Sound is a mechanical wave which propagates longitudinally and carries energy from one place to another place through elastic media. Sound waves propagate by oscillatory motion of the particles in the medium, creating alternating regions of compressions and rarefactions.
Sound needs a medium such as air, water, or soft tissue to travel through. Sound cannot travel through a vacuum.
The speed of sound is determined by the characteristics of the medium through which the sound travels. All sound waves travel at the same speed through a particular medium. For example an ultrasound wave with frequency of 5 MHz and an ultrasound wave with frequency of 2.5 MHz will travel at the same speed through soft tissue.
The ultrasound waves encounter many different media such as muscle, fat, bone, and lung tissue while travelling through the body. The speed of sound is slightly different in each media. The average speed of sound in soft tissue is approximately 1540 m/sec.
The speed of sound is slowest in the air, then fluids and then solids.
air < liquids < solids
Diagnostic Medical Ultrasound is the use of high frequency sound to aid in the diagnosis and treatment of patients. The frequency ranges used in medical ultrasound imaging are 2 – 15 MHz.