Audio signals also help us interact with our environment. Some of these signals are designed: We wake to the buzz of the alarm clock, answer the ringing telephone, and race to the kitchen when the shrill beep of the smoke alarm warns us that dinner is burning on the stove. Other audio signals are not deliberately designed, but help us nonetheless. For instance, we may know the proper sound of the central air conditioning starting, the gentle hum of the PC fan, or the noise of the refrigerator. So, when these systems go awry, we notice it immediately--something doesn't sound right. Likewise, an excellent mechanic might be able to tell what is wrong with a car engine just by listening to it run.