Sound Absorption Research Articles

Micro-perforated plates constructions with approaching the theoretical limits of its sound absorption performance

Micro-perforated plates resonant absorbers have a considerable theoretical sound absorption bandwidth upper limit. Three types of MPPs with straight holes, including circular holes on thick plate, circular holes on thin plate, and slotted holes on thin plate, are investigated with approaching the theoretical limits of its sound absorption performance. It is demonstrated that thin plates are easy to obtain satisfactory sound absorption performance but hard to get high mechanical strength. Slotted holes are beneficial for extending the sound absorption bandwidth of MPP absorbers. MPPs with straight holes usually require a combination of thin plates, minute holes and high perforation ratios, which is practically difficult to apply on a large scale. Two other types of MPPs with variable cross-sectional holes, fabricated using the subtractive processing method and superposition processing method, are then proposed to achieve perfect sound absorption performance and good engineering feasibility.

Experimental study on the acoustic properties of monofilament fabrics

The traditional machining perforated sound absorption structure has high processing cost and inconvenient installation and disassembly, so that how to efficiently and economically manufacture submillimeter micropores still remains a challenge. In this study, a kind of flexible microporous sound absorbing material was obtained by weaving a polymer artificial fiber covered with ethylene polyester fiber, and the relationship between the sound absorption performance of the curtain and the porosity of the pass and fabric was obtained through the finite element simulation model and impedance tube experiment. The mechanism parameters of wide-band flexible microporous material with high sound absorption performance are obtained, which guide the design of a kind of sound absorption material with lower production cost and easy installation and disassembly.

Simplification of Johnson-Champoux-Allard-Lafarge model on fibrous materials

The Johnson-Champoux-Allard-Lafarge (JCAL) model, a pivotal framework in acoustics for analyzing sound absorption in fibrous materials, presents challenges in practical implementation due to its complexity. This paper introduces a refined variant of the JCAL model explicitly tailored for fibrous materials, aiming to balance accuracy and computational efficiency. The proposed model retains essential physical principles while simplifying computational intricacies, enhancing its usability for real-time applications and large-scale simulations. Through comprehensive validation against experimental data, we demonstrate the efficacy of the simplified model in accurately characterizing the sound absorption behavior of fibrous materials. This study contributes to advancing the practical applicability of the JCAL model, facilitating its broader adoption across various acoustic engineering domains. The proposed simplification approach holds promise for accelerating research and development in sound absorption materials and acoustic design methodologies within scientific and engineering communities.

Experimental validity of using a ResNet to predict sound absorption coefficients of finite samples

The validity of using a neural network to predict sound absorption coefficients of finite porous materials is tested with experimental data with a flush-mounted glass wool sample on a baffle. The network is pre-trained and validated with numerical simulations of flushed-mounted finite absorbers using a Delany-Bazley-Miki model. The experimental setup consists of a 12 x 12 microphone array placed above the absorber and a sound source placed at angles of 0, 40, and 75 degrees with respect to the normal of the sample. The sound absorption coefficients predicted at normal incidence by the network are compared with the impedance tube method as a reference result.

Design and analysis of acoustic metamaterial sound insulator for noise reduction at multiple frequencies

In this paper, a design of an acoustic metamaterial sound insulator based on porous material with embedded multiple Helmholtz resonators is proposed for noise reduction at multiple frequencies. The periodic unit cell of the metamaterial is made of the porous layer with embedded four, nine, and sixteen different Helmholtz resonators that are arranged in parallel. The cavities of the resonators have the same volume while the parameters of the necks are varied. The sound absorption coefficient and the transmission loss of the metamaterial obtained using finite element method show four, nine and sixteen resonant peaks that correspond respectively to the number of Helmholtz resonators within the unit cell. When the parameters of the necks and the cavities are identical resulting in identical resonators, the transmission loss and the sound absorption coefficient show only one resonant peak. The proposed metamaterial can attenuate the noise at four, nine, and sixteen different frequencies by choosing the number of resonators that constitute the unit cell and adjusting the parameters of the necks. It can be used in many engineering applications such as aerospace for noise attenuation at multiple frequencies.

An applicability discussion of the in-situ measurement method using the ensemble averaging technique for sound absorption characteristics

In our former papers, the authors have proposed a measurement method for sound absorption characteristics, surface impedance and sound absorption coefficient, of materials using the ensemble averaging technique. We named it EA-method, for short. So far, we have shown the robustness as well as the reproducibility of the results obtained by the method at various environments. The method consists of two kinds according to sensor types employed: i.e. two-microphone and PU-sensor. In this paper, the applicability of the EA-method is discussed onto three types of absorption: porous, panel, and resonator. At first, the outline of the EA-method is summarized and measurement mechanism is explained. Then, the robustness as well as the reproducibility of the method are discussed based on accumulated results of measurements conducted in various environments including reverberation rooms and in-situ conditions.

