Electroacoustic material for optimal apparent sound absorption

This paper presents the results of experimental investigations on the sound absorption of two porous structures and the far-field noise performance of an airfoil employing sound-absorbing porous structures at the trailing edge. The porous structure consists of a microperforated housing with an air gap (PA structure) or an acoustic foam insert (PF structure). Sound absorption coefficients have been characterized using an impedance tube. Compared with a commercial acoustic foam, PF structures have shown a consistently higher (up to 0.5 higher) sound absorption coefficient over 0.5–6.4 kHz. Moreover, the frequency characteristics of PA and PF structures are predictable, which allows their geometric optimization to suit different applications. Far-field noise of airfoils has been measured using a 64-channel microphone array at various flow conditions. The sound-absorbing microtube structure is found to reduce the trailing-edge (TE) noise by up to approximately 12 dB at lower frequencies (1.5–6 kHz), while increasing high-frequency (over 6 kHz) noise levels due to the roughness elements of porous geometries and unsteady flow permeation. The peak frequency of noise reductions is found to not scale with the flow speed, but remains within a narrow frequency band at different flow conditions (U∞=30−50 m/s, αg=0°−10°). This is consistent with the narrow-band feature of microtube structures’ sound absorption, which indicates the contribution of sound absorption to TE noise reduction. The PF structure, due to its broadband sound absorption and tunable frequency characteristics, is considered a promising device for trailing-edge noise control.

The application of solutions based on conventional porous materials for low-frequency sound absorption is often limited to the available space for their installation, as these require a significant thickness to be efficient. A system combining of a microperforated panel with an array of Helmholtz resonators, properly designed, can provide effective noise absorption at low- and mid-frequencies with a limited thickness. In this work, the sound absorption coefficient of several systems of this type was assessed by analytical calculations, numerical simulations and experiments carried out in an impedance tube according to the standard ISO 10534-1:1996. The prototypes were 3-D printed using recycled materials. The absorption coefficient, analytically obtained, was significantly higher than that measured. However, the results from numerical simulations are in good agreement with measurements, showing that the numerical model carried out can be used for further studies to optimize the system design. An example is given by increasing of the number of Helmholtz resonators in the array, that yields an increase in the sound absorption. Results show the potential of this type of solutions to be used for low-frequency noise absorption in applications where the available space is limited.

Acoustic properties of a porous polycarbonate material produced by additive manufacturing

This paper reports the influence of nanosilica on friction and sound absorption responses of concrete for application in rigid pavements. The paper also discusses the feasibility of applying nano-lotus leaf as a coating for concrete material. Previous research on friction and sound absorption characteristics of concrete pavement primarily emphasized creating different surface textures through macrotexture modifications. The microstructure can also have a significant effect on surface texture, friction, and sound absorption characteristics of concrete. This research studied the friction and sound absorption properties of concrete on the basis of microtexture modification that used nanomaterials such as nano-silica and nano-lotus leaf. Several laboratory concretes were produced by using different proportions of nanosilica, which partially replaced cement by weight. The fresh concretes were tested for workability, wet density, and air content. The hardened concretes were tested for compressive strength, friction, and sound absorption. The British pendulum test was used to determine the friction number. An impedance tube was used to determine the sound absorption coefficient. Preliminary results indicate that nanosilica can increase friction and sound absorption of concrete pavements. In addition, the results show that nano-lotus leaf can be applied as a coating on concrete material for rigid pavements to improve retention of their friction properties during the wet season.

A sound absorption material composed of styrene butadiene rubber and high impact polystyrene blend has been developed. With the increased amount of HIPS in SBR/HIPS polymer blends, hardness and Izod impact strength values of the resultant material increased. Sound-absorbing characteristics of SBR/HIPS polymer blends were investigated in the impedance tube, according to transfer function method. The sound absorption coefficients are generally above 0.2 with the maximum of 0.5. It reveals that soft blends have good sound absorption ability in 4000 Hz frequency range. For all groups the least sound absorption rate was seen in the area of 315 Hz at the rate of 0.01. With the increase of frequency for all groups impedance ratio decreased. With addition of SBR rubber to the HIPS, the adhesion and distribution of the present phases were considerably enhanced as well.

This paper presents an investigation on sound absorption performance of aerogel/polymer nonwoven fabrics. Polyester/polyethylene nonwovens embedded with hydrophobic amorphous silica aerogel were chosen for sound absorption measurements. The sound absorption coefficient (SAC) of single and laminated layers of aerogel nonwovens blankets was tested by Brüel and Kjær impedance tube, the noise reduction coefficient (NRC) was used for numerical analysis. A sound absorption index was developed to analyze the effect of aerogel content on sound absorption ability. The effect of air-back cavities on SAC of single-layer aerogel/polymer nonwoven fabrics was investigated. The results show that there is a decrease in SAC with the increase of aerogel content. It is observed that the NRC linearly increased with the increase of layers for all the samples. It was also found that the air-back cavities result in resonance phenomenon, as the increase in thickness of air-back cavities the peak values of SAC shift toward lower frequencies.

