Acoustics Australia最新文献

In this paper, a micro-perforated panel (MPP) composite structure consisting of an inhomogeneous MPP (IMPP) backed with J-shaped cavities of different depths for low-frequency sound absorption is proposed. The goal is to increase the low-frequency (≤ 500 Hz) sound absorption performance of the IMPP. Sound absorption in a frequency range of 300–480 Hz was achieved with parallel-arranged IMPPs backed by J-shaped cavities, with average absorption of greater than 90%. A parametric analysis was used to optimize the structure's geometric parameters for the specified frequency range. The results show that when the length and volume of the back cavity depths increase, the low-frequency sound absorption peaks shift to a lower frequency. Similarly, the sound absorption curves are enhanced and move towards lower frequencies as the thickness of the IMPP increases. The structure was studied using an electro-acoustic equivalent circuit model (ECM) and finite element method (FEM) simulation. Model prototypes were then made using stereolithography (SLA) and verified by a square-shaped impedance tube-based experimental study to determine the normal absorption coefficient. The results revealed that the three types of curves, namely theoretical, FEM simulation, and experimental, were in good agreement.

Effective broadband duct sound propagation control is highly required in many practical engineering applications. In this study, a compact structure constituted by multiple detuned resonators is proposed for broadband duct noise transmission control. The coupling characteristics of two detuned resonators flush-mounted on the sidewall of a duct are firstly investigated. Results show that a coherent perfect absorption (CPA) is induced when these two resonators are precisely designed. Meanwhile, a nearly flat transmission forbidden band is formed, which is very beneficial for duct noise control. Furthermore, it is found that the appearance of the forbidden band is insensitive to the distance between resonators. On this basis, a customized broadband CPA-based structure constructed by detuned resonators is developed, in which the geometric parameters of each adjacent resonator satisfying the CPA condition and the resonators are closely placed. By overlapping the forbidden band of adjacent resonators, a broad duct sound transmission forbidden band is attained. The acoustic performance of the proposed compact design is demonstrated experimentally.

Due to the flexibility of the towed line array, shear currents caused by an internal solitary wave (ISW) distort the array shape. This will lead to a mismatch between the conventional target detection algorithm and the array shape, resulting in a significant decline in the performance of the towed line array gain, azimuth resolution, etc. Based on the improved motion model of towed cable constructed by Ablow and Schechter (Ocean Eng, 10: 443–457, 1983), we propose a motion model of the towed line array under an ISW according to the Korteweg-De Vries (KdV) equation and solve the model by finite difference method combined with the Newton iteration method. In addition, the DFT beamforming theory is used to detect the target signal after the array shape distortion compensation to verify the validity of the model. The data analysis shows that the array shape distortion caused by the ISW is mainly affected by the relative position between the array and the ISW, the amplitude of the ISW, the fluid layer density, the fluid layer depth, the towing velocity, the tangential/normal drag coefficient, the elastic modulus, and the towed cable density. The influence of elastic modulus and the towed cable density on array shape distortion can be ignored. The detection results show that the output signal power is about 6 dB higher than the output noise power, and the array gain and azimuth resolution are improved after array shape distortion compensation.

Human existence is accompanied by environmental sounds as by-products of people’s activities and sounds that are intentionally generated to allow human society to function. The resulting soundscapes are perceived and experienced by listeners within a social context. These soundscapes are composed of a myriad of individual sounds in both the individual and communal social spheres, and may be perceived positively or negatively, with some sounds having significant value attributed to them by certain segments of society. This paper explores the type and classification of sounds in the urban environment and presents a best fit model of sound classification in order to enable a greater understanding of the potential heritage management of this resource.

Controlling low-frequency noise propagated by power transformers in urban and industrial areas poses a technical challenge in the design of sound absorbers. Although existing acoustic metamaterials are largely successful as single-band absorbers, they are far from optimal approaches to absorb power transformer noise. As a common method of providing multi-band absorbers, the parallel configuration of multiple individual units leads to the coupling effect, resulting in the absorption coefficient drop. In this research, a novel design is proposed to convert a single-band Helmholtz resonator-based sound absorber into a multi-band absorber while maintaining its thickness at 55.1 mm, ~ 1/62 of 100 Hz noise wavelength. The proposed design offers simultaneous perfect absorptions at 100, 200 and 300 Hz frequencies as main components of power transformer noise. Design validation is carried out using finite element modelling in COMSOL Multiphysics and the experimental analyses on the 3D-printed sample.

