Noise exposure and its impact on psychological health of agricultural tractor operators

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.

Application of a micro-perforated panel absorber to reduce the curve squeal noise of railways

A micro-perforated panel absorber (MPPA) can reduce noise without the use of sound-absorbing materials. It is made of various materials and is poten- tially applicable in a variety of fields. However, the application of MPPAs is limited owing to the narrow absorption band. In a previous study, to enhance the sound- absorption performance of MPPAs, we proposed a parallel arrangement of multi- layer MPPAs. Based on the results of this previous study, we carried out the present study to apply an MPPA to the curve squeal noise of railways. Squeal noise, which occurs at approximately 500 to 5000 Hz, has high-level peaks. We designed and fabricated an MPPAwith the proposed structure according to the characteris- tics of the squeal noise. The noise reduction was analyzed after an MPPA barrier was installed in the field with a curve radius of 120 m. The reduction in noise level was approximately 17 dB(A) on the first floor and approximately 7 dB(A) on the fourth floor of the building. At 630 to 5000 Hz, the noise reduction level was at least approximately 5 dB(A). To analyze the noise absorption effect of the MPPA bar- rier, a simulation was carried out and subsequently verified by comparing the measurement results with the simulation results. In the simulation, the difference in noise reduction level between the MPPA and reflective barrier was analyzed. The noise reduction level of the MPPA barrier was approximately 7.5 dB(A) higher than that of the reflective barrier on the second and third floors of the build- ing at 630 to 5000 Hz. These results support the viability of MPPA application in re- ducing noise from curve squeal noise.

Prediction and analysis of sound field in long enclosures with a modal-based method

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.

Case study: Experimental characterization of noise and vibration sources in a refrigerator

The sources of noise and vibration of a refrigerator are characterized by means of laboratory measurements. Three different elements are considered: compressor, evaporator fan, and condenser fan. Both the radiated acoustic energy and the mechanical power injected to the refrigerator structure are measured. Acoustic intensity maps on the refrigerator faces at different frequencies are also provided They are helpful in visualizing the more problematic zones in terms of noise emis- sion. The devices are tested in vacuo or working inside the refrigerator. The provided information can be used as input data for vibroacoustic simulation models such as statistical energy analysis. It is also of interest in order to improve the refrigerator acoustic design.

Vibration signal analysis of planetary gear with amplitude, frequency and phase modulation

In this paper, the vibration signal of planetary gear with amplitude, frequency and phase modulation is studied. The proposed mathematical model is employed to in- vestigate the modulation behavior of planetary gear. Based on this model, the ampli- tude modulation (AM) sidebands are analyzed to verify the correctness of theoretical calculation by Inalpolat and Kahraman. Then, the frequency modulation (FM) side- bands and phase modulation (PM) sidebands are also illustrated through an exam- ple analysis. The effects of parameters of planetary gear such as number of planets, teeth of sun and planet phasing relationships on the AM, FM and PM sidebands are analyzed. Finally, the specific expression of transmission error, time-varying mesh stiffness and dynamic mesh force including gear manufacturing error is developed. Time history signal and acceleration spectra of gear mesh interface excitations including AM, FM and PM are investigated for the meshes of sun-planet and ring- planet. The results show that gear parameters have important influence on the mod- ulation behavior. Additionally, manufacturing errors can be introduced to predict the sidebands of planetary gear. The amplitude, frequency and phase modulation study are extremely significant for the noise and vibration reduction, especially the fault diagnosis of planetary gear

Microphone array speech enhancement based on optimized IMCRA

Microphone array speech enhancement algorithm uses temporal and spatial informa- tion to improve the performance of speech noise reduction significantly. By combining noise estimation algorithm with microphone array speech enhancement, the accuracy of noise estimation is improved, and the computation is reduced. In traditional noise es- timation algorithms, the noise power spectrum is not updated in the presence of speech, which leads to the delay and deviation of noise spectrum estimation. An optimized im- proved minimum controlled recursion average speech enhancement algorithm, based on a microphone matrix is proposed in this paper. It consists of three parts. The first part is the preprocessing, divided into two branches: the upper branch enhances the speech signal, and the lower branch gets the noise. The second part is the optimized improved minimum controlled recursive averaging. The noise power spectrum is updated not only in the non-speech segments but also in the speech segments. Fi- nally, according to the estimated noise power spectrum, the minimum mean-square error log-spectral amplitude algorithm is used to enhance speech. Testing data are from TIMIT and Noisex-92 databases. Short-time objective intelligibility and seg- mental signal-to-noise ratio are chosen as evaluation metrics. Experimental results show that the proposed speech enhancement algorithm can improve the segmental signal-to-noise ratio and short-time objective intelligibility for various noise types at different signal-to-noise ratio levels.

