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Chapter 1: Basics
Compression and Rarefaction
How sound waves squeeze and stretch particles
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Compression and Rarefaction
When a sound wave moves through a medium, it creates two alternating zones:
Compression
Particles squeezed together High pressure High density
Rarefaction
Particles stretched apart Low pressure Low density
These zones alternate back and forth as the wave travels.
📷 Ultrasound transducer image
Think of a slinky. Push one end and watch the coils bunch up. Pull it and they spread apart.
At Rest
Grab the left handle and push or pull
Where coils bunch together: compression. Where they spread apart: rarefaction. The pattern repeats over and over.
Now imagine the same thing happening to molecules in a medium. Dense zones alternate with sparse zones.
Speed
The red zones are compressions (high density). The blue zones are rarefactions (low density). Together they form one complete wave cycle.
Sound Waves in Tissue ▸
When the transducer sends a pulse into the body, it creates alternating compressions and rarefactions in the tissue. These pressure changes are what carry the sound energy through the patient.
Why This Matters ▸
The pattern of compressions and rarefactions is how ultrasound carries information. When the wave hits a boundary between tissues, part of it bounces back. The machine reads those returning compressions to build the image.