轿车乘员舱结构噪声分析技术

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作者:长安汽车股份有限公司汽车工程研究院 周建文    来源:汽车制造业
乘员舱是典型的弹性薄壁腔体结构,其内部噪声除了由外部辐射噪声传入车内外,主要由壁板结构振动与车内空气产生强烈耦合作用引起的低频轰鸣噪声。对由结构振动引起的车内噪声问题,一般有有限元法和边界元法。在时间是连续的假定下,声场的控制方程为海尔姆霍兹方程,它由波动方程推导而来。在边界元方法中,海尔姆霍兹方程被转化成了积分方程,由于积分方程中的未知变量只分布于边界表面,因此只有边界才必须被划分为网格。这大大简化了输入数据,减少了计算时间。由此,边界元法被广泛应用于车内噪声问题和辐射噪声问题的分析。 中国环境保护网+O+PI\v4D3f2i1G~

tj1X n8@8a0车内噪声分析方法
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车内噪声计算只考虑车内侧声场,并且是完全封闭的,因此可以采用直接边界元法进行内部声场计算。以下对本文采用的内部噪声分析方法进行了描述。
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1. 直接边界元法
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无源的各向同性流体介质中任一点的辐射声压均可由边界域方程(1)式描述,其中:p为辐射声压;Q为结构表面S上任意点,即边界点;P为空间中任意点,即场点;n为结构表面S的内法向单位矢量;G为格林函数。
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由Euler方程,方程(1)式可写为方程(2)式,其中:ν为边界点的法向速度,ρ为流体介质的质量密度,ω为圆频率。此方程在边界点及场点成立。 中国环境保护网){hMzZ
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则由方程(2)式可得方程(3)式,其中:{atv} 为噪声传递向量,即方程(4)式。 中国环境保护网)I];] \M/[
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由方程(4)式可见,噪声传递向量为结构表面法向速度与场点的声压的传递函数。分别对不同的场点计算,方程(3)式则可写为方程(5)式,
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其中:[ATM]为噪声传递矩阵,由噪声传递向量构成。 中国环境保护网/iO1N@v:D

kFMo%S02. 分析步骤
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车内噪声主要是由发动机、传动系、轮胎、液压系统及结构振动引起,产生的振动、噪声经过悬架系统、车身结构等的放大作用以结构噪声和空气噪声的形式进入乘员舱内,形成车内噪声。其中,动力总成是最大的噪声源,它通过空气和车体结构传递到车内,如图1所示。

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j1g ju{0图1 动力总成到车内噪声的传递路径中国环境保护网r[m$x {s!Me~

通过大量的测试,车内噪声组成在20~1000Hz之间的能量分布如图2所示。可以看出,在低频段结构声是主要噪声成分,在高频段空气声占主要成分。因此,在低频段分析时以结构声为主,即主要考察动力总成引起的振动通过车身结构传递到乘员舱内引起的噪声。

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2 车内结构声和空气声在全频域段的能量分布

6_2Go!dR%eL0车内噪声计算流程如图3所示,车内噪声分析主要包括声模态分析、车内声压分析和面板贡献量分析等。声模态的计算可以看到车内声腔对什么频率最敏感,对车身设计及其他部件设计具有指导作用。车内乘员耳侧声压计算结果与测试值对比,能够检查并标定模型;通过与目标值对比,找出超过目标声压值的频段范围。各激励输入点到耳侧声压的传递路径分析,即TPA(Transfer Path Analysis)分析,可以看出不同的传递路径在各个频段所占的贡献量。再通过IPI(Input Point Inertance)和PCA(Panel Contribution Analysis)分析进一步确定引起噪声过大的原因是悬置接附点激励过大,还是车内壁板的振动贡献量过大造成。

