Sound travels through a medium as a mechanical wave. It cannot travel in a vacuum because it travels by vibrating the particles of the medium to transfer energy from one place to another.
There are two types of waves based on their method of transferring energy. Electromagnetic waves do not require a medium and can transfer energy through a vacuum. Mechanical waves, such as sound, require a medium to transfer their energy. A medium is any substance that has molecules. Air is the most common medium in which sound travels, although it can also travel through liquids, such as water, and solids, such as steel. The source of the sound sets off a chain of events by vibrating the particle of the medium that is closest to it. This first particle that starts vibrating because of the energy it received from the source transfers its energy to the next closest particle, thus causing a second particle to vibrate. The second particle then transfers its energy to its closest neighbor. This process of energy transfer allows sound to travel from its source to the observer. Sound travels fastest in solids, since the particles are packed tightly together, allowing for a more efficient transfer of energy.