If sound travels from fat to soft tissue, how does the speed change?

• A. It increases
• B. It decreases
• C. It remains the same
• D. It depends on frequency

Answer: (A)

The speed of sound in tissue is governed by how stiff (elastic) the tissue is and how dense it is. Soft tissues like muscle are less compressible and generally stiffer than fat, so sound waves move faster in them. When a wave travels from fat into soft tissue, the medium becomes stiffer, so the propagation speed increases. In numbers commonly cited for ultrasound, fat is around 1450 m/s while soft tissue is about 1540–1600 m/s, illustrating the increase. The speed in biological tissues is effectively independent of frequency in the typical diagnostic range, so the main effect here is the higher speed in soft tissue, not a frequency-related change.