Where the flames shoot up, the air inside is being squeezed together. Where they shrink, the air is spreading apart.
But why do the flames change height? It comes down to how tightly the air particles are packed.
When sound pushes air particles together, that area becomes more dense — meaning the particles are packed more tightly in the same space. It's not that more particles appeared; the same particles are just closer together.. When they spread apart, less dense. That's all density means here: how close together the particles are.
Where particles are squeezed together, they are compressed. This is a zone of high pressure and high density.
Where particles spread apart, they are in a state of rarefaction. This is a zone of low pressure and low density.
This back-and-forth pattern is how sound moves through any medium: air, water, tissue. The pattern travels forward as a wave, even though each particle mostly stays in place.
No medium, no particles to squeeze, no sound.
Watch closely. The candle flames rise and fall in a pattern. That pattern is compression and rarefaction made visible.
In every version: sound creates alternating zones of compression and rarefaction. The flames make those invisible zones visible.
Rubens' tube -- the classic physics demo. Gas flames in a tube react to sound.
The fire table -- same idea, scaled up. The most dramatic version.
Same thing in a solid. A compression wave traveling through diamond.
Deep bass = wide compression zones (long wavelength). High notes = narrow zones packed tight (short wavelength).
Drag the handle back and forth. Watch the slices bunch together (compression) and spread apart (rarefaction).
Tap each card to reveal the definition. These four ideas connect everything you've seen so far.
This is compression and rarefaction happening with an actual ultrasound transducer.
Schlieren imaging makes the invisible pressure waves from an ultrasound scan visible.
Everything you learned on the previous tabs -- compression zones, rarefaction zones, pressure differences, density changes -- that's exactly what's happening every time a sonographer places a transducer on a patient.
Space is a vacuum: nothing to compress, nothing to rarefy. No medium means no sound wave can form. This comes up on a later card.
The device that sends and receives sound waves. It converts electrical energy into mechanical vibrations and back again. Future lesson.
When a sonographer uses ultrasound to image what's inside the body. The transducer sends pulses into tissue and listens for echoes. Future lesson.
The distance from one compression to the next (or one rarefaction to the next). Covered on a separate card.
The energy moves forward through the medium, but the particles themselves mostly stay in place. Covered on the acoustic waves card.
A wave where particles vibrate in the same direction the wave travels. Compression and rarefaction are features of longitudinal waves. Covered on the acoustic waves card.