Falling trees in secluded areas and stuff like that... Sound.

As in the sound we hear. Not the common slang expression to express your acceptance.

Sound waves are longitudinal (waves in which particles oscillate along lines in the direction in which the wave travels,) waves produced by variations in air pressure. The wave is a mechanical wave, as it requires a medium in which to occur. Thus, in a vacuum where there is nothing, sound can not travel.

Sound is made by a vibrating source which moves the molecules in air, creating areas of compression and rarefaction. When a molecule moves, it collides with the next one and thus also makes it move. The energy of a sound wave travels away from the source through the series of collisions parallel to the direction of the wave.

Rarefaction is the reduction of a medium's density, or the opposite of compression. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.

Sound travels faster through denser materials because a higher density leads to more elasticity in the medium and increases the ease by which compression and rarefaction can take place.