The Earth's molten core can only be traveled through by compressional waves. P-waves travel fastest, at speeds between 4-8 km/sec (14,000-28,000 km/h) in the Earth's crust. S-waves travel more slowly, usually at 2.5-4 km/sec (9,000-14,000 km/h). Sound waves are usually called P-waves and are heard but not often felt.