Why do empty vessels make sounder?

When an empty metal vessel is struck, the entire surface of the vessel is set into vibration. The air in contact with the vessel is also set into vibration producing sound of high intensity. If the vessel is partly filled, the surface area of the vessel remaining free to vibrate becomes greatly reduced. Since the total area of air that is set into vibration by the vibrating surface of the vessel also becomes much smaller than in an empty vessel, the intensity of sound also gets reduced.