Study On Low Frequency Noise Reduction In Automobile Based On Phononic Band-gaps

With the evolution of the domestic automobile market,more and more attentions are paid on NVH performance,so automobile manufacturers continue to invest in R&D to improve the NVH performance.However,satisfactory results have not been achieved in the low-frequency noise control.The main obstacles are found to be: the vehicle chassis structure is complex and multi-system,including engines,transmission systems,suspension systems,etc.A lot of lowfrequency noise sources,structure transfer paths and influencing factors are involved.The mechanism of the interior low-frequency noise remains unclear.In addition,Traditional means,such as structural optimization design,adding damping material and active noise control,all show their limitations in application.When the low-frequency noise is transmitted into the vechile cavity,it is even more difficult to absorb the noise with common engineering methods such as sound absorption and insulation.Acoustic metamaterials,with their superior low-frequency band-gap,provide new ideas for low-frequency noise control in vehicles.However,the existing acoustic metamaterial structures are mostly applied in regular structures or single sound souces.It remains to be very challenging to apply the acoustic metamaterial in the automobile which is consist of complex multiple subsystems.To apply the acoustic metamaterial structures in vehicle low-frequency noise reduction,the study was carried out from three aspects: the mechanism of the interior lowfrequency noise,the control methods and the structure design of acoustic metamaterial for vehicles.The main results obtained in the study are summarized as follows:(1)The generating and propagating mechanism of the low-frequency noise in vehile was analyzed based on the traveling wave method.To analysis the mechanism of the low-frequency noise in vehicle,a traveling-wave-based approach was proposed.One of the most popular minivans in China was set as the research object.To study its interior low-frequency booming noise during acceleration,experimental analysis and theoretical calculation were conducted.The Low-frequency torsional vibration energy was found to be generated from the engine,transmitted through drivetrain,rear suspension,body and cavity,and finnaly cause the interior low-frequency booming noise.Then,a quantitative mechanism model of the low-frequency noise problem was established based on the traveling wave method.The main transfer path of the low-frequency acousto-vibration energy was identified,and the transfer characteristics of the acousto-vibration energy were analyzed.The quantitative mechanism model was validated with the comparison of calculation results and test results.According to the quantitative mechanism model,serval traditional methods were applied to reduce the minivan interior booming noise.It was shown in the results that structural optimization design,adding damping materials and other traditional methods all have their limitations in low-frequency noise control in the car.It is necessary to explore new control methods for the interior low-frequency noise reduction.(2)The discrete model of generalized phononic cell was established and a control method of low-frequency noise in vehicle was proposed based on the design of phononic band-gap structure.The research on existing acoustic metamaterials is mostly carried out in regular structures or single sound souces.To apply acoustic metamaterials to automobiles,a modeling approach for phononic-cell discrete model was established.In the established approach,the structures in automobiles were represented as a theoretical model composed of many phononic cells.With the phononic-cell discrete model of the driveline,the Lattice and Band Theory was introduced.Then,the calculation method of the torsional band-gap in the driveline was derived,combining the elastodynamic theory and the vibration-acoustic theory.The transfer characteristics of torsional vibration wave in traditional shafts were analyzed based on the torsional band-gap calculation method.It is shown in the results that it is very difficult to suppress the low-frequency torsional vibration waves in traditional structural shafts.To solve the problem,a locally resonant acoustic metamaterial structure was proposed to realize low-frequency torsional band-gaps in the shafts.And this structure provided a new method to low-frequency torsional vibration control in automobile powertrain.Then,the calculation method of the locally resonant acoustic metamaterial structure band gap was derived,and this method provided necessary theoretical supports for the low-frequency noise reduction with locally resonant acoustic metamaterial structure.(3)Based on the design method of phononic band-gap,a torsional vibration damper was developed and the low-frequency noise caused by driveline was reduced.The driveline torsional vibration is the main cause of the minivan interior booming noise during acceleration.To block the transfer of the low-frequency torsional vibration waves in driveline,the torsional vibration damper needs to be designed reasonably.A dynamic model of the torsional vibration damper was built to analyze its dynamic equivalent negative mass density characteristics,and the band-gap calculation method of the locally resonant torsional vibration damper was proposed.The influences of the geometric parameters and the material parameters on the band gap were studied.Based on this band-gap calculation method,a locally resonant torsional vibration damper was designed and applied in the minivan.It was illustrated in the results that the interior low-frequency noise was reduced significantly.This method could be used in the development of the torsional vibration damper to solve the low-frequency noise in the cars caused by driveline torsional vibration.(4)A kind of lightweigh metamaterial panels for reducing low-frequency noise in vehicle were developed.To explore new acoustic metamaterial structures that could be widely used in lowfrequency noise reduction in automobiles,a kind of lightweight metamaterial panel with lowfrequency band-gap was developed.The band-gap calculation method of the metamaterial panel was derived.In the laboratory,the locally resonant acoustic metamaterial panel was prepared and its band-gap was validated.The influences of the geometric parameters and the material parameters on the band-gap were studied.This provided theoretical supports for the design and application of locally resonant acoustic metamaterial panel.The designed acoustic metamaterial panels were applied to 3 different automobiles and all achieved effective low-frequency noise reduction:(1)When the acoustic metamaterial panels were laid on the ceiling of a commercial vehicle cab,the low-frequency vibration of the ceiling was suppressed and the interior noise around 36 Hz was reduced;(2)While the acoustic metamaterial panels were fixed on the inside of the tailgate of a traditional power SUV,the interior low-frequency noise was effectively reduced at around 34 Hz during acceleration and constant speed.The subjective evaluation of the noise in the car was improved from a very unsatisfactory state to a basically satisfactory state.(3)When the acoustic metamaterial panels were installed on the tailgate of a new energy SUV,the low-frequency road noise in the car during driving on rough roads was effectively controlled.Moreover,the noise reduction of the acoustic metamaterial panel and the traditional damping plate were compared.It was illustrated in the results that the acoustic metamaterial panels could achieve better noise reduction performance with the same size and snaller weight.It was also shown that the locally resonant acoustic metamaterial panels had many practical advantages,such as light weight,superior lowfrequency noise reduction,free band gap and shape design,and high reliability.The locally resonant acoustic metamaterial panel contains the potential to be widely used in automotive lowfrequency noise reduction.