Analyses On Cause Of Abnormal Interior Noise Of Metro Vehicles And Control Measures

Metro,due to its high safety,high speed,weather independent operations,large capacity and low pollution,has been developing into the backbone of public transportation in major cities.However,the interior noise may be very serious on some tracks,lowering the running comfort significantly.Nowadays,a low noise design has become an important issue of metro system,and is one of the major measures to improve the competitiveness of metro.This thesis intends to treat the problem of abnormal interior noise occurred on a metro network in China.The cause was isolated based on a series of field tests and related analyses,based on which counter measures are proposed.Such a work provides a reference for better maintenance of the metro system and improved low noise designs of metro vehicles in future.The main contents and conclusions of this thesis are as follows.First of all,the interior noise of the metro vehicles has been measured at speeds of 0 and 75 km/h,corresponding to the stop at stations and the full running speed,respectively.It has been found that the sound pressure of the interior noise at the full speed is 10~20 dBA higher than that at stop.Moreover,the interior noise at the full speed exceeds the limit of 83 d BA at most sections of track.Spectral analyses have shown that the frequencies of the interior noise are mainly 400-800 Hz.The sound resources,identified by a visualization method,are mainly located at the edges of the window,the crevice of the doors and the floor.Measurements of vibrations of key components have confirmed that the noise at the window and the door is transmitted by air and that at the floor is from vibrations transmitted by structures.Besides the noise and vibration measurements of vehicles,the rail irregularity has also been checked using CAT.It has been found that the sections with severe rail corrugation agree well with those with the abnormal interior noise.Furthermore,the passing frequencies of the observed corrugation,being 400-800 Hz and corresponding to the corrugation wavelength of 30-50 mm,are also in good agreement with the main frequencies of the interior noise.To further confirm the relationship between the rail corrugation and the abnormal interior noise,sections of the corrugated rails were ground and then another noise measurement is conducted.The results have shown that the interior noise is reduced by 10 dBA by rail grinding.It is also found that the corrugation component with a wavelength of 125-160 mm contributes little to the interior noise,and the main cause of abnormal interior noise is the component with a wavelength of 30-50 mm.To exclude the contribution of the wheel irregularity on the interior noise,wheels of the tested vehicles were also reprofiled for further measurement.It is found that interior noise is even increased by 0.5-1.5 dBA compared to that before reprofiling.Detailed investigation shows that the noise increase is due to the wheel OOR of 105-140 order occurring after re-profiling.It is also found the sound pressure of interior noise decreased by 5.2 dBA when the speed decreases from 75 km/h to 60 km/h,and decreased by 4.1 d BA from 60km/h to 50 km/h.The sound pressure of interior noise only varies no more than 1 d BA when the same vehicle running over sections of different track structure,suggesting the insignificance of the track structure on noise.Finally,the relationship between the level of the interior noise and the amplitude of rail corrugation component with a wavelength of 30-50 mm was postulated from the test results.On such a basis,a new method is proposed to derive the amplitude of rail corrugation from the level of the measured interior noise.In addition,corrugation limits are proposed for rail grinding to better mitigate the abnormal interior noise,and suggestions are also made from the aspects of train and track designs.