Sound is produced when an object vibrates. Due to vibrations of the object, the molecules (or particles) of air surrounding the object also start vibrating with the same frequency. The molecules vibrate along the direction of motion of the object. The motion is communicated by each molecule to the next and so on. Soon all the air molecules in the vicinity start vibrating. The motion of air molecules near the ear affect that organ and give the sensation of sound. In other words, our ear feels the vibrations of air molecules as sound.

Transmission of sound by the ear to the brain: the outer ear is shaped to collect vibrations of air particles and lead them down the ear canal to the eardrum. The sound vibrations striking this thin membrane set up vibrations. As the eardrum vibrates, three little hinged bone in the middle ear pass the vibration to the inner ear.

The inner ear is filled with liquid inn which are located many end-fibres of the auditory nerve. As the sound vibrations spread through the liquid, they vibrate these nerve endings. The auditory nerve picks up the vibrations and sends electric signals to the auditory centre of the brain, which produces the sensation we recognize as sound.

Ultrasonic and Infrasonic Sounds:

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Our ears respond to vibrations having frequency between 20 Hz and 20,000 Hz. We cannot hear sounds with frequency less than 20 Hz and greater than 20,000 Hz. Sounds of frequency greater than 20,000 Hz are called ultrasonic (ultra means greater and sonic means sound). Sounds of frequency smaller than 20 Hz are called infrasonic (infra means smaller). It does not mean that human voice can produce sounds with a frequency between 20 Hz and 20,000 Hz.

Frequency of human voice lies between 60 Hz and 13,000 Hz. Low frequency sounds, from 20 to 200 Hz, produce deep bass (base) sound. High frequency sounds, above 8000 Hz, produce high-pitched or treble sound.