声学黑洞（ABH：Acoustic Black Hole）是将天文物理学中的黑洞概念引入到波动和声振领域中。物理学家创造了声音世界的人造黑洞，让某种材料以超音速在介质中移动，在介质中穿行的声波无法跟上这种材料的速度，就像鱼儿在快速流动的河流中游动一样，声音最终被类似河流的事件视界捕获。
本期分享西北工业大学航海学院和中国船舶集团有限公司第705研究所团队近期发表在《科学通报》上的研究工作“声学黑洞研究进展与应用”。本文第一作者和通讯作者均为西北工业大学航海学院高南沙副教授。
高南沙1*,张智成1,王谦2,郭鑫羽1,陈克安1,侯宏1
1.西北工业大学航海学院 “海洋声学信息感知”工业和信息化部重点实验室
2.中国船舶集团有限公司第705研究所
E-mail: gaonansha@nwpu.edu.cn文章来源：《科学通报》Chinese Science Bulletin, (2021)https://doi.org/10.1360/TB-2021-0439
声学黑洞结构作为一种新型的弯曲波调控技术，可以有效地降低结构中弯曲波的传播速度，减小边界末端的反射，形成具有高能量密度的区域，因此在减振、降噪、波动调控以及能量回收等方面具有广阔的应用前景。不同于以往复杂的减振降噪复合结构，声学黑洞因其结构与材料单一，在实际应用方面具有一定的优势。时至今日，针对声学黑洞结构已经进行了大量的基础理论研究和实验探索，并取得了一定的阶段性研究成果。本文首先介绍了声学黑洞的起源和基本原理，然后全面介绍了理论计算和实验研究方法，详细地综述了声学黑洞结构的四个主要功能性分类，即减振、降噪、波动调控和能量回收，并总结了现在研究存在的问题，最后对声学黑洞的发展前景进行展望，并指出了未来研究的重点和方向。
声学黑洞是天体物理学中黑洞概念的声学类比。1946年，Pekeris[1]发现声学中的波动也有与黑洞类似的效应,即声波在特定的非均匀分层流体中传播时，声速随介质深度的增加而减小到零，不会发生声波的反射，其表现为声波不能逃离开放区域的边界。1988年，Mironov[2]发现弯曲波在楔形结构中传播时也有类似现象，即理论上认为在特定的条件下，弯曲波在指数型表面剪裁的薄板上将不会发生反射，形成零反射条件。1989年，Krylov[3]首次研究了具有楔形几何特征的一维梁结构，即通过改变楔形结构的几何外形参数来操纵弯曲波的传播，至此声学黑洞的概念正式提出。本文将声学黑洞定义为: 通过几何参数或者材料特性的变化而形成一种对弯曲波有汇聚效果的结构，在绝对理想情况下，弯曲波波速在声学黑洞区域内逐渐减小至零。
近年来，基于声学黑洞的应用不断丰富。本文将从对弯曲波的控制角度出发，按照声学黑洞的功能性划分为减振、波动调控、降噪、能量回收四个部分。
声学黑洞用于弯曲波减振经历了数十年，以上研究反映出声学黑洞结构的有效性。目前，大多数研究依然集中在针对经典几何结构的应用场景中，对于复杂结构的减振需求，并考虑不同的边界条件，声学黑洞的相关研究较少。因此，如何通过声学黑洞的巧妙设计来解决工程实践中的减振需求, 是今后声学黑洞发展的重要一步，同时也可以填平理论和实践的鸿沟。
波动调控主要研究集中在声子晶体、声学超材料以及超表面，声学黑洞的引入无疑更加丰富了波动调控的手段。从学科角度看，基于声学黑洞的波动调控是近年来新兴的研究热点，未来的发展可能在于波动调控结构一体化设计。相对于传统降噪材料或者结构，声学黑洞因具有刚性基体而具备防火、防潮的物理性能，在宽频噪声控制中具有良好的发展前景。目前的研究大多集中在通过控制弯曲波的减弱来影响振动辐射噪声，并且大多数研究停留在声波导管和声学混响室测试方面，在实际具体应用场景中的使用效果有待进一步研究。此外, 低频降噪受制于声学质量作用定律，一直以来是难以克服的棘手问题。在声学黑洞结构中，特征频率的设计对于低频降噪至关重要，提出具备低频特征频率的声学黑洞结构无疑是一种解决方法。此外，与经典降噪结构复合后的协同作用能否对低频声波进行抑制也值得不断地深入研究。
