What term describes a decrease in the density of air molecules?

The term that describes a decrease in the density of air molecules is rarefaction. In a wave, specifically a sound wave, molecules in a medium (like air) move in response to oscillations. When a sound wave travels, it creates regions where air molecules are pushed closer together (compression) and regions where they are spread further apart. The latter of these regions, where the density of air molecules decreases, is known as rarefaction. This concept is essential in understanding how sound waves propagate, as these fluctuations in air pressure create the audible sounds we experience.

The other terms have distinct meanings: compression refers to the increase in density where air molecules are closer together; resonance involves the amplification of sound waves at specific frequencies; and refraction describes the bending of waves when they pass from one medium to another. Understanding these concepts helps in grasping the nature of sound and its transmission through different environments.