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Basic Knowledge about High Intensity Ultrasound

Sound waves are pressure waves. They consist of a continuous progression of positive and negative pressures. High intensity ultrasound uses frequencies in between 20 kHz to 100 kHz. The human ear responds to sound waves between 20 Hz to 18 kHz. Diagnostic medical applications employ frequencies ranging from 5 MHz to 10 MHz.

Figur 1. Because the compression phase has a positive derivative of the pressure, i.e. it is compressed when the pressure function is going "up" in pressure and decompressed when the pressure is going "down".

The relationship between frequency f, propagation velocity v, and wavelength (λ) is the following:
(λ) = v / f

Propagation velocity and wavelength are varying for different mediums:

Creation of Sound Waves

The pressure waves are produced by a sound generator called resonator or sonotrode in high power ultrasound. The resonator is mechanically coupled with a converter.

Mechanical displacement in the converter is created by piezo crystals, which change in volume as an alternating voltage is put across. The alternating voltage is created by the generator. Converter and resonator form a unit that oscillates in resonance. The frequency of a transducer is dependent of its geometric and material characteristics. Every transducer works optimal at the resonance frequency.

Attenuation

Sound waves propagate in all mediums, not in vacuum, and attenuation varies due to different material properties. The energy lost by attenuation transforms into heat in the medium. This attribute is applied for wax melting and plastic welding.

Reflexion and interference

Sound waves can increase or decrease intensity through constructive or destructive interference, respectively.

Sound waves are reflected at a surfaces. Constructive standing waves are achieved if the resonator and the surface have a space of (λ), (λ/2), (λ)/4. Standing waves can strengthen or weaken desired effects.

Cavitation in fluids

Sound waves create cavitations in fluids. During the compression phase, the material compresses, in the rarefaction phase it expands. When the negative pressure exceeds the adhesive force between the molecules, they get torn apart and a cavitation is created. Gasses and liquids solved in the fluid diffuse into the cavitation. In the following compression phase the cavitations are compressed, but only a fraction can diffuse back to the fluid. Because of the difference in surface area for the compression (small bubble) and rarefaction (big bubble) state of the cavitation bubble, the bubble grows larger for each cycle.

Cavitations grow larger each decompression cycle until they reach a critical, unstable size and collapse. The energy in the center of collapse can reach pressures up to 2000 atm. and temperatures of 5000 K for a fraction of a microsecond. In addition the collapse produces a shock waves and swirl in the medium.

The creation of cavitations is dependent on vapor pressure, temperature, surface tension, surface pressure, amplitude and the sound wave frequency.

Applications

High intensity ultrasound is efficient, trusted and environment-friendly. The method increases the chemical reaction rate and saves costs and time.

The high energetic cavitation collapse results in acceleration of the chemical reactions. Swirls and shock waves enable ultrasonic cleaning, enhance mixing and emulsification and increase extraction rates. Cavitation bubbles lead to degassing of the fluid and formation of spontaneous nucleation in crystallization processes. The energy dissipation is used in plastic welding and wax melting.

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