1,721,109 research outputs found
Directivity of sound radiated from baffled rectangular plates and plate strips
No full textRectangular plates are important components in structures such as vehicles and bridges. The noise radiated by vibrating plates is mainly determined by three factors: the mean-square vibration, the radiation efficiency of the plates and the directivity of the sound. Although the first two factors have been widely investigated, much less attention has been paid to the directivity. The aim of this study is to investigate the directivity indices for rectangular plates subjected to either a single point force or multiple incoherent forces. Particular attention is given to plates with a large aspect ratio, referred to as plate strips. New definitions of directivity index are introduced that are more appropriate to such plate strips. The vibration of the plates is calculated from a modal superposition method based on approximate modal solutions of the plates with various boundary conditions. The Rayleigh integral method is used to obtain the sound pressure radiated from the vibrating plates, assuming that they are set in an ideal infinite baffle. Directivity indices of sound are firstly determined for plates with a small aspect ratio, and then for plate strips with a larger aspect ratio. Examples are given to illustrate the effects of the structural boundary conditions, and the effect of the baffle. For distributed incoherent excitation, as often found in practice, it is shown that the sound directivity pattern in the far field corresponding to the width direction varies insignificantly along the length of the strips; this is different from a single point force excitation. It is also found that the noise radiation from different concrete plates is approximately omnidirectional in the plane perpendicular to the longitudinal direction. Plates and plate strips are much less directional at high frequencies than the corresponding rigid piston. Nevertheless, the maximum value of directivity increases at high frequency as the number of modes in a one-third octave band increases. The dominant directions of sound radiation from plates are mainly controlled by the acoustical wavelength and vibrational wavelength, while the structural boundary conditions and the presence of the baffle have only a minor influence on them. Finally it is shown that, for the prediction of sound directivity of plate strips under distributed incoherent excitation, a two-dimensional vibro-acoustic model can be used in place of the three-dimensional one
The horizontal directivity of noise radiated by a rail and implications for use of microphone arrays
No full textStructural waves propagating along a railway rail form an extended source of sound radiation. Using an equivalent source model the distribution of this sound in a horizontal plane is investigated and shown to consist mainly of sound propagation at a particular angle to the normal. This direction is determined by the ratio of the wavenumbers in the rail and in air. Due to the extended nature of the rail as a source, the spatial distribution of the sound field in the direction along the track does not lend itself to the use of a simple directivity factor. The consequences for the measurement of noise from the rail using a microphone array are then explored. It is shown that a microphone array focussed normal to the rail does not detect most of the sound radiated by the rail. By turning the focus angle, the sound detected becomes a maximum when this angle corresponds to the angle of propagation of the sound radiation. Measurements on a test track using artificial excitation and measurements during the passage of a train confirm these conclusions<br/
Time-domain prediction of impact noise from wheel flats based on measured profiles
No full textRailway impact noise is caused by discrete rail or wheel irregularities, such as wheel flats, rail joints, switches and crossings. In order to investigate impact noise generation, a time-domain wheel/rail interaction model is needed to take account of nonlinearities in the contact zone. A nonlinear Hertzian contact spring is commonly used for wheel/rail interaction modelling but this is not sufficient to take account of actual surface defects which may include large geometry variations. A time-domain wheel/rail interaction model with a more detailed numerical non-Hertzian contact is developed here and used with surface roughness profiles from field measurements of a test wheel with a flat. The impact vibration response and noise due to the wheel flat are predicted using the numerical model and found to be in good agreement with the measurements. Moreover, compared with the Hertzian theory, a large improvement is found at high frequencies when using the detailed contact mode
A non-reflecting boundary for use in a finite element beam model of a railway track
No full textSome beam-like structures such as a railway track are effectively infinite in nature. Analytical solutions exist for simple structures but numerical methods like the finite element (FE) method are often employed to study more complicated problems. However, when the FE method is used for structures of infinite extent it is essential to introduce artificial boundaries to limit the area of computation. Here, a non-reflecting boundary is developed using a damped tapered tip for application in a finite element model representing an infinite supported beam. The FE model of the tapered tip is validated against an analytical model based on Bessel functions. The reflection characteristics of the FE tapered tip are quantified using a wave/FE superposition method. It is shown that the damped tapered tip is much more effective than its constant counterpart and achieves reduction of the model size. The damped tapered tip is applied to a simple FE railway track model and good agreement is found when its point mobility is compared with an analytical infinite track model