Sound absorption coefficient in the audible and ultrasonic frequency range measured in a reverberation chamber with an omnidirectional parametric loudspeaker

An omnidirectional parametric loudspeaker (OPL) is a sound source that contains hundreds of ultrasonic transducers set on the surface of a sphere. Each transducer emits an ultrasonic collimated beam consisting of an audible exponential sweep sine (ESS) modulated in amplitude. This results in hundreds of ultrasonic sound waves emitted in all directions, but also audible sound waves generated by the parametric acoustic array (PAA) phenomenon, by which a modulated signal returns to the audible range thanks to nonlinear propagation in the air. This work examines how the OPL performs in assessing room acoustics, specifically in determining material sound absorption coefficients within reverberation chambers. The ISO 354 standard requires measuring reverberation times with the chamber empty and with the material sample. These are measured using ESS, which has proven effective for the OPL performance. This signal concentrates all the sound power into a single frequency, which improves ultrasonic power levels and thus the signal-to-noise ratio of the audible sound field. The absorption coefficient is next evaluated following the standard, and the results are compared with those obtained using a standard dodecahedral loudspeaker. Finally, the absorption coefficient in the ultrasonic range is examined to evaluate the OPL performance in this frequency range.

Comparison of sound insulation performance between closed-cell foam and automotive carpet

In developing sound insulation materials for automobiles, multiple sound insulation and sound absorption materials are laminated to achieve high performance. Generally, steel sheets, needle felts, and carpets are used to create sound insulation properties. In a luxury car, multiple layers with sound absorption and sound insulation properties are placed on the carpet to add higher sound insulation properties, but this increases the mass of the carpet. To solve this problem, we are investigating the the potential to realize both lightweight and high sound insulation properties using closed-cell form. In this study, we experimentally evaluated the sound transmission loss of closed-cell foam by comparing it with those of carpets with the same configuration used in standard and luxury cars.Furthermore, a numerical study is conducted. As a result, the closed-cell foam obtained higher sound transmission loss than the automobile specification carpet in a wide frequency range from 400 Hz to 5000 Hz. On the other hand, the sound transmission loss decreased by the same degree for both the automotive carpets and the closed-cell foam at 250 Hz.

A causal model for acoustic assessment of informal learning environments

Irrelevant and intelligible speeches are the most effective distractive noise sources in open-plan environments. They are inevitable in informal learning environments as group assignments are very common. Informal learning can be defined as any additional learning activity that takes place outside the classroom and is conducted individually and collaboratively on campus. This study will examine the impact of sound-absorbing materials on students' attention in informal learning environments in a causal manner. To examine the effect of materials, 3D models will be designed by changing the sound-absorbing materials, using reverberation time as a reference metric by applying the auralization process. The control and treatment models will be provided. The statistical significance of the impacts will be investigated using pairwise correlations, ANOVA, and multivariate regression methods.

Vegetal wools for building application: investigation of optimization approaches for the enhancement of low-frequency sound absorption

Bio-based building materials, such as vegetal wools for thermal and acoustic applications, are increasingly used in construction. Indeed, as they store atmospheric carbon dioxide, they have a lower carbon footprint than conventional materials. Despite high-level multi-functional properties, they suffer from poor low-frequency absorption for thin panels (less than 5 cm thick). Therefore, it seems relevant to investigate the optimization possibilities available, including meta-material approaches, and to adapt them to the specificities of vegetal wools, such as a low air resistivity for hemp and flax wools which are our main concern. To meet this challenge, a state-of-the-art is being carried out to identify the optimization methods best suited to the specific characteristics of vegetal wools. This preliminary work is based on the implementation of an initial optimization concept using a multilayer poroelastic material with a perforated resistive layer. The material structure is optimized using analytical modeling to target low frequencies. Experimental verifications are then carried out to determine the sound absorption coefficient of vegetal wools with optimized configurations.

Analysis of in-situ acoustical performance of a removable urban pavement with a porous top layer

A demonstrator of a new removable urban pavement with functionalized surface was implemented in the inner city of Nantes within the I-STREET CUD-SF project. Designed for easy maintenance operation, it is made of hexagonal cement concrete slabs with a dense body and a 4 cm thick porous top layer of grading 0/6 and porosity 25% for drainability. The noise performance of the pavement was assessed at urban speeds by simultaneous Coast-By and Close-Proximity noise measurements performed with a dedicated test vehicle after closure of the road to traffic. The results are compared with noise levels obtained with three reference surfaces, a DAC 0/10 and two VTAC of grading 0/4 and 0/6. The tested pavement gets ranked differently depending on the noise measurement method considered. The results are analyzed in the light of 3D texture measurements as well as sound absorption measurements performed in situ by the extended surface method and with Kundt's tube on core samples. The influence of texture irregularities at the junction between slabs and sand used for slabs stabilization is addressed. Finally, the Coast-By noise levels are compared with those obtained with the HyRoNE prediction model from 3D texture and sound absorption. The discrepancies observed are discussed.

