たわみ振動板と反射板を用いた超音波空気ポンプ

摘要

An ultrasonic air pump using a vibrator and a reflector was discussed. The ultrasonic air pump consists of the bending vibrator with the PZT element and the reflector paralleled to the vibrator with the small air gap. The intense sound pressure field can be generated in the air gap between the vibrator and the reflector by generating the bending vibration on the vibrator, and the air flow can be eventually induced along the air gap. In this report, a method for generating the large sound pressure in the air gap by corresponding the resonance frequencies of the vibrator and the air layer was investigated. The vibrator consists of a 20-mm-length and 30-mm-width aluminum plate and a rectangular PZT element, and the fundamental bending vibration was generated in the length direction of the vibrator. The acrylic plate was paralleled as the reflector with the small distance of 1 mm compared with the wavelength in air. By using the designed prototypes of the vibrator, the sound pressure distributions in the air gap were measured at each resonance frequency. It was found the larger sound pressure can be generated in the air gap with the larger correlation between the vibration mode of the vibrator and the sound pressure distribution. The maximum flow velocity of 0.23 m/s, which is 1.4 times larger than the previous design, could be obtained at the frequency of 26.2 kHz with the vibrator having the thickness of 2 mm. The relationship between the sound pressure distribution and the configuration of the reflector was also investigated, and the one-way air flow could be achieved with the vibrator having the asymmetric configuration in the length direction.%たわみ振動板と反射板を用いた超音波空気ポンプについて検討した.圧電素子を接着した振動板にたわみ振動を励振し，振動板に平行に反射板を設置すると，振動板と反射板間の空気層中に強い音圧を発生させることができ，結果的に空気の流れが生じる.本報告では振動板の共振周波数とモード形状を空気層音場のそれらと一致させることにより，空気層内により高い音圧を発生させる方法を提案した.振動板にPZT素子を接着した幅30mm，長さ20mmのアルミニウム板を用い，長さ方向に対し1次たわみ振動を励振した.反射板として，アクリル板を空気中の音波の波長より短い厚さ1mmの空気層を隔てて振動板と平行に設置した.設計に基づいた振動板を製作し，各音場モードの共振周波数で振動板を駆動したときの音圧分布を測定した.振動板の振動モードと空気層中の音場モードとの相関が大きいほど，空気層中に高い音圧が発生することがわかった.厚さ2mの振動板にその共振周波数26.2kHzで1次たわみ振動を励振した時，最も相関が大きい（3，1）モードの音場を励振すると，従来値と比べて約1.4倍大きい最大流速0.23m/sが得られた.また反射板形状が空気層音場や流れに与える影響についても検討した.反射板を長さ方向に対して非対称な形状とすることにより，一方向に空気流れを発生することができた.