It all depends on the desired properties. Some common materials used for this purpose include ceramics, metals and polymers like polyvinyl chloride (PVC). 

Materials that can slow down sound include those with high acoustic impedance (basically resistance to the flow of sound waves), such as stiff foam, elastomers, and dense metal foams. To speed up sound, materials with low impedance, such as light-weight metal foams and porous ceramics can be used.
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