You may find this http://www.kettering.edu/physics/drussell/Demos/reflect/reflect.html" . At the boundary, the incident want splits into a reflected wave and a transmitted wave. The total power that was contained in the initial wave is now split among the reflected and transmitted wave, so the amplitudes must necessarily be different. Waves on a string travel at a speed inversely proportional to the string mass, so if the transmitted wave is on a less dense string, it will be going faster. If you are more interested in pressure waves through bulk matter, than it gets more complicated because there are different types of waves possible and the phase of the matter matters.