Sound is a longitudinal wave and requires a material medium. Sound disturbances can make solids vibrate, including the ear drum; these processes operate over limited frequency ranges. Normal human hearing is approximately 20 Hz to 20 kHz; higher frequency gives higher pitch and greater amplitude generally gives louder sound. Example: State the normal human hearing range. About 20 Hz to 20 kHz. A complete explanation links the relevant physical cause to its observable effect. Justify your answer.