Acoustic impedance
Acoustic impedance describes how much a material resists being set into motion by a sound or vibration wave, equal to its density times its sound speed. When a wave hits a boundary between two materials, the difference in their impedances decides how much of the wave reflects and how much passes through. Similar impedances let energy pass, and very different impedances reflect most of it.
Example: Ultrasound technicians spread gel on the skin because air reflects nearly all the sound, while the gel lets it pass into the body.