Waves..

We hear daily many types of sound. It may be the sound of a human or an animal or a vehicle. The ordinary sound we hear is the result of the sound waves traveling in air around us. These waves are produced by the vibrations of an object. Every vibration in a thing necessarily create sound waves in the air. But we cannot listen to the sound of the frequency is 16 vibration per second. Beyond a limit that the vibrations itself more or less of which could not create vibrations in the air. The limit is called hearing limit. It ends at the frequens of 20,000 vibrations per second. That is, we cannot hear sound waves of higher frequencies.

  Sound waves having frequency less than 16 vibrations in infrasonic and more than 20,000 are called ultrasonic.