Wolfram Demonstrations Project: "Waves on a String with a Mass in the Middle"

To use this demonstration, you must download and install the Mathematica Viewer from the Wolfram Demonstrations Project. Waves are partially reflected by local changes in the medium through which they propagate. This is illustrated here by the introduction of a point mass on a vibrating string. The transmitted wave becomes smaller in amplitude as the mass becomes larger. Why? Is there a phase shift associated with reflection/transmission? Why?