What Are You Talking About?

Human hearing spans a pretty wide range of sound frequencies. At the low end is 20 herts, that's 20 full vibrations per second. As we get older, our ability to perceive the highest pitches diminishes. If you want to keep those punk kids away from your convenience store, you can play a piercing, high pitched sound over the speakers and only the kids will be able to hear it. All adults will have lost that ability.