Ultrasounds are mechanical sound waves. Unlike acoustic phenomena, the frequencies that characterize ultrasounds are higher than those normally heard by a human ear.

The frequency conventionally used to discriminate acoustic waves from ultrasonic waves is set at 20 kHz up to several gigahertz, higher than the upper audible limit of human hearing (below 20 kHz in healthy young adults).

Ultrasounds are generated using piezoelectric materials, which have particular mechanical-electrical characteristics. These particular materials such as quartz or barium titanate have the characteristic of generating an electrical potential difference if compressed or stretched in the transverse direction; vice versa, if a potential difference is applied to their extremes, these are compressed or dilated in a transverse direction.

This last feature is used to generate these mechanical waves above the audibility range (ultrasounds). Depending on the type of material: different frequencies of ultrasound, different propagations in the materials, and therefore different power characteristics of the generating machines are obtained.

A second system for generating ultrasounds is based on magnetostriction: a ferromagnetic core subjected to an alternating magnetic field (maximum 200 kHz) is put into vibration at ultrasonic frequencies. This system is, for example, used for the production of industrial ultrasonic washing machines.

Like any other type of wave phenomenon, ultrasounds are subject to reflection, refraction and diffraction phenomena and can be defined by parameters such as frequency, wavelength, propagation speed, intensity (measured in decibels), the attenuation (due to the acoustic impedance of the medium crossed). Few people can perceive them clearly and can become disturbing listening to them.

Ultrasound applications

Ultrasounds are mostly used in the medical and industrial fields as they are widely used in ultrasound scans, in non-destructive tests and many appliances used for surface cleaning of small objects. Ultrasounds are also used to nebulize water in some types of humidifiers.

They are often prescribed as physiotherapy following trauma or fractures, although there are conflicting studies on their effectiveness. Even sonar employs frequency ranges that often border on the ultrasound range.

The main applications beyond those mentioned above also relate to the mechanical field, especially in the welding of plastic materials and non-destructive testing of welds. The welding of plastic materials employing ultrasound is often used when a certain aesthetic quality is required but above all speed of execution. Two plastic objects (preferably of the same material so that the molecular friction is high) are put in contact with each other, and a metal parallelepiped (sonotrode) leans on one of them emitting ultrasounds and then putting it in vibration.

The friction generated will melt the plastic parts in contact by joining them. The shape and frequency at which the sonotrode will vibrate depend on the geometry of the object to be welded. The aesthetic quality is excellent even if water tightness is not ensured, so if it is a fundamental requirement, it is preferable to consider another type of welding (eg, hot-blade welding).