This boy is cupping his hands behind his ears in order to hear better. Do you think this will help? To answer this question, you need to know more about sound, the ear, and how we hear.
The Sounds We Hear
Sound is a form of energy that travels in waves through matter. The ability to sense sound energy and perceive sound is called hearing. The organ that we use to sense sound energy is the ear. Almost all the structures in the ear are needed for this purpose. Together, they gather sound waves, amplify the waves, and change their kinetic energy to electrical signals. The electrical signals travel to the brain, which interprets them as the sounds we hear.
The Figure below shows the three main parts of the ear: the outer, middle, and inner ear. It also shows the specific structures in each part of the ear. The roles of these structures in hearing are described below and also in these animations: http://www.medindia.net/animation/ear_anatomy.asp and http://www.youtube.com/watch?v=tkPj4IGbmQQ
The outer ear includes the pinna, ear canal, and eardrum.
- The pinna is the only part of the ear that extends outward from the head. Its position and shape make it good at catching sound waves and funneling them into the ear canal.
- The ear canal is a tube that carries sound waves into the ear. The sound waves travel through the air inside the ear canal to the eardrum.
- The eardrum is like the head of a drum. It is a thin membrane stretched tight across the end of the ear canal. The eardrum vibrates when sound waves strike it, and it sends the vibrations on to the middle ear.
Q: How might cupping his hands behind his ears help the boy pictured above hear better?
A: His hands might help the pinna of his ears catch sound waves and direct them into the ear canal.
The middle ear contains three tiny bones (ossicles) called the hammer, anvil, and stirrup. If you look at these bones in the Figure below, you might notice that they resemble the objects for which they are named. The three bones transmit vibrations from the eardrum to the inner ear. The arrangement of the three bones allows them to work together as a lever that increases the amplitude of the waves as they pass to the inner ear.
Q: Wave amplitude is the maximum distance particles of matter move when a wave passes through them. Why would amplifying the sound waves as they pass through the middle ear improve hearing?
A: Amplified sound waves have more energy. This increases the intensity and loudness of the sounds, so they are easier to hear.
The stirrup in the middle ear passes the amplified sound waves to the inner ear through the oval window. When the oval window vibrates, it causes the cochlea to vibrate as well. The cochlea is a shell-like structure that is full of fluid and lined with nerve cells called hair cells. Each hair cell has many tiny “hairs,” as you can see in the magnified image below. When the cochlea vibrates, it causes waves in the fluid inside. The waves bend the “hairs” on the hair cells, and this triggers electrical impulses. The electrical impulses travel to the brain through nerves. Only after the nerve impulses reach the brain do we hear the sound.
- Sound is a form of energy that travels in waves through matter. The ear gathers and amplifies sound waves and changes them to electrical signals. The brain receives the signals and interprets them as the sounds we hear.
- The outer ear includes the pinna, ear canal, and eardrum. These structures gather sound waves, funnel them into the ear, and pass the vibrations to the middle ear.
- The middle ear contains three tiny bones that amplify the vibrations as they transmit them to the inner ear.
- In the inner ear, the vibrations are changed to electrical signals by hair cells lining the cochlea. The electrical signals then travel to the brain.
hearing: Ability to sense sound energy and perceive sound.
sound: Transfer of energy from a vibrating object in longitudinal waves that travel through matter.
At the following URL, review the information about the ear and hearing. Be sure to watch the hearing animation. Then fill in the blanks in the statements below. http://www.bbc.co.uk/science/humanbody/body/factfiles/hearing/hearing_animation.shtml
- The outer ear canal ends at the __________.
- The __________ is the cavity between the eardrum and inner ear.
- The snail-like __________ is filled with fluid.
- The spiral organ of __________ consists of hair cells.
- Nerve impulses travel along the cochlear nerve to the __________.
- Because we have two ears, we are able to locate the __________ of sounds.
- Summarize how we hear sounds.
- Identify the structures of the outer ear and state their functions.
- The three tiny bones of the middle ear work together as a lever. A lever is a simple machine that may increase the force applied to it. How does this relate to the function of the middle ear?
- Loud sounds can damage the hair cells lining the cochlea of the inner ear. Explain how this might affect the ability to hear sound.