oscillation in a targeted frequency band. We numerically and experimentally achieve an average absorption coefficient larger than 97% within the ultra-broadband extending from 860 to 8000 Hz, proving the validity of the deep learning based acoustic metamaterials. The excellent ultra-broadband and near-perfect absorption performance allows the absorber for versatile applications in noise-control engineering and room acoustics. Our work also reveals the significance of modulating coupling effects among resonators, and the deep learning approach may blaze a trail in the design strategy of acoustic functional devices.

Biomimetic Metamaterial Design Simulation and Evaluation for Building Acoustics by Impedance Measurements

Abstract This study investigates the potentials of acoustic metamaterial (AMM) applications in room and building acoustics by means of impedance tube experiments. With their extreme properties in either sound absorption or transmission loss, AMMs can perform better than traditional acoustic materials in buildings. Importantly, they are also more sustainable and hygienic than fibrous and porous materials. Depending on the matrix material used, AMMs can vary in transparency and color. Considering both their acoustic and aesthetic values, this study develops different types of metamaterial for possible uses as a partition wall, a surface layer, or a design element. The proposed metamaterials are primarily based on the exploration of ratios and forms from nature—the golden ratio, web-labyrinthine structures, genetic and neural systems such as deoxyribonucleic acid (DNA) molecules, and the synapse structures in the brain—reproduced on a subwavelength scale. These abstractions are then combined with the 3-D space coiling and 3-D labyrinth approaches of AMM design. Modules of the proposed AMMs are manufactured in a 3-D printer and tested in an impedance tube to estimate their normal incidence sound absorption coefficients and transmission loss characteristics. Based on the results obtained, the modules with the higher performances are used in the design of partition walls of varying heights. Two real-case architecture studios are simulated with and without the proposed AMM interventions over field test–tuned acoustical models of the studios to assess the effectiveness of such an approach in a possible acoustical design problem.

Micro-perforated panels for noise reduction

Space limitations in duct applications, new industrial and health safety regulations require new absorber configurations. Micro-perforated panels (MPP) as used in micro-perforated absorbers (MPA) allow new sound absorber concepts in the category of metamaterials. In this contribution we investigate MPA designs for the low and mid-to-high frequency range and apply finite element (FE) simulations to precisely design absorber configurations for applications in room acoustics and axial fans. The investigations show that the MPA’s cavity configuration must be customized for the desired frequency range and has significant influence on fan performance when applied in a duct flow setup.

An investigation of sound radiation from the double bass using acoustical holography

Studying radiation from musical instruments helps increase understanding and improve applications in auralizations, room acoustics, and microphone placement techniques. This work reviews a string bass's directivity measurements and demonstrates how numerical techniques such as spherical near-field acoustical holography enhance understanding of both the instrument's near-field and far-field radiation. The technique also allows calculation of the sound intensity field around the instrument, providing additional insights.

Sound insulation via a reconfigurable ventilation barrier with ultra-thin zigzag structures

Acoustic ventilation barriers (AVBs) constructed from metamaterials can effectively insulate sound waves while maintaining airflow, but previous schemes suffer from complex geometric structures. Here, we propose a reconfigurable AVB with subwavelength thickness (0.15λ), which is composed of simple zigzag structures with regular intervals and can be assembled into a full enclosure with an arbitrary shape. It is the interaction between the scattered wave of the zigzag structure and the un-scattered wave through the air channel that leads to a transmission dip and hence a sound insulation. Numerical simulations and experimental demonstrations consistently verify that the AVB possesses excellent omnidirectional sound insulation, while the width of the air channel can reach three times the unit width. As the number of cavity increases, the acoustic insulation bandwidth is broadened from 0.875/λ to 2.75/λ. As an example, a circle AVB is further demonstrated to shield acoustic waves emitting from either the interior or exterior. Experimental measurements indicate that the average sound transmission losses can reach roughly 19 dB and 15 dB, respectively. We believe that the proposed AVB may find potential applications in architectural acoustics, room acoustics, and duct noise control.

