Analysis And Control Of Vibra-acoustic Characteristics Of Mini-car Body Structure In Low Frequency Range

This paper was based on the university-industry cooperation project” Interior NoiseAnalysis and Control for Mini-car in Middle and Low Frequency Range”. Market reflectedmini-car interior noise was high when at high speeds and affected ride comfort and marketcompetitiveness. This paper was aim to solve the mini-car high interior noise problem.Carried out tumbler tests on car interior noise at constant speeds running and accelerationrunning in the semi-anechoic chamber to determine the sources and main peak frequenciesof the interior noise. Tests results showed that the vibration and acoustic characteristics ofthe mini-car body structure and interior acoustic cavity didn’t match well in low frequencyrange, causing acoustic resonance in the frequency vicinity of250Hz.By using structural and acoustic finite element methods, built FE models of the closedbody structure and the interior acoustic cavity considering chairs, calculated and analyzednatural frequencies and mode shapes of these two FE modes in low frequency range, andevaluated these two models’ vibration characteristics. Analysis results indicated that localmodes of the front roof panel and rear floor at about250Hz were concentrated along withlarge modal displacements, and coupled with the acoustic cavity. The local acoustic-structurecoupling resonance resulted in high acoustic pressure in low frequency range.Considering the mini-car in mass production already, the interior acoustic cavity wasdifficult to change, so sensitivity analysis method was applied. Thorough the sensitivityvalues of the main local mode frequency of the closed body structure around250Hz withrespect to the thickness of the main body panels, the panels affecting this local modefrequency mainly were determined. The results showed, for the245.9Hz modal frequency,the thickness of the front roof panel was the most sensitive, followed by the firewall, theright rear door outside panel, and the rear floor.According to the sensitivity analysis results above, considering the feasibility and economy of the vibration and noise reduction measures, two measures to control mini-carinterior noise around250Hz were ascertained: one raising local stiffness of the closed bodystructure and the other pasting unconstrained damping layer on the front roof panel and therear floor where local vibrations were prominent. Performed natural vibration characteristicsanalysis of the closed body structures which respectively taking these two noise andvibration reduction measures. The analysis results certified local vibrations of the front roofpanel and rear floor were substantially suppressed. Conducting tumbler tests on interiornoise at constant speeds for the test vehicle with damping layer measure, and comparingwith the original structure, the interior noise was significantly decreased with the largestdecline of2.2dB(A). Consequently, taking noise and vibration reduction measures reducedthe interior noise and improved ride comfort of this mini-car.