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Effect of sound insulation on noise reduction in an agricultural tractor cab
Abstract and Figures
Tractor cab interior noise is a risk factor that degrades operators’ work performance and threatens their health; therefore, the noise must be reduced to ensure farmworkers’ safety and efficiency. Cab interior noise can be classified as structure-borne noise and air-borne noise. Structure-borne noise has been extensively studied. However, although air-borne noise greatly contributes to cab interior noise, detailed frequency-domain analyses have not been performed. In this study, the components of cab interior noise were identified in the frequency domain through an order analysis, which helped improve the sound insulation of the cab and reduce the effects of air-borne noise. A test was performed while driving a tractor on a chassis dynamometer in a semi-anechoic chamber for reproducible measurement and evaluation. The A-weighted sound pressure was transformed by a fast Fourier transform algorithm, and its order was tracked by the engine speed signal. In addition, a direct path was identified by acoustic images using a sound camera. The contributions of major noise sources were identified through an order analysis. We proved that air-borne noise significantly contributes to the interior noise of tractor cabs and that improvement of the cab sound insulation is an effective noise-reduction technique.
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The noise level in a tractor cab directly affects the driving experience. Reducing the noise level in a tractor cab is an important means of improving the image of the product brand and protecting the physical and psychological health of employees. To reduce medium- and low-frequency noise in a tractor cab, the influence of the installation of coupled resonance acoustic materials (CRAMs) on tractor driver ear noise is investigated in this study. Theories for sound absorption by perforated plates and sound insulation by thin membranes are used to design a CRAM composed of a perforated plate, elastic membrane and additional mass. The acoustic impedance and sound absorption coefficient of the CRAM are obtained by an equivalent circuit method, and the resonance frequency of a local resonance system composed of an elastic membrane and additional mass is calculated. The effects of different hole diameters and porosities of the perforated plate on the CRAM sound absorption coefficient are compared. The results of tests performed using impedance tubes over a wide frequency range of 50 Hz~1600 Hz show that the maximum sound absorption coefficient is 0.99 and several transmission loss peaks of 15 dB~26 dB. Measurement of the acoustic response of the CRAM installed on the cab front panel shows that an ear noise reduction of approximately 1.3 dB(A) for a tractor working speed of 2200 rpm. The results show that the designed CRAM has a relatively good effect on reducing driver ear noise over a wide frequency band and is a feasible scheme for noise reduction in a tractor cab.
Acoustics represents a quality feature of a passenger vehicle. During the first development phases, prediction tools of the acoustic performance are required to assess in the design process. A combination of numerical and experimental data provides a good compromise between accuracy and modeling effort. A key part of the airborne noise transmission chain are the different passive acoustic treatments applied to minimize the noise impact. These treatments usually include one or more layers of poroelastic media. They exhibit a highly dissipative behavior, making them suitable as noise barriers but, at the same time, complex to model. This article aims to identify the parameters that define the appropriate numerical modeling approach for vibroacoustic systems with poroelastic acoustic packages. Two different concepts for the noise reduction based on poroelastic materials have been investigated, namely the insulation through a spring–mass component and the absorption of a porous layer. The essential principles of the theory of poroelasticity are recalled and three material formulations are derived. The application range of each material model is examined with the help of two configurations: a flat plate and a simplified model of a vehicle front end. The acoustic response of the system is solved with the help of the finite element method using the different material formulations for the description of the poroelastic layers, and the results are compared to measurements conducted in a window test bench. Finally, the main findings are summarized and recommendations towards a more realistic representation of the complete transmission chain are presented.
This paper proposes an ontology-based noise source identification method, establishes an ontology knowledge expression model in the field of noise, vibration, and harshness (NVH), and provides an extensible framework for sharing noise diagnosis knowledge in the field. Based on the key features extracted from the noise and vibration signals at different positions and the prior knowledge, mechanical engineers can construct an ontology rule and locate noise sources by identifying the intrinsic relationship between signal characteristics through ontology reasoning. A case study is conducted to demonstrate the effectiveness of the proposed method in resolving the problem of integrating multisource heterogeneous knowledge and exchanging noise diagnosis knowledge information in the field of NVH for agricultural machines. Thus, our study facilitates the sharing and reuse of knowledge and advances the development of intelligent noise diagnosis expert systems to a certain extent.
