Compressions are the parts where the coils are close together
In longitudinal waves in a spring, the parts where the particles are closest together are called compressions, while the parts where the particles are farthest apart are called rarefactions. These alternating compressions and rarefactions create the wave motion that travels through the medium.
The type of wave that passes through the spring in the frog toy is a longitudinal wave. This is because the particles of the spring vibrate parallel to the direction of wave propagation, causing compressions and rarefactions to travel through the material.
Longitudinal wave
rarefactions
rarefactions
Sound generally refers to a disturbance that is audible to the human air. In air, like in any gas, such disturbances propagate as a compression and rarefaction 'pulse'. Watch a traffic jam from a distance - you see the 'bunching' of cars propagate away from the head of the traffic. Then, when the initial blockage is removed (light goes green) the bunched cars separate out again (the rarefaction). In solid media you can have both compression/rarefaction waves and also transverse waves. These can be visualized by (mentally) taking a long and rather slack spring, stretching it out, and shaking one end from side to side.
A longitudinal wave is a wave where the particles of the medium move parallel to the direction of the wave propagation, like compressions and rarefactions in a spring. A transverse wave is a wave where the particles of the medium move perpendicularly to the direction of wave propagation, like the motion of a string being plucked.
In a longitudinal wave moving along a spring, the areas where the coils are the farthest apart are called rarefactions. These are regions of lower density and pressure in the wave where the coils are spread out.
To model a compressional wave using a coiled-spring toy, you can compress one end of the toy causing the coils to bunch up together. When released, the energy will travel through the coils in a wave-like motion resembling a compressional wave. As the coils expand and contract, they represent the alternating compressions and rarefactions of the wave.
It is compressions
A compressional wave moves by particles in the medium vibrating back and forth in the same direction as the wave. This creates areas of compression and rarefaction as the wave passes through the medium. The energy of the wave is transferred through these compressions and rarefactions.
Compressions and rarefractions make up sound waves. These look like squashed up coils of a spring and then stretched out coils. Try using a slinky on the ground to show it. Grab a friend, and hold both ends of the slinky stretched across the room, then push at one end. You will see the compression move along the slinky. Do it over again rapidly and you will see the series of compressions, which mirrors the behavior of a sound wave.