Now the “speed of sound” is also often referred to with ultrasound. So why is the speed or velocity of sound so important?
Well, the exact speed of sound in specific tissue does not actually mean much to you clinically. However, the change in speed between two different mediums is extremely important. This is the essence of how ultrasound waves reflect and refract to create important ultrasound artifacts. So while you don’t need to know the exact speed of sound in certain tissue you do need to understand how the speed of sound changes between different mediums such as soft tissue, fluid, air, and bone.
The average speed/velocity of sound in all mediums is 1540 cm/s. However, depending on what medium the sound waves travel through, it can drastically change the propagation speed of sound as it passes through.
Two of the factors that affect the speed of sound are the stiffness and density of the material it is traveling through. The stiffer the medium, the faster the sound waves will travel and that is why sound waves travel faster in solids than in liquids or gases. So the ultrasound propagation speed from slowest to fastest is: Lung (air) << Fat < Soft tissue << Bone. This happens because stiffer mediums have tighter particles to propagate the ultrasound wave and therefore the velocity is greater.