Sound is a form of mechanical energy. Mechanical forces can be steady, like the weight of this journal in your hand, or they can vibrate, like your car when it goes over speed bumps. This sets up small oscillations of air molecules that in turn cause adjacent molecules to oscillate as the sound propagates away from its source. Since a pressure wave consists of a molecular disturbance, sound waves cannot travel through a vacuum. Sound waves travel fastest in solids, slower in liquids and slowest in air.
Sound vibrations extend from a few cycles per second to millions of cycles per second.
Human hearing is limited to a range of between 20 to 20,000 cycles per second. Sound at a vibration rate of greater than 20,000 cycles per second is called ultrasound. Other mammals can hear ultrasound, some such as whales approach 100,000 cycles per second. A sound is characterized by its frequency and intensity. The frequency of a sound contributes to its pitch and is measured by counting the number of cycles per second in the vibration.