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@Y5GMp/t%vQRNI0图3 车内噪声分析步骤

f @R3f1d5Pk0关于动力总成系统对整车系统的激励一般通过理论计算得到,但这种方法误差较大。通过在测
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n }MYc4X6In0功机上测试动力总成悬置点被动侧在加速工况下的激励作为车内声压响应的输入激励会更为准确。 中国环境保护网-xF ax BQJ

6E)P@:s!e%S{/qh0如图4所示为加速工况下动力总成后悬置在某个方向的加速度图示,可以看出,动力总成二阶激励是主要的激励成分,所以可以选取二阶激励作为模型的激励输入。

newmaker.com 中国环境保护网 \0v:P7}hdJEU0?
图4 动力总成后悬置被动侧在某个方向的加速度

\ ^ _p5b:q[y [H)Y0车内噪声分析
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U,p0`Y([.[01. 声模态计算
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在声学空腔建模时,由于仪表板下部没有封闭,仪表板内部没有形成独立的空腔,所以车内空腔建模没有考虑仪表板的影响。通过计算得到车身空腔模态频率,在设计中可根据声模态及振型调整动力总成、悬架、车身等各系统的频率,避免在这些系统的激励下产生空腔共鸣,引起让人不舒适的轰鸣声。 中国环境保护网*?*H2}4?+n ` X!q
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2. 乘员耳侧声压计算 中国环境保护网W7{8Wge6X
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乘员耳侧声压是指在动力总成激励下,乘员耳侧的噪声声压响应。根据对竞争车和参考车的测试,设定乘员耳侧声压目标值,如图5中黄线所示,由此确定超过目标值的频段,并在后续的分析中找出引起在这些频段内声压过大的原因。

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(|{8L [*P$`6^0图5 驾驶员耳侧声压

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-?.P [ _cP|'Lga0轿车乘员舱内噪声是由组成乘员舱的所有板件振动引起的,车身壁板的不同区域对于乘员舱内部空间任意位置声压的贡献是不同的。这种贡献不仅与幅值有关,还与相位有关。在汽车设计阶段进行壁板声学贡献量分析可以判断造成车内噪声的主要板件及其贡献量,以便在开发初期就进行车身改进设计或者进行其他降噪处理。 中国环境保护网9~[fy [!`4\i oT9Sx

0K,Y+M)b0V:dY04. TPA及IPI分析 中国环境保护网 C)e$}Tm
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传递路径分析TPA有助于找出主要的声传递路径,并针对主要的传递路径进行分析,找出原因。如图6所示的TPA分析Bar图,可以看出在30~40Hz以及160~180Hz之间分别为后悬置Z向和前悬置Z向到车内噪声的传递路径贡献量大。图7 所示的TPA分析矢量图更加明显的显示在35Hz处,后悬置Z向对车内噪声声压有最大的传递贡献量。引起这一路径贡献量大的原因可能会有输入点激励过大、后悬置接附点车身侧Z向在该频段共振等原因。在确定输入点激励没有比别的输入点显著过大的情况下,对后悬置接附点车身侧Z向进行了IPI分析,分析结果如图8所示。

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图6 TPA分析Bar图

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$RBJV$JV"z.u$i K"C0图7 TPA分析矢量图

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图8 IPI分析图

2\0hGoP0从图8可以看出,在35Hz后悬置接附点车身侧Z向具有较大的激励响应,有可能是与后悬置连接的车架出现共振或局部动刚度偏低等原因造成。
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总结 中国环境保护网 C!d t:Fp;mpT

,I$u T5P3X)P0噪声问题在汽车工业中已经引起了人们的普遍关注和重视,特别是近年来随着人们对轿车乘坐舒适性的要求越来越高,减振降噪已经成为汽车开发中越来越重要的环节。因此,在汽车设计初期就预测车内振动噪声水平,提出有效减振降噪方案,可以大大提高开发效率,降低开发成本。本文介绍了应用边界元方法对车内噪声分析的常用方法,并通过实例介绍了这些方法的应用,对整车设计提供了解决车内噪声过大问题的思路和方法。(end)

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