Acoustic materials based on the Gosper curve

In this work, slotted acoustic materials based on a space filling curve called the Peano-Gosper curve are proposed and investigated. The slits in such materials form a complex pattern because they are divided by walls built along lines generated by the Gosper curve algorithm. The pattern can be twisted around an axis normal to its surface to increase the tortuosity inside the material, and therefore, modify its acoustic properties, which can be controlled by the turning angle or pitch of the twist. A highly efficient semi-analytical model has been developed to accurately predict the acoustic properties, in particular the sound absorption of such materials. It only requires a representative part of the pattern, or better, scanning the surface of the fabricated material so that the actual geometry and dimensions (in particular slit widths) are well reproduced in a two-dimensional finite element mesh generated on a representative fluid domain. The mesh is used to solve a dedicated Poisson problem and determine a few key parameters, and the rest of the modelling is based on analytical formulas. Material samples with straight and twisted slit patterns were 3D printed and then measured in an impedance tube to confirm semi-analytical sound absorption predictions.

Low cost modelling tools for the design of additively manufactured acoustic materials

Additive manufacturing (AM) methods have unlocked new capabilities in the design of acoustic materials and their topologies. This has enabled the design of devices with breakthrough performance in sound absorption and insulation while achieving subwavelength characteristics. However, material fabrication through AM presents its own challenges. These challenges include inherent defects which can be specific to the printing method employed. Examples of defects in low-cost AM include void formation, surface roughness and poor bonding between layers. Each of these features of AM have different acoustic impacts which may degrade the performance of a design. To optimize the production of additively manufactured acoustic materials it is imperative that designers can exploit features, such as surface roughness, to enhance rather than degrade the overall material performance. The aim of this paper is to investigate the effects of surface roughness in extrusion-based AM on acoustic material performance. A novel, low-cost numerical methodology is used to capture the influence of surface roughness in a subwavelength acoustic absorber. With this approach the impact of roughness can be captured in the design process. The intention is to develop efficient low-cost design methodologies that can be exploited for industrial development of novel additively manufactured acoustic materials.

General ventilation filters: what acoustic characteristics?

In the field of general ventilation, air filters are widely used, and introduced in ventilation ductworks, or air-handling units. These filters are passed through by air flow and noise. However, their characteristics of sound attenuation and sound generation are very poorly known, used and tested. This presentation reports a study done in collaboration with French air filtering manufacturers members of CETIAT. More than thirteen 592x592 mm filters representing four different types of air filters commercially available have been tested in laboratory, to characterise the insertion loss and the noise generation. Those types of filters are mini pleated (50 & 100 mm thickness), V-bank, bags and pleated, with different materials such as glass fiber and synthetic fiber. In addition, the effect of the dust loading has been investigated the sound characteristics as well as the influence of a series installation of a prefilter and a filter. These experimental data will be helpful for acoustic studies of ventilation networks. Some results about sound absorption can be related to the characteristics of the filters, thickness or shape but also to the loading and the use in series, whereas no obvious relation can be found for sound generation.

Sound absorption metamaterials designed by machine learning and their applications

Acoustic metamaterials are artificial materials composed of acoustic functional elements along the spatial order, which have extraordinary acoustic properties that are not possessed by homogeneous materials. Since the conception of phononic crystal in the 1990s, acoustic metamaterials have gradually matured, which prospects industrial application. In our report, we present an acoustic metamaterial design method based on deep learning for sound absorption. The design method is firstly set up with the array of Fabry-Perot resonators, then adapted for the array of Helmholtz resonators. The method uses a tandem inverse design model based on the convolution neural network(CNN), which is capable of on-demand design. It can generate acoustic metamaterial design of high absorption in targeted frequency range. Three designs of acoustic metamaterials for different purposes are presented. Firstly, sound absorption metamaterials with selective bandwidth absorption, are used to control tonal noise from electric transformers. Secondly, metamaterials with broadband absorption are used in vehicle cabin noise control. Lastly, full-band near-perfect absorption materials are created to construct acoustic anechoic chambers.

