A word about Sound and how it relates to coffee roasting: Sound is a good indicator of bean temperature during roasting. There are two temperature thresholds called “cracks” that roasters listen for. At about 200–202 °C (392–396 °F), beans will emit a cracking sound much like popcorn does when it pops, only much quieter.
This point is called “first crack,” marking the beginning of light roasts. When the beans are at about 224–226 °C (435–439 °F), they emit a “second crack.” During first and second “crack” pressure inside the bean has increased to the point where the structure of the bean fractures, rapidly releasing gases, thus an audible sound is emitted.