High-intensity aerial converged ultrasonic waves (frequency: 20 kHz) were used to study acoustic waves entering a perforation. As a result, it was elucidated that the acoustic waves favorably entered the perforation with a sufficiently smaller diameter than the wavelength of the acoustic waves (approx. 17.4 mm). The sound pressure increased in the ratio of the 0.5th power of the electric power supplied to the sound source. When the electric power supplied to the sound source was 70 W, a very powerful ultrasonic field of approx. 174 dB was created at the bottom of the perforation. The entering acoustic waves were greatly distorted and included many harmonics of integral orders.
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