The Rubens' Tube: Soundwaves in Fire!

Step 2 Nerdy stuff - a bit on waves.

Visualizing sound as a wave can be a bit misleading. Sound is often a often portrayed as a sine waveform because it's easier to illustrate than the longitudinal compression wave that it actually is.
The first image shows the typical way a sound wave, or just about any wave, is portrayed visually. Because sound is basically a vibration created by changes in air pressure, the peaks on the waveform correspond to the highest pressure and the troughs correspond to the lowest pressure. The wavelength is measured by the distance between two peaks, or two troughs.

The second image is more accurate way to visualize compressional waves, including sound waves. Each dot could represent a molecule of the matter (such as air) that the wave is traveling through. At the points where the pressure is highest you can see that the density of the material is relatively higher than the areas with lower pressure. These are called condensation and rarefaction, respectively.

The third image shows the previous two stacked, it's easy to see how the peaks of the wave correspond to condensation, and how the troughs correspond to rarefaction