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Hard Materials
A hard, relatively nonporous material, such as concrete, is as dissimilar as possible to the medium the sound waves move through -- the air. That dissimilarity keeps the sound wave -- which is traveling on or through the air -- from seeping through the material's surface. Since the sound wave can't seep through the surface, it doesn't get absorbed within it -- it bounces off, and is reflected back as an echo.
Examples of Hard Materials
In a natural environment, materials hard enough to bounce sound waves off of might include rock (such as a mountain or a cave) or ice (such as a glacier or frozen-over lake). In a man-made environment, similarly hard materials include concrete buildings or the asphalt of the sidewalk or street.
Smooth Materials
When a sound wave, which is made up of kinetic energy, collides with a surface, it will release its kinetic energy as heat at the site of that collision. If it collides with a gritty surface with many hills and valleys of molecules, it will collide against a greater number of molecules, and more of its energy will be converted to heat. Smooth materials, on the other hand, allow relatively few opportunities for small, individual collisions between the surface's molecules and the sound wave -- and therefore less of the sound wave's energy is converted to heat, and more of it bounces back off of the surface as an echo.
Examples of Smooth Materials
In a man-made environment, materials that are smooth enough to produce an echo include glass, metal and concrete buildings, the brick and metal interior of a warehouse, marble buildings and floors, or, if the room is shaped in a way that the acoustics produce an echo, plastered and painted interior walls. In a natural environment, materials that are smooth enough to produce an echo include a muddy river or lake bottom, limestone cave walls that have been dug and smoothed by the sea, or the interior of a cave that has been smoothed by flowing groundwater.