鎌倉 友男¹, 野村 英之², 小塚 晃透³

Tomoo KAMAKURA¹, Hideyuki NOMURA², Teruyuki KOZUKA³

¹電気通信大学 産学官連携センター, ²電気通信大学 情報理工学研究科, ³愛知工業大学工学部

¹Center for Industrial and Governmental Relations, The University of Electro-Communications, ²Graduate School of Informatics and Engineering, The University of Electro-Communications, ³Aichi Institute of Technology

キーワード : nonlinear acoustics, waveform distortion, acoustic radiation pressure, acoustic streaming, ultrasound

音波は代表的な波動であって，我々はコミュニケーションの媒体として，特に意識することなく利用している．一方で，非破壊検査やイメージングにおける情報収集手段として，積極的に音波，特に周波数の高い超音波を利用している．波動には共通した現象として回折や散乱があるが，放射圧も共通の現象の一つである．音波の放射圧とは，音波を受ける物体の表面に働く直流的な圧力であり，その大きさは媒質の境界を挟む音響エネルギー密度の差で与えられ，2次的な微小量である．実用的には，その物体の表面に働く放射圧を積分した放射力を我々は利用する．放射力は小さいといえども，例えば音響エネルギーを局所に集中させることで，微小物体を非接触で動かしたり，浮揚させたりするほどの力となる．本稿では，音響エネルギーに関わる非線形現象のうち，音波の波形歪み，放射圧，そして音響流の古典的現象を概観する．

A familiar example of waves are sound wave in air that are used as a medium for communication without being aware of their presence. At the same time, sound waves, particularly ultrasound are positively utilized as a collection tool of information in non-destructive testing and acoustical imaging. Generally, waves exhibit phenomena like diffraction, reflection, and interference. Radiation pressure is also one of the phenomena that waves have in common with each other. Acoustic radiation pressure is exerted on an interface that separates two acoustically different fluids and is provided by the difference of the acoustic energy densities in both fluids. Hence, it is regarded as a non-zero net pressure of the second order of smallness. Practically, acoustic radiation force is of importance in applications where evaluation is performed by integrating the radiation pressure over an obstacle surface. Although the radiation force is small as well, it can be utilized for non-contact levitation and manipulation of small obstacles by focusing ultrasound waves on them. This report overviews second-order classical phenomena caused by acoustic energy; specifically, the phenomena of waveform distortion, radiation pressure, and acoustic streaming.