Comparative analysis of acoustic testing methods of a multi-layered material: uncovering the membrane effect

This paper presents a comprehensive investigation into the differences observed when evaluating the acoustic properties of a multi-layered material using different standardized methods. The primary focus is on the membrane effect of the upper aluminum layer, which has been shown to contribute to the overall acoustic absorption. This phenomenon, crucial for applications requiring enhanced acoustic performance at lower frequencies, is observed in tests conducted with large sample sizes but remains undetected in impedance tube measurements. Through a series of experiments, including reverberant chamber, impedance tube, and in-situ absorption measurements, this study investigates the complexities of accurately estimating sound absorption characteristics in multi-layered materials. Additionally, 2D colormaps to visualize the spatial distribution of sound absorption across different samples are examined, offering deeper insights into the uneven absorption patterns that standard testing methods may overlook. The experimental findings highlight the importance of selecting an appropriate evaluation technique for multi-layered materials, particularly in applications where acoustic performance is critical at lower frequencies.

A study on the difference in noise reduction between rear and side microphone on CPX measurement method through sound absorption frequency analysis

The road noise evaluation method in Korea is based on the 'noise vibration process test standard' by the Ministry of Environment. In this evaluation method, the noise felt at the sound-receiving point is measured as LAeq. Recently, in order to clarify the noise measurement standards for low-noise pavements, an international standard (ISO11819-2, CPX method) was first introduced in the Porous Pavement Guidelines of the Ministry of Land, Infrastructure and Transport. According to ISO, CPX measurement method adopts side microphones as mandatory, and the rear microphone as optional. Due to the difference in location noise level is different, causing confusion in the road noise evaluation method. Researchers have published several studies in the past that the rear microphone is highly correlated with the equivalent noise level method based on the Korean standard. As a follow-up study to the above results, this study examines the characteristics and causes of the frequency spectrum of the rear and side microphones in the case of dense asphalt pavement and porous asphalt pavement and derives the results through frequency analysis of the sound absorption coefficient test to determine and to contribute improving reliability and institutional rationalization of the CPX measurement method.

Towards topology optimisation of poroelastic materials using field-based heuristics

In passive noise control, it is well known that the shapes of acoustic porous material absorbers can be modified to achieve favourable resonances resulting in high sound absorption. However, finding the optimised shapes for a given sound field environment is challenging as it involves solving an inverse problem. Recently, the use of topology optimisation for designing the shapes of acoustic porous material absorbers has gained interest. Previous studies suggest that conventional gradient methods can often result in local optimal results that may be poorer in overall absorption than what can be achieved by using heuristic optimisation techniques. In this study, the use of field variables such as acoustic pressures and velocities in developing better heuristic optimisation methods are explored with the aim of avoiding cumbersome gradient computations. The acoustic shapes produced by the field-based heuristics are compared with traditional approaches to develop insights as to what patterns in the shape designs contribute to improved sound absorption in various target frequencies.

Virtual assessment of phone booths' acoustic performance in laboratory and office environments

The irrelevant speech noise is one of the significant issues affecting workers' productivity and comfort. Acoustic furniture is the most common solution addressing this problem: mobile phone booths and partially closed workstations are two examples. The acoustic performance of such devices is defined with laboratory tests according to ISO 23351-1:2020. The sound power level measured in reverberation rooms with and without the product determines the reduction of speech A-weighted sound power level (DS,A). However, predicting acoustic performance in real-world scenarios is still challenging. The present work explores the discrepancies between DS,A, and apparent speech reduction (D'S,A) through numerical models. The study assesses the performance difference between a calibrated virtual reverberation room and virtual open-plan offices maintaining the same acoustic properties, i.e., sound transmission and sound absorption, of a typical two-person phone booth. Moreover, the study investigates the fluctuation on D'S,A changing main room acoustic criteria, e.g., the reverberation time (T) and the total absorption area (A). Results help define ray-based simulations' reliability in predicting furniture ensembles' acoustic performance.

A fundamental investigation on the sound absorption characteristics of acoustic meta-surface type baseboards

The sound of footsteps generated in corridors propagates into surrounding spaces, sometimes becoming a problem of noise pollution. Therefore, the aim of this study is to reduce footstep noise by incorporating sound absorption mechanisms into the baseboard of corridors. We focus on the acoustic metasurface that has gained significant attention in recent years in order to absorb a relatively wide range of frequencies. In this study, we proposed acoustic metasurface type baseboards by utilizing multiple resonators with different resonance frequencies. The proposed models feature parallel arrangement of multiple resonators with straight or inclined slits. The absorption performance of these models is evaluated through theoretical analysis, numerical simulations, and experiments. Results suggest that acoustic metasurface type baseboards is found to be able to absorb the dominant frequencies in the sound of footsteps. In addition, increasing the number of resonators and incorporating slanted perforations in the neck contribute to improved absorption performance. Furthermore, the possibility of thinning the models through slanted perforations is also suggested.