We have seen various ways of producing sound. We know that when a string or any other structure vibrates, it may develop standing waves. These are waves that occupy a “fixed” position in any structure, whether it is a string, a plate, a container filled with a liquid, or a blood vessel. Nodes are the spots in which minimum motion occurs. When sets of waves are superposed, interference patterns result. A hologram is an interference pattern of light waves on a photographic plate. When two differing frequencies are superposed, beat frequencies result. Coherency is an in-step or in-phase behavior
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