MP Board · Class 9 · Science · Sound(a) Describe the structure and functioning of the human ear by explaining its three main divisions: Outer ear, Middle ear, and Inner ear. (b) Explain step-by-step how sound pressure waves are converted into electrical signals by the human ear and transmitted to the brain.

(a) Structure and Functioning of the Human Ear

\nThe human ear is a sensitive sense organ that enables us to hear. It is divided into three main parts:

1. Outer Ear (Pinna and Ear Canal):

   • Pinna: The outer visible part of the ear collects sound waves from the surroundings.
   • Auditory Canal: The collected sound passes through a tube-like structure called the auditory canal.
   • Eardrum (Tympanic Membrane): At the end of the canal is a thin, stretched membrane called the eardrum. When a compression of a sound wave reaches the eardrum, the pressure outside increases and pushes the eardrum inward. When a rarefaction reaches, the pressure decreases and pushes the eardrum outward, causing it to vibrate.

2. Middle Ear (Ossicles):

   • The middle ear consists of three tiny interconnected bones called Ossicles: the Malleus (Hammer), Incus (Anvil), and Stapes (Stirrup).
   • These bones receive vibrations from the eardrum and amplify them several times through lever action before passing them to the inner ear.

3. Inner Ear (Cochlea and Auditory Nerve):

   • Cochlea: A coiled, fluid-filled tube that receives the amplified mechanical vibrations from the middle ear.
   • Auditory Nerve: Connects the cochlea to the brain to transport the processed signals.

(b) Step-by-Step Working Mechanism

1. Collection: The pinna gathers sound waves and directs them into the ear canal.
2. Vibration: Sound waves strike the eardrum, setting it into vibration corresponding to the frequency of the sound.
3. Amplification: The three bones in the middle ear (hammer, anvil, and stirrup) amplify these vibrations efficiently.
4. Signal Conversion: The amplified vibrations reach the oval window of the cochlea in the inner ear. The fluid inside the cochlea starts vibrating, which bends microscopic hair cells inside the cochlea. These hair cells convert the mechanical fluid movement into electrical nerve impulses.
5. Transmission: The auditory nerve carries these electrical impulses to the auditory cortex of the brain, which interprets them as recognizable sounds.