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People hear sound waves with frequencies between 20-20000Hz. Sound waves above 20000Hz are called ultrasonic waves. Sound waves propagate longitudinally following a sine curve, alternating between stronger and weaker intensities. When a weak sound wave acts on a liquid, it creates negative pressure, increasing the liquid's volume and the space between molecules, forming numerous tiny bubbles. When a strong sound wave acts on a liquid, it creates positive pressure, compressing the liquid and collapsing these tiny bubbles. Research shows that the collapse of each bubble in a liquid subjected to ultrasonic waves generates a high-energy shockwave, instantaneously creating temperatures of several hundred degrees Celsius and pressures of thousands of atmospheres. This phenomenon is known as "cavitation." Ultrasonic cleaning machines utilize the shockwaves generated by the collapse of bubbles in liquids to clean the inner and outer surfaces of workpieces.

Secondly, ultrasonic waves propagating through a liquid cause the liquid and the cleaning tank to vibrate at the ultrasonic frequency. The liquid and the tank have their own natural frequencies, which are the same as the sound wave frequency. This vibration is what people hear as a humming sound.

1. What are ultrasonic waves? Waves are categorized into three types: infrasound, sound, and ultrasound. Infrasound has a frequency below 20Hz; sound waves have a frequency between 20Hz and 20kHz; and ultrasonic waves have a frequency above 20kHz. Infrasound and ultrasound are generally inaudible to the human ear. Ultrasonic waves, due to their high frequency and short wavelength, exhibit good directionality and strong penetration capabilities, which is the reason for their application in ultrasonic cleaning machines.

2. How does ultrasonic cleaning work?

Ultrasonic cleaning utilizes the cavitation, acceleration, and acoustic streaming effects of ultrasonic waves in liquids to directly and indirectly act on the liquid and contaminants, causing the contaminants to disperse, emulsify, and detach, thereby achieving cleaning. In current ultrasonic cleaning machines, cavitation and acoustic streaming are the most widely utilized effects.

(1) Cavitation: Cavitation involves the transmission of ultrasonic waves into a liquid with alternating high-frequency compression and rarefaction at a rate exceeding 20,000 times per second. During rarefaction, vacuum nuclei bubbles form in the liquid. During compression, the collapse of these bubbles generates a powerful shockwave that removes contaminants from the surface of the cleaned object, achieving precise cleaning.

During ultrasonic cleaning, the visible bubbles are not vacuum nuclei but air bubbles, which inhibit cavitation and reduce cleaning efficiency. Only when air bubbles are completely removed from the liquid can the vacuum nuclei bubbles generated by cavitation achieve their cleaning effect.

(2) Acoustic Streaming: The phenomenon where ultrasonic waves in a liquid generate flow along the direction of wave propagation is called acoustic streaming. At a sound intensity of 0.5 W/cm2, acoustic streaming is visible to the naked eye, producing a flow perpendicular to the vibrating surface at a speed of approximately 10 cm/s. This acoustic streaming stirs microscopic oil contaminants on the surface of the cleaned object. The cleaning solution on the surface of the contaminant also undergoes convection, mixing the contaminant-dissolving solution with fresh solution to accelerate the dissolution process and significantly aid in the removal of contaminants.

(3) Acceleration: The acceleration generated by the movement of liquid particles. For ultrasonic cleaning machines with higher frequencies, cavitation becomes less significant, and cleaning relies mainly on the acceleration of liquid particles under ultrasonic action to impact and clean contaminants with ultra-high precision.

3. Composition of an ultrasonic cleaning machine: An ultrasonic cleaning machine primarily comprises an ultrasonic cleaning tank and an ultrasonic generator. The cleaning tank is made of high-quality stainless steel that is robust, elastic, and corrosion-resistant. An ultrasonic transducer is mounted at the bottom. The ultrasonic generator produces high-frequency, high-voltage signals that are transmitted to the transducer via a cable. The transducer and vibrating plate work together to produce high-frequency resonance, causing the solvent in the cleaning tank to be affected by the ultrasonic waves and clean the contaminants.