From the sound of birds to the sound of broken glass - the sound is beyond our ears. It arises from the mechanical vibration of an object. For example, if a guitar string is plucked, its vibration will cause pressure and density fluctuations in the air, starting from the sound source and propagating in all directions in the form of waves. This applies not only to air, but also to any elastic medium, i.e. gas, liquid and solid.
Ultrasound is an example of how humans cannot hear all types of sound. Its high-frequency sound waves are quietly applied in industry and medicine.
Sound waves are defined by their frequency (waves per second). For example, ultrasonic waves range from 20 kHz to 1 GHz. Frequencies from 20 kHz to 70 kHz are used for ultrasonic welding. In the lowest area, the human ear can hardly perceive it. Because the range of audible sound is only from 16 Hz to 20 kHz. The vibration on it is considered vibration at best.
