The sound absorption effect of a Helmholtz resonator was induced by creating an aperture in a nonwoven fabric sheet with a back air space, thus obtaining a sufficient sound absorption effect in a broad frequency range. In this study, transfer matrices were used to obtain the sound absorption coefficient. Transfer matrices were used to represent the following mathematically: a nonwoven fabric model based on the simplified limp-frame model, an acoustic element that considers the vibration of the nonwoven fabric sheet, the air space behind the nonwoven fabric sheet, and the aperture of the nonwoven fabric sheet. Equivalent circuits combining these transfer matrices were used to obtain sound absorption coefficient. The Helmholtz resonator sound absorption effect was more pronounced for the nonwoven fabric sheets with greater ventilation resistance. The results showed consistency between the experimental and theoretical trends. As for the sound absorption coefficient derived using the theoretical models for nonwoven fabric sheets, the results of the simplified limp-frame model were the closest to the experimental values. As the length of the back air space increased, the peak sound absorption value shifted toward low frequencies due to the sound absorption principles of both the nonwoven fabric sheet with a back air space and the Helmholtz resonator.

In this paper, the band gaps in a mass-spring structure composed of resonator with elastic support are investigated. The band structure, effective parameters, and transmission spectra of the acoustic metamaterials are calculated numerically. The numerical results show that the elastic support and resonator in the structure generate the first and the second band gap, respectively. The effective parameters are analyzed to reveal the nature of this phenomenon and calculated to obtain the edge frequencies of band gaps. The transmission spectra are in accordance with the complex band structure. It is found that the attenuation factor is non-zero in band gap, and it determines the shape and magnitude of transmission ratio in band gap. The influences of the structural parameters on the band structure and attenuation factor are investigated numerically. Numerical results show that the band structure can be modulated by changing the structural parameters, and the topological structure will be changed when those parameters change greatly.

To study the damping effect of different track structures on the environmental vibration, field tests of train-induced vertical acceleration were performed on four types of track structures, namely, ordinary track (OT), medium vibration-damping fastener track (MVDFT), floating-ladder track (FLT) and steel-spring floating slab track (SSFST). The measurement points were set on the rails, track slabs, and tunnel walls. Eventually, the time domain, frequency domain, and one-third- octave frequency division vibration level were adopted to investigate the vibration characteristics of four track structures. The results were obtained as follows. (1) The root-mean-square acceleration values of the four track structures at different measurement points were analyzed in the time domain. Overall, the results could not effectively reflect the damping capacity of the vibration-reduction measures. (2) Based on the one-third octave frequency analysis, regarding 10À6 m/s2 as the decibel reference value, we found that three vibration-reduction measures' vertical vibration level of the tunnel wall was significantly lower than that of OT. Specifically, the damping effect of SSFST was the most obvious. The Z-weighted vibration levels of the three vibration-reduction measures on the tunnel wall have been reduced by 8.919, 11.745 and 13.744 dB, respectively. (3) According to the results of the vibration-transmission characteristics in the frequency domain, the main vibration-frequency regions of the four track structures at the track slab and tunnel wall were all in the range of 1 to 20,00 Hz. Concerning OT, MVDFT, FLT, and SSFST, the peak values of the vibration transmission from the rail to the tunnel wall sequentially decreased, and the damping effect sequentially increased. This paper compared the damping capacity of the above four track structures at 1 to 80 Hz according to ISO 2631-1. In addition, their more detailed vibration characteristics at 1 to 2000 Hz were also considered. It aims to provide a more comprehensive study for the application of subway vibration-reduction measures.

In this paper, an in-plane Z-shaped structure with multi-span elastic supports is proposed to investigate the natural frequency and transmission response through employing in-plane governing equation and transfer matrix method. Based on the solutions of transverse vibration and torsional vibration, the total transfer matrix for the Z-shaped structure with multi-span elastic supports is derived. Furthermore, natural frequencies and mode shapes of the Z-shaped structure are calculated. Finite element simulation method (FEM) is conducted here to verify the theoretical results. In order to effectively evaluate the vibration reduction performance, transmission response of the Z-shaped structure under different boundary supports and multi-span elastic supports is analyzed. This work is significant for the vibration reduction of in-plane Z-shaped structure, especially the multi-span elastic support case in engineering applications.

Predictors of people's responses to noise inside cars are used by car companies to identify and address potential noise problems. Because significant advances have been made in the reduction of engine, powertrain, and tire/road noise, it is now important to pursue reductions in wind or aerodynamic noise. While models of loudness are commonly used to predict people's responses to stationary wind noise, some wind noises are less acceptable than is predicted by the loudness metric. Additional sound characteristics may account for this. Two listening studies were designed to examine the usefulness of including sound quality metrics in addition to loudness and sharpness in models used to predict acceptability for non-stationary wind-noise sounds, particularly noise with the kind of variations that are expected from wind gusts. Test sounds were based on recordings made in cars in a wind tunnel. A simulation method was implemented to generate sounds with controlled gusting features by modifying stationary noise recordings. A gusting metric was designed to predict subjects' acceptability responses based on the strength, modulation rate, and duration of the gusts.