Structure design for reducing noise is essential and widely studied because cab interior noise seriously affects the ride comfort of the driver and passengers, especially for heavy commercial vehicles. This paper proposes an enhancement investigation on the low-noise structure performance of a cab in a heavy commercial vehicle based on an approximation model. A series of vibroacoustic tests with varying running speeds are implemented first to acquire the vibration signals at four cab suspensions and the sound pressure signal at the right ear of the driver. The finite element model and structure–acoustic coupled model of the cab of a heavy commercial vehicle are established sequentially. When the vibration accelerations measured in the tests are converted into excitation signals using the cab structure–acoustic coupled model, the sound pressure of the right ear of the driver is predicted, and the accuracy of the cab structure–acoustic coupled model is then verified. After panel acoustic contribution analysis, an approximation model of critical panel thickness and the peak noise near the right ear of the driver is established in accordance with the radial basis function. Genetic algorithm is utilized to solve an optimization model to obtain optimal panel thicknesses. By comparing the right ear sound pressure before and after optimization, the results confirm that optimized panel thicknesses can reduce sound pressure level of the critical frequency.
Helmholtz resonators are widely used as acoustic dampers to mitigate noise in gas turbines and automotive exhaust systems. In order to increase the damping performance and broaden the damping frequency range, multiple Helmholtz resonators are typically implemented. In this work, two connected Helmholtz resonators by sharing the same flexible or rigid sidewall, i.e. parallel-coupled are designed and tested on a cold-flow pipe system with a mean flow present. Two different geometric shapes of the coupled Helmholtz resonators are considered. One is cylindrically shaped and the other is rectangular. Both experimental and 3D numerical investigations are conducted to study the effects of the geometric shape and the mean grazing flow on the noise damping performance of the resonators. The numerical model is built in frequency-domain and solving linearized Navier-Stokes equations. To characterize and quantify the resonators' noise damping performance, transmission loss in dB is determined. It is experimentally found that the cylindrical shaped resonators are associated with much larger transmission loss. Compared with the rectangular-shaped resonators, approximately 12 dB more sound pressure level reduction is achieved by the cylindrical resonators, which is independent on the rigidness of the shared sidewall. The rigidity of the shared sidewall is shown to shift the frequencies corresponding to the maximum damping peaks, depending on the sidewall's flexural rigidity. Numerical results confirm that the geometric shape does play an important role in determining the resonator's damping. Further investigation of the mean grazing flow effect is conducted. It is shown experimentally and numerically that when the Mach number of the mean grazing flow is lower than 0.01, the noise damping performance of the coupled Helmholtz resonators is little changed over the frequency range from 150 to 600 Hz. However, as the mean flow Mach number is increased, the damping performance becomes deteriorated as observed numerically. The present work reveals the critical roles played by the geometric shape and the mean grazing flow on the aeroacoustics damping performance of the coupled-resonators.
A farm tractor is the most important component of farm mechanization. Farm tractor noise, on the other hand, has a negative impact on human health. The exhaust system produces the most noise of all the noises produced by a tractor. A muffler is an acoustic filter that is used to reduce exhaust noise. Perforated ducts, baffles/perforated baffles, expansion chambers, and longer inlet/outlets are used to make mufflers. A person working in a workplace with a noise level of 90 dBA is limited to 8 h of work each day under the Occupational Safety and Health Act (OSHA, 1983). As a result, utilizing the finite element approach and acoustic theory, an existing muffler (Reactive type muffler) was optimized to a combined resistance muffler after installing in a 47.6 PS, 2200 rated RPM three-cylinder diesel engine. Except for the frequency range of 138–455 Hz, 515–851 Hz, and 1524 Hz, the optimized combined resistance muffler had a considerable increase in transmission loss in all other frequency ranges. The improved muffler's mean transmission loss was 3.77 dB higher than the original muffler's in the 0–2000 Hz range, which indicates notable improvement in the noise canceling effect. The noise reduction of this combined resistance muffler was tested at operator ear level (OEL). Near rated engine speed, the combined resistance muffler suggests a noise reduction of 1.2 dBA–1.9 dBA over the existing muffler. The upgraded muffler also served as a benchmark for similar muffler structural improvements.
In order to evaluate environmental comfort of tractor cabs, temperature, relative humidity and noise within the cab were taken from 31 tractors during plowing and rotovating operations. The temperature and humidity were evaluated with regard to the comfort zone of KS B ISO 14269-2 and PMV of ISO 7730. The noise was evaluated with regard to the permissible sound level of OSHA for daily exposure of 8 hours. The collected data indicated that thermal environment of the cabs was out of the comfort zone, which meant tractor operators worked under uncomfortable thermal conditions. Difference in the thermal comfort by tractor power and maker, and type of works was not found. However, 25% of the studied tractors showed PMV in a range of -0.5 to +0.5, which indicated their operators worked under the comfort criteria. PMV was improved when the cab was air-conditioned. Levels of measured cab noise were lower than the permissible criteria, and 76.7% of the studied tractors had cab noise ranged from 75 to 85 dBA. There was a tendency that high powered tractors, rotovating operations and locally-made tractors had greater cab noise levels. However, their differences were insignificant