Robust high-efficiency and broadband acoustic absorber based on meta-molecule cluster sets

We demonstrate a broadband high-efficiency acoustic absorber with subwavelength thickness. The acoustic absorber is based on meta-molecules can achieve near-perfect absorption. By adjusting structural parameters of meta-molecules, the acoustic absorber can offer absorption tenability within a wide frequencies range under a fixed thickness. Therefore, we can realize broadband and multiband absorption by assembling cluster sets based on different meta-molecules. Moreover, an optimization method for effectively filling the ‘acoustic absorption valley’ is ascribed to utilize the coupling effect between porous materials and meta-molecule cluster sets. Theoretical analysis, numerical simulations and experimental measurements show that the absorber can realize sound absorption of broadband and high-efficiency. Our design may offer applications in noise reduction and room acoustics.

Broadband low-frequency sound transmission loss improvement of double walls with Helmholtz resonators

Helmholtz resonators are commonly used as narrowband sound absorbers in room acoustics applications. Previous research has shown that Helmholtz resonators can also be used to improve the sound transmission loss of double walls. The focus of this paper is the broadband improvement of the transmission loss of double walls in the low frequency region by tuning the Helmholtz resonators inside the cavity to frequencies lower or higher than the mass-air-mass resonance frequency of the double wall. A new analytical model using the effective material parameters (bulk modulus and density) of a fluid volume containing Helmholtz resonators is developed to describe the vibro-acoustic behavior of double walls with Helmholtz resonators. Using this model it can be shown that by tuning the Helmholtz resonators properly, the mass-air-mass resonance frequency of the unmodified double wall can be shifted significantly, leading to an improvement of the transmission loss of the double wall roughly between the mass-air-mass resonance frequency and the resonance frequency of the resonators. This improvement, however, comes along with a decrease of the transmission loss at high frequencies due to a decoupling of the Helmholtz resonators which is also covered by the proposed analytical model. Parametric studies are performed to identify relevant design parameters to optimize the transmission loss improvement by the Helmholtz resonators. Finally, experimental results of different double wall designs with integrated Helmholtz resonators are presented to validate the proposed analytical model and demonstrate the effectiveness under diffuse field incidence.

Diffusion equation modeling for sound energy flow analysis in multi domain structures.

This study investigates reliable models and methods to be applied in sound field analysis of multi-domain structures. The case structures are two monuments, namely, Süleymaniye Mosque and Hagia Sophia in İstanbul. These are both multi-volume spaces with many smaller sub-volumes coupled to each other by coupling apertures in form of arches. A key concern of the study is to examine energy flow decays and understand the mechanism of multi-slope sound energy decays. The methodology involves diffusion equation model (DEM) application in a finite-element scheme for sound energy flow analysis. Energy flow decays, energy flow dips, and spatial flow vectors are examined for single versus multi-domain DEM solutions. It is concluded that specification of different domains with individual diffusion coefficients is a critical setting such that, if not assigned correctly, may mislead the results. The energy flow vector analysis has enabled us to comprehend the architectural features in relation to such energy flow decay dip occurrence. The computational efficiency of DEM is also discussed. The DEM application in this study has proved to be a powerful and practical method in room acoustics applications, specifically for multi-rate decay investigations.

Passive volumetric time domain simulation for room acoustics applications.

A major design consideration for volumetric wave-based time-domain room acoustics simulation methods, such as finite difference time domain (FDTD) methods, must be sufficiently general, or robust, to handle irregular room geometries and frequency-dependent and spatially varying wall conditions. A general framework for the design of such schemes is presented here, based on the use of the passivity concept, which underpins realistic wall conditions. This analysis is based on the use of conservative finite volume methods, allowing for a representation of the room system as a feedback connection of a lossless part, corresponding to wave propagation over the interior, and a lossy subsystem, representing the effect of wall admittances. Such a representation includes simpler FDTD methods as a special case, and allows for the determination of stability conditions for a variety of time-stepping strategies.

