Very simply, sound is the vibration of any substance. The substance can be air, water, wood, or any other material, and in fact the only place in which
sound cannot travel is a vacuum. When these substances vibrate, or rapidly move
back and forth, they produce sound. As described in the How We Perceive Sound:
The Ear section, our ears gather these vibrations and allow us to interpret
them. To be a little more accurate in our definition of sound, however, we must
realize that the vibrations that produce sound are not the result of an entire
volume moving back and forth at once. If that were the case, the entire
atmosphere would need to shift for any sound to be made at all! Instead, the
vibrations occur among the individual molecules of the substance, and the
vibrations move through the substance in sound waves. As sound waves travel
through the material, each molecule hits another and returns to its original
position. The result is that regions of the medium become alternately more
dense, when they are called condensations, and less dense, when they are called
rarefactions.