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1. 南京航空航天大学 航空学院 机械结构力学及控制国家重点实验室, 南京 210016;
2. 南京航空航天大学 航空学院 直升机旋翼动力学国家级重点实验室, 南京 210016

Broadband noise reduction inside helicopter cockpit with acoustic black hole effect
WANG Xiaodong1, QIN Yifan2, JI Hongli1, LU Yang2, QIU Jinhao1
1. State Key Laboratory of Mechanics and Control of Mechanical Structures, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China;
2. National Key Laboratory of Rotorcraft Aeromechanics, College of Aerospace Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China
Abstract: Acoustic Black Hole (ABH) effect allows alteration of the phase velocity and group velocity of wave propagation in a structure by changing the impedance to concentrate the waves in local areas of the structure and dissipate energy with a little damping. With the advantages of high efficiency, light weight, and wide frequency, the ABH provides a new perspective for structural vibration and noise control, exhibiting strong potential and application prospects. To reduce the broadband noise inside helicopter cockpits, this paper presents two kinds of structural design schemes based on the ABH effect after considering noise sources and transmission paths. The coupling model of the helicopter cockpit is established using the finite element software. The vibro-acoustic characteristics are then analyzed, and the mechanism of the ABH induced cabin noise reduction is explained. The effect test and performance evaluation are carried out on the established experimental platform. Results show that the embedded ABH structure can effectively reduce the medium-high frequency noise inside the cockpit, while its insufficient performance on low frequency noise is compensated by the additional ABH structure, therefore widening the effective frequency band. The average noise level can be reduced by 3-10 dB in the one-third octave band after employing both the embedded ABH and the additional one. Moreover, the total mass is slightly decreased compared with the traditional structure. This research contributes to the application of ABH new technology to vibration and noise reduction of helicopter engineering in the future.
Keywords: acoustic black hole    noise reduction    broadband    helicopter    cabin noise

1 基于ABH效应的直升机驾驶舱降噪方案 1.1 声源分析与结构设计

 图 1 直升机驾驶舱简化模型示意图 Fig. 1 Schematic diagram of simplified helicopter cockpit model
 图 2 基于ABH效应的降噪结构设计方案 Fig. 2 Structure design scheme of noise reduction based on ABH effect
 $h(r)=\left\{\begin{array}{ll} h_{1} & r \leqslant r_{1} \\ a\left(r-r_{1}\right)^{m}+h_{1} & r_{1} \leqslant r \leqslant r_{2} \end{array}\right.$ （1）

 $h(r)=\left\{\begin{array}{ll} A\left(R_{1}-R_{2}\right)^{m}+H_{1} & 0 \leqslant r \leqslant R_{1} \\ A\left(r-R_{2}\right)^{m}+H_{1} & R_{1} \leqslant r \leqslant R_{2} \\ H_{1} & R_{2} \leqslant r \leqslant R_{3} \end{array}\right.$ （2）

1.2 ABH内的波操控原理

 $\nabla^{2}\left(D \nabla^{2} w\right)-(1-v)\left(\frac{\partial^{2} D}{\partial x^{2}} \cdot \frac{\partial^{2} w}{\partial y^{2}}-\right.\\ \;\;\;\;\;\;\;\left.2 \frac{\partial^{2} D}{\partial x \partial y} \cdot \frac{\partial^{2} w}{\partial x \partial y}+\frac{\partial^{2} D}{\partial y^{2}} \cdot \frac{\partial^{2} w}{\partial x^{2}}\right)+\rho h \frac{\partial^{2} w}{\partial t^{2}}=0$ （3）

 $k=\left[\frac{12 \rho \omega^{2}\left(1-v^{2}\right)}{E h^{2}(r)}\right]^{1 / 4}$ （4）

 $c=\left[\frac{E h^{2}(r) \omega^{2}}{12 \rho\left(1-\nu^{2}\right)}\right]^{1 / 4}$ （5）

2 数值仿真与分析

 图 3 声振有限元模型 Fig. 3 Vibro-acoustic FE model

 Emb-ABH Add-ABH h1= 0.4 mm H1 =0.2 mm r1= 10 mm R1=5 mm r2= 120 mm R2=55 mm rd= 60 mm R3=61 mm m =2 Rd=10 mm a = 0.000 3 A =0.001 12

 参数 驾驶舱结构 阻尼层 弹性模量E/GPa 71 0.1 密度ρ/(kg·m-3) 2 820 1 780 泊松比ν 0.33 0.45 损失因子η 0.002 0.28
2.1 降噪效果

 $\overline{\left\langle p^{2}\right\rangle}=\frac{1}{2 V} \int_{V} p p^{*} \mathrm{d} V$ （6）

 图 4 空间均方声压对比 Fig. 4 Comparison of space mean quadratic sound pressure

2.2 控制机理

 图 5 1 678 Hz和294 Hz的速度场 Fig. 5 Velocity fields at 1 678 Hz and 402 Hz
 图 6 结构的模态损失因子对比 Fig. 6 Comparison of modal loss factors of structures

3 实验测试与讨论 3.1 实验平台与测试原理

 图 7 实验平台 Fig. 7 Experiment set-up

 图 8 测试原理图 Fig. 8 Test sketch
3.2 中高频减振降噪验证

 图 9 噪声测试结果对比 Fig. 9 Comparison of noise test results

 图 10 振动测试结果 Fig. 10 Comparison of vibration test results

3.3 低频特性的拓展

 图 11 振动噪声测试结果对比 Fig. 11 Comparison of vibration and noise test results

3.4 宽带降噪效果

 图 12 1/3倍频程噪声(#1位置)测试结果对比 Fig. 12 Comparison of noise test results (position #1) in the one-third octave band
4 结论

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http://dx.doi.org/10.7527/S1000-6893.2020.23831

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#### 文章信息

WANG Xiaodong, QIN Yifan, JI Hongli, LU Yang, QIU Jinhao

Broadband noise reduction inside helicopter cockpit with acoustic black hole effect

Acta Aeronautica et Astronautica Sinica, 2020, 41(10): 223831.
http://dx.doi.org/10.7527/S1000-6893.2020.23831