Research On Reverberation Characteristics And Acoustic Response Characteristics Of Train Surface In Metro Tunnel

When the metro train runs in the tunnel,the reverberant sound produced by the reflection of the tunnel wall will increase the interior noise and exterior noise of the train,reduce the ride comfort of the train and even cause harm to the health of drivers and passengers.Reverberation time is an important parameter to measure the reverberation characteristics of sound field,which can reflect the reverberation strength in space.Studying the reverberation characteristics of tunnel and controlling the reverberation time of tunnel can reduce the reverberation sound in the significant frequency band on the train surface,which can reduce the interior noise of the train.Therefore,it is particularly important to study the effect of reverberation time on the noise of the vehicle body surface.The main research work and conclusion are as follows.(1)Based on metro line test,the characteristics of metro trains noise in different operating environments are analyzed.The tunnel reverberation test is carried out by using interrupted noise method and integrated impulse response method respectively,and the sound absorption coefficient of tunnel wall is calculated based on the theory of image source method and reverberation time.When metro train runs on the ground surface,the sound pressure level of bogie area is obviously higher than those of side wall area and roof area.After the train entering the tunnel,the sound pressure levels of bogie area,side wall area and roof area increase by 2.4 d BA,22.0 d BA and 17.0 d BA,respectively.The average reverberation time measured by the two methods is close to each other,but the interrupted noise method shows more fluctuation than the integrated impulse response method.The sound absorption coefficient of tunnel wall has a minimum value in the 1/3 octave band with center frequency of 160 Hz.(2)Based on the ray tracing method,the simulation calculation model of tunnel reverberation time is developed and used to calculate the reverberation time in empty field.The accuracy of the model is verified by the experimental results.The reverberation time characteristics of train surface in tunnels with different sound source,different tunnel sections and different sound absorption materials laying schemes are calculated and analyzed.The results show that the more the number of sound sources,the smaller the range of reverberation time and the more uniform the distribution.In tunnels with different shapes sections,the average reverberation time of the response surface at the bottom of the train is quite different,while the response surfaces at the roof and side are very close.The reverberation time of sound absorption materials laid on the top of tunnel wall decreases the most,which is the best laying scheme.(3)The sound response prediction model of train body surface is developed.By using acoustic boundary element method and ray tracing method,the sound responses of train body surface in the 100～1000 Hz frequency band and above 1000 Hz frequency band are calculated,respectively.The sound responses of train surfaces in tunnels under different operating environments,different tunnel sections and different sound absorption materials laying schemes are calculated and analyzed.When the metro train runs in the tunnel,the higher the sound source frequency,the more uniform the noise distribution on the surface of the train body.For tunnels with different section shapes,the total sound pressure level on the train body surface in the tunnel with rectangular section is the largest,and the low-frequency noise sound pressure levels on the bottom and side of the train body in the horseshoe-shaped single-line section tunnel are lower than other sections.Laying sound absorption materials on the tunnel wall mainly reduces the surface noise of the train body at 250～1250 Hz,and laying sound absorption materials on the bottom of the tunnel wall has the best noise reduction effect.At this time,the sound pressure levels of the bottom,top and side of the train body are reduced by 5.1 d BA,5.3 d BA and 4.5 d BA,respectively.(4)Based on the above calculation results,the variations of reverberation time and train body surface noise under different sound absorption material laying area are analyzed,and the variation law of train body surface noise with reverberation time is obtained by combining the characteristics of the two parameters under the same scheme.When the average reverberation time of the response surface decreases,the noise sound pressure level of the significant frequency band on the train body surface decreases.When the reverberation time change rate is below 20%,the average sound pressure level change rate of the response point is below 4%,and when the reverberation time change rate is between 20% and 35%,the sound pressure level change rate increases obviously,and some frequency bands can reach up to 10%.When the reverberation time change rate is small,the the train surface whose sound pressure level changes from large to small is side,top and bottom.The above research methods and results provide systematic design and reference for reverberation time prediction and sound response calculation of train body surface in tunnel.