a. Any energy (disturbance) introduced into the medium causes the molecules closest to the
disturbance (energy) to be pushed together. This we call compression (fig 1-2).
Figure 1-2. Compression
b. Because they always try to stay about the same distance from each other, the compressed
molecules now attempt to return to their original state. Here they expand their distance between each
other to what it was before the disturbance. This expansion we call rarefaction (fig 1-3).
Figure 1-3. Rarefaction
c. As the compressed molecules return to their original state, the molecules right next to them
become compressed and then they expand. This occurs down the line in the medium until the collisions
of the molecules from the compressions and rarefactions reach the eardrum (fig 1-4).
Figure 1-4. Rarefactions and compressions
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