Vibration and Wave in physics

• A vibration source in a mechanical medium will cause wave propagation.
• Ultrasound is a mechanical wave, cannot exist in vacuum.
• Ultrasound transfer energy within the medium, does not transfer mass.
  
• Longitudinal wave: particle movement in the same direction to the wave propagation.
  
• compression: Ultrasound propagation causes local medium density and pressure varying with time. The density or pressure will increase when it is compressed.
• rarefaction: When low pressure pass through, the local density will drease.
• Transverse wave: particle movement in the direction perpendicular to the wave propagation.