What will change the velocity of a periodic wave?

Changing the medium of the wave.

Explanation:

Hi, a Periodic wave is a wave with a repeating continuous pattern or cycle that the source produces.

They are determined by the properties of the medium. So, if we change the medium of the wave It´s velocity will change.

The mediums are distinguished by their properties - the material they are made of and the physical properties of that material such as the density, the temperature, the elasticity, etc.  

Feel free to ask for more if needed or if you did not understand something.

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