How do we hear?

Your ears pick up sound which travels in invisible waves through the air. Sound occurs when a moving or vibrating object causes the air around it to move.

Sound waves travel down the ear canal and hit the eardrum in the middle ear. This causes the eardrum to vibrate. Three tiny bones in your middle ear link the vibrating eardrum to the cochlea in the inner ear.

The cochlea is filled with liquid that carries the vibrations to thousands of tiny hair cells sitting on a membrane that stretches the length of the cochlea. The hair cells on the membrane fire off tiny electrical signals. These electrical signals travel up the cochlea nerves of the auditory pathway to the brain. All this happens in a fraction of a second.

The parts of your ear

Your ear is made up of a conductive pathway which includes the outer and middle ear and the neural nerve pathway that includes the inner ear and auditory nerve.

The outer ear

The outer ear consists of the:

• external flap of skin (pinna) and cartilage
• ear canal that leads down to the eardrum.

The pinna is the external flap of skin that helps you know the direction of sound. It serves to collect or funnel sounds into your ear canal yet it is not very important for good hearing.

The ear canal varies in size and shape from person to person. It runs nearly horizontally toward the centre of the head for about 2.5cm (in adults) and ends at the eardrum.

The skin along the outer part of the canal has tiny hairs and produces a waxy substance called cerumen. This earwax discourages foreign objects from entering the ear, and keeps the skin of the canal from drying out.

The middle ear

The middle ear consists of the:

• eardrum
• air-filled cavity that includes three middle ear bones
• oval and round window membranes
• eustachian tube.

The eardrum and middle ear bones

The cone-shaped eardrum is stretched across the ear canal and is quite stiff, yet flexible. Behind the eardrum three bones are connected to form the ossicular (pronounced oss-ick-you-lar) chain. They are the:

• Hammer (malleus)
• Anvil (incus)
• Stirrup (stapes).

The hammer is connected to the eardrum on one end and through the anvil to the stirrup at the other end. The stirrup is the smallest bone in the body, smaller than a grain of rice. It rests against the oval window membrane leading into the inner ear.

The stirrup moves in and out of the oval window membrane like a piston as the drum moves in response to sound.

The round window membrane is located just below the oval window and is flexible. When the stirrup moves in and out it pushes the fluid in the cochlea and the round window allows the fluid to be displaced.

The middle-ear cavity, filled with air, is connected to the back of the nose and throat by the eustachian tube. This tube adjusts the air pressure in the middle-ear space to match the air pressure on the outside of the eardrum and is normally closed. In a plane when you take off or land, it helps to yawn or swallow because these actions usually open the eustachian tube to adjust the air pressure in the middle ear space.

The inner ear

The inner ear is made up of the:

• Cochlea
• Semicircular canals.

The cochlea is a tiny spiral-shaped structure, about the size of a pea. It is nestled in the bone of the skull and filled with fluid. A thin membrane with around 15,000 microscopic hair cells sits in this fluid. Each cell is tuned to a particular sound or frequency.

The tiny hair cells connect to the cochlea nerve that sends messages to the brain.

The semicircular canals are mainly responsible for the sense of balance.