The multilayer sound absorber (MSA) is typically a composite structure that consists of a micro-perforated panel (MPP) with circular perforations, a layer of porous material, and an air gap. This combination enhances the sound absorption coefficient (SAC) across a broad frequency range. However, the sound absorption mechanisms of the MSA when incorporating polygonal cross-section perforations have not been thoroughly investigated. In this paper, the acoustic impedance of MPP with square or equilateral triangular cross-section perforations is simply given based on a complex density function and the radiation impedance in short tubes with square cross-sections. Theoretical acoustic impedance models are established for three configurations: CMSA (circular perforations), SMSA (square perforations), and EMSA (equilateral triangular perforations), using the transfer matrix method (TMM). To validate these models, SAC measurements were conducted via an impedance tube, showing good agreement with theoretical predictions. Further analysis of the acoustic performance revealed that the SMSA configuration exhibits a higher resonance peak and a broader absorption band. Finally, a simple optimization of the hole diameter and air gap was performed using an oppositional Runge Kutta optimizer with cuckoo search (OCRUN). The results demonstrate that the EMSA achieves quasi-perfect sound absorption within specific frequency bands using a smaller air gap compared to both CMSA and SMSA.

Selective frequency band filtration enables targeted acoustic wave attenuation, with applications in noise control, telecommunications, and vibro-acoustic systems. In passive noise cancellation earmuffs, selective frequency filtration can be used to address the challenge of workers needing to remove earmuffs for communication exposing them to harmful noise levels. By integrating periodic lattice structures, earmuffs can selectively allow safe low- to mid-frequency sounds to pass through while blocking harmful high-frequency noise. This study proposes a novel maltese-cross-honeycomb unit cell design, integrating Bragg scattering, and local resonance mechanisms through the Helmholtz resonator concept into a lattice structure to achieve broadband and selective sound filtration. Sound absorption and transmission loss were experimentally characterized using an impedance tube, analysing the effects of thickness, cell arrangement, and cell opening orientation. A validated finite element modelling model was used to study structural displacements and sound pressure levels (SPLs) within the impedance tube, correlating SPL values with absorption coefficients. The proposed metamaterial design exhibits a peak sound absorption coefficient of approximately 0.99 with high sound absorption retention across the frequency band of 3000–6400 Hz, with a transmission loss of 20–25 dB. On contrary, the published triangular reentrant honeycomb design could not exhibit retention of sound absorption across the frequency range, and the transmission loss value of only 10–12 dB was achieved. The optimized metamaterial design was then adopted as an insert for earmuff application demonstrating required hearing protection and speech intelligibility (up to 95 dB pink noise). Based on these findings, the maltese-cross-honeycomb unit cell metamaterial design is proposed to be adopted for earmuff applications to show selective frequency filtration by allowing low to mid-frequency waves (speech up to 3000 Hz) to pass through while blocking off the unwanted high-frequency sound waves.

The effects of chemically treated natural fibres (rice straw and kenaf) embedded as filler into polypropylene matrix were investigated for its sound absorption properties to regulate the industrial noise. In this respect, untreated natural fiber as well as treated natural fiber reinforced with polypropylene composites were fabricated and compared. The composites were prepared by compression moulding technique. Its sound absorbing characteristic was investigated with the Impedance tube, according to a transfer function method. A two microphone setup was fabricated according to American society for testing materials ASTM E1050-10 and it is used to measure sound absorption coefficients of composites in the frequency range of 300 Hz to 2000 Hz. The sound absorption coefficients of the composites increased with the frequency. However, at 1000 Hz, the sound absorption coefficient decreased for all treated samples and then increased again which is due to specific character of natural fibers. This point of inflexion was due to the specific characteristic of natural fiber reflecting sound at around 1000 Hz, but absorbing sound in the middle and high frequencies. The results indicates that the process of chemical treatment enhanced the sound absorption coefficients by 12.5% for rice straw reinforced Polypropylene and 15.78% for kenaf fiber reinforced Polypropylene composites respectively.

