Why do ultrasonic cleaners sometimes make humming sound?

The sound that people hear is the sound wave signal with a frequency of 20-20000 Hz. The sound wave higher than 20000 Hz is called ultrasonic wave. The transmission of sound wave propagates according to the longitudinal direction of the sinusoid, that is, one layer is strong and another weak, and transmitted in turn, when the weak sound wave signal acts in liquid, a certain negative pressure is generated to the liquid, that is, the volume of the liquid increases, the molecular voids in the liquid increase, and many tiny bubbles are formed, and when a strong acoustic signal acts on the liquid, will cause the positive pressure in liquid, that is, the volume of the liquid is reduced by compression, and the tiny bubbles formed in the liquid are crushed.

The research proves that:

A. When the ultrasound acts in the liquid, the rupture of each bubble in the liquid will produce a shock wave with a great energy, which is equivalent to instantaneously generating a high temperature of several hundred degrees and up to thousands of atmospheric pressure. This phenomenon is called " cavitation effect ". Ultrasonic cleaning is the use of shock waves generated by the collapse of bubbles in a liquid to clean and flush the inner and outer surfaces of the workpiece.

B. Ultrasonic wave causing the liquid to vibrate together with the cleaning tank at the ultrasonic frequency. When the liquid and the cleaning tank vibrate, it has its own natural frequency, and the vibration frequency is the sound frequency, so people hear the humming sound.