Ultrasound in air has been applied to detecting objects, activating oxidation, levitating materials, agglomerating particles and so on. For these applications, the bending vibrations of a thin shell or plate are of great use since the mechanical impedances of these vibrations are less than those of the other vibrations. However it is difficult to apply these vibrations to focusing ultrasound in some direction or area because of the complexity of the sound field shaped by these vibration. This paper proposes a radiator using the bending vibration of a conical shell that may have high electroacoustic efficiency. The radiator can shape the sound field with a sharp main lobe in the axial direction when the component of the wavelength along the conical shell referred to the axial direction is equal to the wavelength of sound in air. The vibration, however, has not been analyzed exactly. So the vibration is simulated by the quadratic approximation FEM that is suitable for analyzing bending vibrations, and the directivity characteristic is analyzed by a method. On the basis of these results, The radiator using a conical shell is fabricated, and the vibration and directivity characteristic are investigated, In consequence, it is found that the analytical results are in good agreement with the experimental results, and that the radiator is suitable for focusing ultrasound in the axial direction in air and is useful for many applications.
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