声学黑洞在振动能量回收应用中起步较晚，研究相对较少，但是近几年开始崭露头角。目前对声学黑洞结构能量回收的研究中，回收元件以及回收电路对声学黑洞效应的形成及影响机制仍不明晰。在将声学黑洞结构运用于能量回收的实践中时，首要任务仍然是要更加明确能量转化机理，其次是如何通过声学黑洞结构设计提高能量体转化率。此外，对于宽频范围内的能量收集一直以来都是研究重点。简而言之，通过声学黑洞结构与能量回收系统的协同一体化设计，实现高效宽频的振动能量回收是近几年有较大潜力的发展方向。
图1声学黑洞基本分类以及相关应用。(a)-(c) 三种典型声学黑洞示意图; (d)声学黑洞的拓展应用举例
图 2激光超声扫描技术测试声学黑洞板波场，其中采用压电材料产生对薄板的激励。改自文献[32]图3 声学黑洞减振结构。(a)螺旋一维声学黑洞，改自文献[34];(b)不同构型和不同曲率的螺旋一维声学黑洞，改自文献[35];(c)双叶型声学黑洞梁，改自文献[36]; (d)空间V 字弯折的双叶型声学黑洞梁，改自文献[18];(e)二维内嵌声学黑洞结构的阻尼板，改自文献[43]图4 声学黑洞波动调控结构。(a)椭圆声学黑洞透镜，改自文献[47];(b)圆形声学黑洞压痕对弯曲波能量汇聚，改自文献[28];(c)-(d)声学黑洞Luneburg 透镜，改自文献[32];(e) 基于声学黑洞的宽频带声透镜，改自文献[31]图5 声学黑洞降噪结构及实验。(a)声学阻抗管测试声学黑洞小样品隔声量，改自文献[49];(b)混响室测试声学黑洞板大样品隔声量，改自文献[51]
综上所述,振动噪声问题由来已久，始终是困扰着人类社会发展的难点之一,因此减振降噪的革新探究从未停止。近年来,通过引入声学黑洞来操纵弯曲波已经成为振动噪声的热点研究领域之一。本文全面综述了声学黑洞的原理、分析方法以及功能性分类,表明声学黑洞结构具备减振降噪效果明显、结构灵活等特点,并在波动调控和振动能量回收领域有着极大的潜力。作为一种新兴研究方向,针对声学黑洞的基础理论和实际应用拓展仍需要开展深入且广泛的研究, 其中重点方向包括:1.声学黑洞声振耦合机理以及与功能材料的匹配问题需要进一步明晰；2.声学黑洞中的非线性波动行为需要进一步完善；3.针对不同应用场景,研究具有声学黑洞复合结构的承载一体化设计方法和理论；4.非理想型声学黑洞结构的理论建模以及应用拓展。随着声学黑洞的研究不断深入，对于减振降噪的基础理论以及复合结构设计也是一种补充和拓宽。从应用实践需求角度出发,声学黑洞结构的研究有望应用于工业设备设计、建筑声学以及功能复合材料设计中。1 Pekeris C L. The propagationof sound in a half-space of variable sound velocity under conditions of theformation of a shadow zone.J Acoust Soc Am, 1946, 18: 295–3152 Mironov M A. Propagation offlexural waves in a plate whose thickness decreases smoothly to zero in afinite interval. Sov Phys Acoust, 1988, 34:318–3193 Krylov V V. Conditions forvalidity of the geometrical-acoustics approximation in application to waves inan acute-angle solid wedge. Sov PhysAcoust, 1989, 35: 176–1804. Pelat A, Gautier F, Conlon SC, Semperlotti F. 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