Material density and sound transmission
Higher density materials will transmit less sound.
Sound is transmitted faster through solids than through liquids and gas. This is because sound is actually kinetic energy that is being transmitted from one molecule to another. This transfer of kinetic energy is easier if the molecules are closer to one another like in solids or liquids than in gasses. The transmission of sound also depends on the elasticity and density of the medium.
Elasticity of materials is measured when the medium can still maintain its original form and shape when a force is applied to it and then removed. A high elastic medium like steel will be able to transmit sound faster than a low elastic medium like rubber.
Another factor affecting the transmission of sound is density. High density materials will normally have bigger and heavier molecules. Since bigger molecules have a heavier mass, they will require more kinetic energy to vibrate and transmit the sound. This will make the sound transmission slower and will also dampen the sound.