On the basis of a rigorous mathematical description of the motion of monochromatic waves, a method is proposed for determining the acoustic permeability of a wall containing an arbitrary number of layers. The method is based on finding exact distributions for incident and reflected waves in each of the layers under consideration and “stitching” linear combinations of these distributions at all common boundaries of adjacent acoustic zones. As a result, to determine the coefficients of linear representations of solutions, a system of algebraic equations is obtained, which can be solved in any and standard way. The study completes the enumeration of all frequencies from the considered spectrum. As an illustrative example, the axisymmetric problem of sound output from a single-layer pipe of a gas pipeline is considered.

To ascertain the influence of occupational exposure to SPL on the hearing threshold of indoor cycling (IC) instructors. This is a cross-sectional study involving eleven IC instructors at fitness centers in Guarapuava, PR, Brazil Questionnaires were applied to assess occupational characteristics and reported auditory and extra-auditory symptoms. The hearing threshold was evaluated by pure tone audiometry testing at seven moments: after 14 h of acoustic rest, and before and immediately after exposure to SPLs of 95, 85 and 75 dB(A), making a total of 77 measurements of the group of instructors. Differences in means were assessed using the Kruskal–Wallis test. The k-means and composition method was applied using principal component analysis to verify the main factors associated with the broadband multi-frequency measurement at the hearing threshold. The average bilateral hearing threshold showed a significant increase at the intensity of 95 dB(A), at all the tested frequencies (p < 0.05) and at most of the frequencies, at 85 dB(A). Conclusions exposure to the equivalent sound pressure levels of 85 dB(A) and 95 dB(A) significantly altered the average hearing threshold of indoor cycling instructors.

Achieving low-frequency and wide band gaps with a simple and small structure can be a challenging task. In this paper, a new type of six-ligament chiral structure with simple geometric parameters and smaller geometric dimensions is designed. The side length of the chiral unit cell is only 20 mm. The proposed structure and the design idea it embodies may be of assistance to deal with the challenges mentioned above. A numerical simulation is performed to analyze the relationship between the geometric parameters and band gap characteristics of the structure. Based on this relationship, a geometrically optimized structure is proposed and the simulation results show it has better band gap characteristics which can achieve low-frequency and wide band gaps. Moreover, frequency response analysis is used to verify the accuracy of the simulation. Finally, the band gap mechanism is analyzed by analyzing the mode diagrams of the structure.

A single layer of four parallel-arranged inhomogeneous microperforated panels (iMPP) absorber is proposed to achieve low-frequency sound absorption and wider frequency bandwidth. The hole diameter of the four parallel-arranged iMPP is set to be equal to or less than 1 mm. The theoretical formula for calculating the absorption coefficient under normal incident sound is established based on an electrical equivalent circuit model (ECM). The parametric study has been performed on the MATLAB software, and the expected results are obtained. The results indicate that the proposed model can produce a wider absorption bandwidth of 195–455 Hz in the low-frequency region with an average absorption coefficient of more than 90% (α = 0.91). To achieve the desired effect, the absorption coefficient and the bandwidth can be tuned by adjusting the aperture size, perforation ratio, thickness of iMPP with depth and width of the back cavity. Also, it is found that iMPP can produce wider bandgaps with good absorption peaks in the low-frequency region by designing sub-MPP of smaller hole diameter, large perforation ratio, and with large cavity depths and the sub-MPP of large hole diameter, small perforation ratio, and with short cavity depths. The finite element method has been employed on COMSOL Multiphysics 5.5a to simulate the acoustic absorption performance of the model and compared with the ECM-based predicted and square impedance tube-based experimental results. Compared with other homogeneous MPPs of different arrangements, this absorber provides exceptional absorption performance in a low-frequency range due to its lightweight structure, and convenient manufacturing availability, this enhanced form of iMPP absorber has great potential in acoustics and noise control applications.