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With an increasing temperature, the speed of sound increases until it reaches a maximum value at a certain temperature.Above this temperature the speed decreases . Why does the speed decrease above this temperature?


Temperature indicates the random bouncing around of the molecules in a material. As temperature increases, so does the amount of motion: the molecules crash together more often, and in less organized patterns. This is how heat energy moves through a material. The faster molecules give energy to the slower molecules. Sound is an organized pattern of motion. As the molecules move back and forth together, they transfer the pattern through the material.

When a material is very cold, the molecules do not move very fast. Even the energy of the sound wave cannot speed them up much. As the temperature rises, the molecules speed up. This transfers the sound pattern through the material more quickly. When a material is very hot, the molecules already have so much energy that the sound energy makes very little difference. The disorganization of the motion increases the time needed for the sound pattern to transfer from molecule to molecule. This then slows the speed of sound.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College

You did not specify whether you are referring to the state of the substance -- solid, liquid, or gas -- nor the temperature range. Without that info it is not possible to "explain" the observation.

Vince Calder

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