To generate ultrasound, a high-frequency generator is needed that converts the normal mains frequency of 50 - 60 Hz into high-frequency vibrations between 20 and 40 kHz. So-called transducers, which can be installed under the cleaning bath, for example, then convert these electromagnetic vibrations into mechanical vibrations of the same frequency. In simple terms, the sound transducer is nothing more than a resonance system that is excited to natural oscillations by suitable energy supply. Known electroacoustic sound transducers such as telephones or loudspeakers, which work in the auditory frequency range, fail in the ultrasonic range for various reasons. Therefore, the indirect piezoelectric effect is used here. If an electrical voltage is applied to a piezoelectric material, the material reacts by increasing in length, and by shortening when the polarity of the voltage is reversed. In this way, an electrical oscillation in which the polarity of the voltage is constantly reversed can be converted into a mechanical alternating movement.
Sintered ceramics made of lead zirconate titanate (PZT), which have an efficiency of more than 90 percent, are used for the production of high-performance ultrasound devices.
At low ultrasonic operating frequencies in the range of 25 kHz, the cleaning effect by cavitation is very efficient. The cavitation bubbles can become large (tensile and pressure phases last longer) and thus contain a lot of energy or release it when imploding. In comparatively higher frequency ranges of 40 kHz, a field of smaller but numerous cavitation bubbles is obtained and a particularly gentle cleaning of sensitive parts is achieved.
Depending on the area of application, frequencies of 25 kHz (rough cleaning in mechanical engineering) and 40 kHz (sensitive parts, e.g. from the medical, optical and electronics sectors) are used today.
Further information on the generation of ultrasound by means of sonotrode-based Ultrasonic homogenisers e.g. for cell disruption.