Characterisation of forces, dynamic response, and sound radiation from an articulated switch sleeper in a turnout system
No full textPre-assembled turnouts have recently been introduced with split concrete sleepers joined by elastic coupling plates. Their effect on the forces, vibration response, and sound radiation as trains run through a turnout is studied here using a calculation model for a whole turnout based on the finite-element method. The non-linear characteristics of the ballast's support under the sleepers, as well as the special construction of the articulated joints, are included. It is shown that compared with the normal integral sleeper design, the track with articulated switch sleepers has a lower rigidity, which changes more evenly along the track and improves its dynamic performance. Furthermore, the fatigue strength of an articulated switch sleeper is higher due to its lower stress amplitudes. Additionally, when the vehicles are running on the main turnout route, as the switch sleepers’ vibration behaviour is modified, the articulated joints effectively reduce the impact loading of the ballast at the end of the sleeper under the turnout branch track. This will extend the life of the ballast bed. Finally, the sound radiation from the articulated switch sleeper is very low in the hinged short part and is reduced by about 0.6 dB for the total length.<br/
A wavenumber domain numerical analysis of rail noise including the surface impedance of the ground
No full textIn most previous studies of sound radiation from railway rails, the rail has been regarded as located in free space, disregarding the influence of the ground. However, in order to predict the noise from the rail more precisely, the effect of the ground should be included in rolling noise predictions. In this study, the rail noise is investigated by means of a wavenumber domain numerical method, including the presence of the ground. For rails attached to a rigid ground or located at a certain distance above it, the influence of the ground is examined in terms of the radiation ratio and longitudinal directivity. From the prediction of radiated power, it is found that the vertical and lateral bending waves of the rail radiate most of the noise for the corresponding direction. Hence, a simplified calculation is proposed that only includes these waves, instead of a full three-dimensional analysis. An absorptive ground is also modelled by applying impedance boundary conditions at the ground surface to investigate the influence of the ground on the rail noise. Finally, for the vertical and lateral bending waves in the rail, the cross-sectional directivity of the noise is predicted for various surface impedances of the ground. It is found that the simplified calculation proposed in this study is valid for the prediction of noise from the rail. Also the presence of the ground and its impedance condition have considerable effects on the level and directivity patterns of the noise radiated from the rail.</p
Implications of the directivity of railway noise sources for their quantification using conventional beamforming
No full textThis paper investigates the effect of the directivity of railway noise sources on the results of an identification procedure based on beamforming using a microphone array. Usually when performing pass-by noise tests, a single-microphone noise spectrum is obtained for a time window corresponding to the length of the whole train, or of a single vehicle. In this context, a source quantification algorithm should be able to evaluate the contribution of each noise source over this time window. However, different railway noise sources have different directivities, and it is shown that these need to be taken into account to achieve accurate source quantification. By making use of monopoles, dipoles and quadrupoles, it is shown that a different compensation is needed according to the directivity. For the particular case of the noise radiated by the rail, this has a complex directivity pattern that is only partially captured by a microphone array. It is demonstrated that the overestimation of the wheel contribution found in previous research may be attributed to a misinterpretation of part of the rail contribution from the beamforming map.</p
A study of the effect of grinding machine parameters on acoustic rail roughness and surface quality
No full textRail grinding is performed by infrastructure managers to control, reduce or prevent the growth of rail defects, such as rolling contact fatigue and corrugation. This is done using preventive methods (to attempt to prevent defects from forming) or corrective methods (to remove defects present in the rail). Trials were undertaken on preventive rail grinding machines used by Network Rail, with the aim of improving the finished quality of the rail whilst still achieving the metal removal and reprofiling required. An important aspect considered in the trials was the acoustic rail roughness and its relationship with grinding surface quality indices. The results demonstrated that, in the case of the operational machines used by Network Rail, the largest impact on the overall surface quality was the age and conditioning of the grinding stones. The trials also demonstrated the differences in Standard requirements for achieving good surface quality indices for grinding and good acoustic roughness levels. They further highlighted the importance of identifying rail corrugation prior to preventive grinding to reduce the likelihood of the grinding signature increasing roughness at corrugation wavelengths
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