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.