Gender differences in perceived loudness have been observed between Japanese men and women in previous studies. Female participants tended to assign higher loudness scores to sounds of the same sound pressure level than did male participants. However, this difference was not clearly observed when using a ratio scale for the evaluation of loudness, such as in a magnitude estimation. To discuss factors affecting gender differences in perceived loudness, the same rating experiments used in previous studies were conducted with Chinese men and women. A rating experiment using a verbal interval scale showed that Chinese women tended to rate the same sounds as louder than did Chinese men. The gender effect was replicated in an experiment using an adjustment method that measured the limits of the sound pressure level perceived as soft or loud; women selected lower levels than did men for the highest limit of soft sounds and the lowest limit of loud sounds. These tendencies were the same as those of Japanese men and women observed in previous studies. Rating experiments adopting the methods of magnitude estimation and magnitude production for the ratio scales showed that the power exponent a in Stevens' power law was larger for female participants than for male participants under some conditions. However, the observed gender difference in the power exponent a for Chinese was not consistent, while it was not clear for Japanese. The gender difference in criteria for verbally categorizing loudness may be responsible for the gender difference in perceived loudness.

Because of the electric power transmission system has no sound masking effect compared with the traditional internal combustion power transmission system, electric powertrain noise has become the prominent noise of electric vehicles, adversely affecting the sound quality of the vehicle interior. Because of the strong coupling of motor and transmission noise, it is difficult to separate and identify the compositions of the electric powertrain by experiments. A psychoacoustic model is used to separate and identify the noise sources of the electric powertrain of a vehicle, considering the masking effect of the human ear. The electric powertrain noise is tested in a semi-anechoic chamber and recorded by a high-precision noise sensor. The noise source compositions of the electric powertrain are analyzed by the computational auditory scene analysis and robust independent component analysis. Five independent noise sources are obtained, i.e., the fundamental frequency of the first gear mesh noise, fundamental frequency of the second gear mesh noise, double frequency of the second gear mesh noise, radial electromagnetic force noise and stator slot harmonic noise. The results provide a guide for the optimization of the sound quality of the electric powertrain and for the improvement of the sound quality of the vehicle interior.

Constant exposure of noise to the auditory system of the agricultural tractor opera- tor can cause physical and psychological problems. A field study was conducted in the Faisalabad and Narowal districts of Pakistan to examine the spread of tractor noise and its psychological effects on the safety of tractor operators driving tractors without cabins and other noise reduction measures. Four of the most common imple- ments used in Pakistan, including land scraper, cultivator, disk harrow and seed drill, were used to detect the changes in tractor noise at two speeds of 5.3 km/h and 10.6 km/h in all four directions. Lowest noise was produced during the field operation of the seed drill at a mean noise value of 81.9 dB(A) among all four implements. Disk harrow and cultivator were found to be the most noisy implement, and during oper- ation, the tractor operator was directly exposed to high noise levels of 86.9 dB(A) and 84.9 dB(A), respectively. This noise pollution caused psychological problems in agri- cultural tractor operators, as demonstrated by the highly positive correlations of de- pression, aggression, anxiety and stress. This trend had a negative effect on their social interactions relative to the comparable population of office employees. In or- der to mitigate the adverse health effects, tractor operators must be equipped with sound proof cabins or at least ear plugs because of direct exposure to high noise levels. Furthermore, a safe distance of 48 m or at least a warning distance of 26 m from the tractor must be maintained by farm workers.

In this paper, the efficacy of porous ceiling treatment to reduce noise levels inside a typical tunnel is examined with a validated modal-based prediction method. It is found that, for a point source, the effect of increasing porous ceiling thickness on sound pressure level (SPL) attenuation along the tunnel is limited. A porous ceiling with thickness of 0.3 m is comparable with an infinite porous ceiling in middle and high frequency ranges. For a line source, the effect of ceiling thickness on SPL reduc- tion in this typical tunnel is limited. Sound pressure level reduction of 4 dBA is real- ized with 0.3 m porous ceiling, which is the same as infinite ceiling and only 1 dBA smaller than the theoretically optimized value. These results suggest that, in the event only ceiling treatment is considered, 0.3 m porous material is sufficient for noise re- duction in this typical tunnel.

Underwater acoustic detection sensors are mounted on the outside of the submarine; the acoustic window for protecting these sensors must be structurally robust while also minimizing any deterioration of sensor's sound detection performance. These two conditions are typically satisfied simultaneously by using composite materials with acoustic window materials. However, since such composite material is manufactured by laminating fibers, there is the probability that delamination occurs, in which an air layer is formed inside, due to manufacturing process errors. Delamination inside the acoustic window degrades the sensor's acoustic performance and results in a failure of military operations. In the case of composites composed of sandwich structures located in the central part, the possibility of internal delamination is higher than in a single composite material. Therefore, it is very important to discriminate the presence or absence of internal delamination after producing an acoustic window. This article uses numerical and analytical methods to determine the internal delamination of the acoustic window fabricated with a sandwich structure. In addition, the results were analyzed and compared through ultrasonic measurement and acoustic transmission loss test.