How do you think scientists discovered the layers of our earth

Scientists learn about the layers deep within the Earth's crust by studying how seismic waves travel through the Earth. If there is an earthquake somewhere, seismagraphs in many locations will record it. By looking at the time of arrival of the main set of waves, and how the frequencies of the waves are arranged within the set, scientists can learn about the density and other properties of the layers. The relative differences in arrival times of the S and P seismic waves at several recording stations tell scientists about the different speeds those waves were traveling at, which in turn gives information about the density of the material the layer is made of, and how thick the layer is in several directions.

If you line up a rod of metal and a wooden dowel, and hit one end of each with a hammer and press your ear to the other end. Does the sound wave reach the other end of each rod at the same time? If you can find rods that are the same size but of different metals (or maybe try plumbing and electrical conduit pipes, which you can probably find in steel, copper, and maybe galvanized aluminum) hit one end of each with a hammer. Do they ring at the same frequencies? Why or why not?