sound waves through the ground

[HUPTON]

sound waves travel through the ground when the ground is impacted upon. they reflect when they hit a harder medium. why?

[bernardo]

Sound waves go through a material if it is slightly flexible, where the molecules shift sideways a bit.  If the wave comes to a more solid material, the wave can't continue easily through the new layer because it would take much more energy to vibrate the molecules.  In order to disperse the energy of the wave, it has to go somewhere else - by bouncing back and sideways.  It's a bit like a sound wave in air hitting a brick wall and the noise being much quieter inside the building.

[Tim Baxter]

That would be like an echo then.....?