Diffusion equation modeling in sound field analysis of disproportionate rooms and coupled spaces

This study discusses alternative models and methods to be applied in room acoustics estimations for specific room types including disproportionate rooms and rooms with coupled volumes. A recent prediction method, namely, diffusion equation model (DEM) in room acoustic applications is utilized in the methodology, and this method is compared to common models as of statistical theory, image-source or ray-tracing techniques. Both long enclosures as of subway stations and coupled volumes as of multi-domain monumental structures are special cases with specific interior sound fields. Statistical theory is not always reliable in such extraordinary room forms, while ray tracing may tend to over or under estimate certain acoustical parameters. Thus, the application of DEM in a finite element scheme for detailed sound energy decay and sound flow analysis are held over some case structures. The results are compared to field tested data and ray-tracing solutions. Pros and cons of DEM in comparison to different methods are searched in detail considering the efficiency in visualization, computational speed, and reliability of acoustical parameter results for specific room shapes.This study discusses alternative models and methods to be applied in room acoustics estimations for specific room types including disproportionate rooms and rooms with coupled volumes. A recent prediction method, namely, diffusion equation model (DEM) in room acoustic applications is utilized in the methodology, and this method is compared to common models as of statistical theory, image-source or ray-tracing techniques. Both long enclosures as of subway stations and coupled volumes as of multi-domain monumental structures are special cases with specific interior sound fields. Statistical theory is not always reliable in such extraordinary room forms, while ray tracing may tend to over or under estimate certain acoustical parameters. Thus, the application of DEM in a finite element scheme for detailed sound energy decay and sound flow analysis are held over some case structures. The results are compared to field tested data and ray-tracing solutions. Pros and cons of DEM in comparison to different methods...

Diffraction simulation from wedges to finite-sized plates based on the physical theory of diffraction

Efficient predictions of sound diffraction around objects are of critical significance in room-acoustic simulations. An advanced diffraction theory has recently been investigated for potential applications in room acoustics for some semi-infinite, canonical wedges and for finite-sized rectangular plates [Rozynova and Xiang, J. Acoust. Soc. Am. 144 (to be published)]. The physical theory of diffraction (PTD) still relies on both geometrical and physical principles, yet emphasizes the physical one. Important features of the PTD approach are its computational efficiency and the high degree of accuracy for the diffracted sound field. This paper reviews the fringe field predictions of canonical semi-infinite wedges and further discusses solutions of diffraction problems on finite, rigid rectangular plates. The PTD is applied to approximate the solutions of a finite-sized, rigid rectangular plate that achieves high numerical efficiency. The PTD simulation allows sound diffraction contributions to be determined independently from two pairs of edges of the rigid plate, while ignoring the edge waves around the corner in far-field. This paper uses numerical implementations of the PTD predictions to demonstrate the simulation efficiency of the PTD in finite-sized objects. The numerical simulations are also validated by some preliminary experimental results carried out using an acoustic goniometer.

Cross-stitched fabric as potential sound absorber for room acoustics applications

Textiles materials are widely used in noise and vibration control engineering. A thin fabric absorber consists of an air cavity and a backing rigid wall, and its main applications include ceiling, curtain, and sound barrier, etc. The main aim of this work is to study the acoustical and sound absorption properties of cross-stitched fabric, which has a similar structure with micro-perforated panel. The sound absorption coefficients of cross-stitched fabrics with different specifications were measured by impedance tube method, where the air cavity distance is 1 cm, 2 cm, and 3 cm, respectively. These distances are suitable to the binding and layout design of cross-stitched fabric painting in practical applications. As industrial textiles, cross-stitched fabric can be made into diversity of artistic patterns through manual or mechanized embroidery technology. This work has also studied the effects of multifarious embroidered patterns on the sound absorption coefficients of fabrics with different air cavity distance. In summary, cross-stitched fabric with exquisite patterns is expected to be a promising sound absorber for room acoustics.