Sound waves travel by pushing particles together and then letting them spread back apart. Where particles are squeezed together, they are compressed.
Where they spread apart, they are in a state of rarefaction.
This back-and-forth pattern of compression and rarefaction is how sound energy moves through tissue, air, or any other material.
Push the handle back and forth. Watch the slices bunch together (compression) and spread apart (rarefaction). The sensor in the middle detects which one is passing through.
Propagation Lab
Tap to start
Longitudinal: Push left/right. Slices compress and rarefy.
The same energy that bunches and spreads particles can also be drawn as a wave. Drag up and down to see the transverse view of the same motion.
Transverse view: Same energy, drawn as up/down displacement.
In Ultrasound
The transducer sends pulses of sound into the body. Those pulses are longitudinal: the tissue particles vibrate back and forth along the same direction the pulse travels.
Compression▸
Where particles are pushed together. This is a zone of high pressure. On the interactive, you saw the slices bunch up. In the body, this is tissue molecules being briefly squeezed closer than normal.
Rarefaction▸
Where particles spread apart. This is a zone of low pressure. The slices that moved further apart on the interactive represent molecules briefly stretched beyond their resting position.
Energy Transfer▸
The sound energy moves from the transducer through the tissue and back, but the particles themselves stay roughly in place. They vibrate, then return to their resting position. Only the energy travels. This is exactly what you saw: the slices shifted but returned, while the compression pattern moved across the screen.
The key idea: sound in the body is a series of compressions and rarefactions traveling through tissue. The echo that bounces back creates the image.
Quick Check
When particles are pushed together, that zone is called:
In a longitudinal wave, particles move:
When a sound pulse travels through tissue, what actually moves from point A to point B?