Sound absorption properties of a natural waste of a low density date palm fibre perforated panel have been presented. The measurements were carried out for a single layer sample using an impedance tube. A simulation was performed using Delany and Bazley model for the sound absorption coefficient for the single layer date palm fibre. The experiment was conducted for the panel with and without perforated plate facing. Three air gap thicknesses were used between the date palm fibre sample and the rigid backing of the impedance tube to study the effect of the air gap on the sound absorption. The results showed an improvement on the sound absorption coefficient at higher and lower frequency ranges when facing the panel with perforated plate. Further studies could be conducted to check the effect of different densities and perforations on the sound absorption of the date palm fibres.

The purpose of this study is to investigate the role of density and pile height on sound absorption coefficient in Double Base Persian (DBP) rug and possibility of prediction the acoustic behavior of DBP rug using the mathematical model. For this aim, in the first step, three double base rug samples were produced at different base densities (2, 4, and 6 warp yarn/cm) and the sound absorption coefficient of samples was measured with Impedance tube in two thicknesses (15 & 13 mm) to study the pile height effect. Moreover, the sound absorption of the double base zone was also measured by shaving off the pile from the double base rug samples. Three rug samples at different base densities were produced with very thin warp and weft yarns to avoid the base effect in this sample. Besides, the macroscopic empirical model (Johnson–Champoux–Allard (JCA)) was implemented on obtained data. The results showed that the sound absorption of the double base rug samples increases with increasing the pile height and base density. The role of the base zone in the sound absorption of the rug is bolder than the pile zone. What leads to improve the rug sound absorption by increasing density is increasing the sound absorption of base zone and the pile density changes do not play a major role in increasing the rug sound absorption. In addition, by assuming DBP rugs as a two-layer porous (pile + base zone) absorber, JCA model shows a good consistency with experimental data.

Tunable acoustic composite metasurface based porous material for broadband sound absorption

Environmental pollution is escalating due to inadequate waste management, with the open burning of agricultural waste being a significant contributor. This process releases various harmful gases into the environment. This study introduces an innovative approach to creating sound absorption materials using agricultural by-products, specifically paddy straw and coconut coir, along with newspaper by-products. The research was conducted in two phases: first, the production of sound absorption panels with different densities and adhesive quantities, and second, the evaluation of these panels’ sound absorption capabilities through laboratory experiments. The impedance tube test was used to determine the sound absorption coefficient (SAC). The results showed effective sound absorption, especially at lower frequencies ranging from 125 Hz to 6300 Hz. Notably, paddy straw and coconut coir exhibited significant sound absorption values at 1,000 Hz (0.59 and 0.52, respectively). This study highlights the potential of paddy straw and coconut coir as sustainable, cost-effective materials for sound absorption panels. These natural materials demonstrate excellent sound-absorbing properties, making them suitable for various applications such as classrooms, sound recording rooms, auditoriums, and theaters at low to medium frequencies.

Measurements of acoustic properties of sound absorbing materials in impedance tubes show poor reproducibility, which was demonstrated in round robin tests. The impedance tube measurements are standardized but lack precise definitions of the actual measurement setup, specimen preparation, and other factors that introduce uncertainty in practice. In this paper, machine learning models identify those factors that mostly affect the sound absorption coefficient from a large data set of more than 3000 absorption spectra measured in one impedance tube. The specimens are manufactured from one polyurethane foam, and different cutting technologies, different operators, different specimen diameters, different specimen thicknesses, and two different approaches to mount the specimens in the impedance tube are considered. Explainable machine learning techniques allow the identification and quantification of the most influential factors and, furthermore, the frequency ranges that are the most affected by the choice of these setup factors. The results indicate that besides the specimen thickness, also the operator affects the absorption coefficient by a directional and non-random relationship. Hence, it needs to be controlled carefully. The method proves to be a promising pathway for knowledge discovery from acoustic measurement data using explainability approaches for machine learning models.

The third octave sound absorption coefficient testing is conducted to compare the sound absorption properties metal foam and flexible cellular materials, by using sound absorption tester with the method of trasfer function sound absorption tester with the method of trasfer function. The sound absorption mechanisms are discussed by changing the parameters of sound absorption structure, such as the thickness of matrix materials and the thickness of cavity. The results show that pearl wool and glass wool exhibited excellent sound absorption properties. The peak value of sound absorption coefficient for pearl wool reaches to 0.991, and for glass wool, 0.985. The average sound absorption coefficient for pearl wool is 0.729, and for glass wool, 0.679. Among of three metal foams, the foamed aluminum material exhibited optimum sound absorption properties, and is superior to flexible sound absorption materials. The peak value of sound absorption coefficient reaches to 0.993, and the average value reaches to 0.781. This can be attributed to the flow resistance, porosity, thickness, cavity and structure factor, which influence the sound absorption of open cell materials.