Acoustics Australia最新文献

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.

During an experiment conducted in the deep South China Sea, a bottom-mounted horizontal linear array (HLA) collected the radiated noise from a moving cooperative ship at a distance less than 13 km. Bearing-time records of the HLA through plane-wave beamforming showed significant bearing estimation errors and two or three split bearing tracks of the ship that leads to misjudgment of target numbers. To reveal the physics underlying the experimental phenomena, the directional response of the HLA to an acoustic source at close range was developed by normal mode theory. Numerical simulations were conducted to analyze the acoustic intensity distribution characteristics and the arrival structures of the acoustic field. Comparative results of beamforming obtained with normal mode and ray theory show that the ray model provides an easy way to predict the split bearings for HLA beamforming. When sources are near the endfire of the array, significant bearing estimation errors and the beam splitting effect for a large aperture array were the main issues. For sources near the broadside of the array, the beam broadening effect was the most significant concern. The effect of the array length on the beamforming anomalies and the array gain loss was also investigated. With the present theories, the HLA beamforming anomalies during the experiment were well explained.

During daily work, bus drivers are exposed to various environmental factors, and their mental responses to these risk factors are still poorly understood. This study aimed to investigate the cognitive performance of bus drivers with respect to noise and vibration exposure. The study was conducted in 103 healthy city bus drivers. Based on their work schedule, the drivers' exposure to noise and vibration was measured using the Svantek SV 104 noise dosimeter and the SV 106 vibration meter, respectively. The simple Stroop test was used to measure drivers' selective attention capacity and skills as cognitive performance indicators. Drivers' job stress was determined using a standard questionnaire. The drivers' exposure levels to noise, whole-body vibration, and hand-arm vibration was 79.50 ± 3.51 dB, 0.620 ± 0.159 m/s², and 0.438 ± 0.064 m/s², respectively. Significant differences were observed in the interference score (IS) and interference time (IT) after driving (p < 0.05), which indicates a decrease in the number of true responses and an increase in response time. The multiple linear regression model showed that noise and vibration, as main environmental stressors in the presence of other individual's covariates such as age, work experience, and job stress, have significant effects on cognitive performance based on changes in IS and IT during driving (model accuracy; r = 0.61 and r = 0.57). The traffic load was also significantly associated with changes in IS and IT (p < 0.05). The possibility of mental function loss while driving indicates that appropriate occupational health surveillance must be implemented for bus driving occupations.