Vibration of an object gives rise to sound. Do we hear the sound of all vibrating objects ? No , we do not hear the sound of all vibrating objects. We humans do not hear the sound that is created with less than 20 vibrations per second. Such words are inaudible. Such words are called audible words. Again, we do not hear the sound caused by too much vibration. No more than 20,000 vibrations per second. Such words are also inaudible. This is called a sound-answer word. So the limit of audibility for human beings is the sound created by 20 to 20,000 vibrations . The amount of vibration that an object makes per second is called the frequency of that object. The unit of expression of this frequency is hertz. If an object vibrates 20 times per second , its frequency is 20 Hz . So the range of the audible frequency in the human ear is 20 to 20,000 Hz . The sound of this frequency is called audible sound.