Singular vector filtering method for mitigation of disturbance enhancement in multichannel active noise control systems

In the design of multichannel active noise control filters, the disturbance enhancement phenomenon will sometimes occur, i.e., the resulting sound is enhanced instead of being reduced in some frequency bands, if the control filter is designed to minimize the power of error signals in other frequency bands or across all frequencies. In previous work, a truncated singular value decomposition method was applied to the system autocorrelation matrix to mitigate the disturbance enhancement. Some small singular values and the associated singular vectors are removed, if they are responsible for unwanted disturbance enhancement in some frequency bands. However, some of these removed singular vectors may still contribute to the noise control performance in other frequency bands; thus, a direct truncation will degrade the noise control performance. In the present work, through an additional filtering process, the set of singular vectors that causes the disturbance enhancement is replaced by a set of new singular vectors whose frequency responses are attenuated in the frequency band where disturbance enhancement occurs, while the frequency responses in other frequency bands are unchanged. Compared with truncation approach, the proposed method can maintain the performance in the noise reduction bands, while mitigating the influence in disturbance enhancement bands.

Identification of uncertainty levels of acoustic properties of biocomposites under different mounting conditions in impedance tube tests

Impedance tube method is widely used to measure acoustic properties of materials. Although this method yields reliable acoustic properties for soft textured materials, uncertainty levels of measured acoustic properties for hard materials, including biocomposites, can be quite large, mainly due to uncertain mounting conditions. Here, the effects of mounting conditions on the acoustic properties of biocomposites in an impedance tube are investigated. First, nominally identical biocomposite samples with a diameter equal to the inner diameter of impedance tube are manufactured and their acoustic properties are determined. As hard materials practically cause fitting problems in the impedance tube, the diameters of samples are reduced, as in practice, by small amounts and acoustic properties of modified samples are determined. Furthermore, in order to match the diameters of samples to the inner diameter of impedance tube, different materials such as tape, petroleum jelly and cotton are applied around samples to close the air gap between the samples and the tube's inner wall. All the results are compared, and the uncertainty levels caused by different mounting conditions on the acoustic properties of biocomposites are identified. The results show that the transmission loss (TL) measurements are dramatically affected by the mounting conditions while the sound absorption conditions are less sensitive to the mounting conditions. The deviations in the measured TL levels are highest for the samples with tape and wax (10–15 dB). On the other hand, the deviations in the measured sound absorption coefficients are highest for the samples with cotton and tape (1–2%).

Prediction and measurement of acoustic transmission loss of acoustic window with composite sandwich structure

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.

Sound transmission paths through a statistical energy analysis model of mechanically linked aircraft double-walls

Sound transmission loss (TL) through mechanically linked aircraft double-walls is studied with a statistical energy analysis method. An overview of the method is given with details on acoustic and structural transfer path analysis. The studied structure is composed of a thick composite sandwich panel representative of a skin panel, lined with an acoustic insulation layer (glass wool), and structurally connected via vibration isolators to a thin composite sandwich lining panel representative of a trim panel. Two types of vibration isolators are considered: a soft and rigid mechanical link. Various experimental methods were used to assess the accuracy of this model. This study shows the robustness of the simple four-pole modeling of isolators, which depends mainly on the importance of correctly determining the experimental dynamic stiffness of typical aircraft vibration isolators. The prediction of the TL while acceptable was, however, found less satisfactory for the soft configuration. This is traced to the uncertainties on the used coupling loss factor. Finally, a transfer path analysis is performed to identify the contribution of each transmission path in the entire frequency range of interest. Results show that non-resonant airborne transmission dominates in low frequencies, the airborne radiation is significant in the critical frequency region of the panels, while the structure-borne radiation increases the noise transmitted in the mid- and high-frequency ranges.