what is the amplitude of a compressional wave
So a compressional wave can be drawn, as in this picture: http://images.tutorvista.com/cms/images/39/longitudinal-wave.PNG Compression refers to exactly what it is, a compressed region. This is where the average distance between lines is decreased. Rarefaction is where the lines appear to be normally spaced. The ideas of measurement in transverse and compressional waves are actually very similar. So when it comes to amplitude, what do we do? Well, in transverse waves, we normally measure the amplitude to be from the equilibrium position to a crest. So basically, we measure the maximum displacement the wave has during it's motion. Therefore, let's try to find what we would use in a compressional wave situation. Since a "crest" seems to align with a "compression" we should assume that to be a good guess. Similar to how a crest is maximally displaced, we should make sure that we choose a point within a compression that is maximally displaced. For that reason, you would consider the crest to be the point right at the center of the compression. The actual mathematics about how to determine the amplitude through calculation is exactly the same as in the transverse case. In fact, if you plot a longitudinal wave and consider the center of compression to be a crest, then you will actually get an image of a typical transverse wave.