<unk> , simply known as<unk> , is a type of mechanical<unk>  called sound characterized by vibrating or moving particles within a medium.<unk>  is distinguished by vibrations with a frequency greater than 20,000 Hz, compared to audible sounds that humans typically hear with frequencies between 20 and 20,000 Hz.<unk>  requires matter or a medium with particles to vibrate to conduct or propagate its<unk> . The<unk>  generally travels through most mediums in the form of a wave in which particles are deformed or displaced by the<unk>  then reestablished after the<unk>  passes. Types of waves include shear, surface, and longitudinal waves with the latter being one of the most common used in biological applications. The characteristics of the traveling<unk>  greatly depend on the medium that it is traveling through. While<unk>  waves propagate through a medium, the amplitude of the wave is continually reduced or weakened with the distance it travels. This is known as attenuation and is due to the scattering or deflecting of<unk>  signals as the wave propagates and the conversion of some of the<unk>  to heat<unk>  within the medium. A medium that changes the mechanical<unk>  from the vibrations of the<unk>  into thermal or heat<unk>  is called viscoelastic. The properties of<unk>  waves traveling through the medium of biological tissues has been extensively studied in recent years and implemented into many important medical tools.
Ultrasound energy
Ligature 
Cryonics
Nuclear technology
Surgical lubricant
Assistive technology
Bioelectromagnetic medicine
Exomedicine