Closer Look at the Cochlea
enter and travel through the cochlea as vibrations in the
fluid within it. The center of the cochlea's coiled chamber
is not hollow, but instead contains a long filament that extends
much of the chamber's length.
one side of the filament is the basilar membrane. This membrane
contains a series of fibers of varying lengths. These lengths
are tuned to different sound frequencies. The longest of these
fibers responds to low pitches and is located near the tip
of the filament. The shorter fibers respond to high pitches
and are located near the point where the filament meets the
the tuned filaments vibrate, they stimulate nearby nerve cells.
These cells produce impulses that are transmitted along the
auditory nerve to the brain.
students that sound is a vibration that is transferred by
matter. In order to make a sound, you need to coax matter
into vibrating. When you blow into a soda container, you create
vibrations in the enclosed air space. This semi-trapped air
begins to vibrate. As it vibrates, it produces waves that
spread into the surrounding air. We detect these waves as
the distinct tone of the soda-container whistle.
you add liquid to the container, the vibrating space gets
smaller. Smaller spaces produce short waves. Short waves have
high frequencies. That's why the pitch, or tone, goes up as
you add water to your whistle-blowing container.
a different sound-making process at work when we strike a
glass container with a spoon. This time, the vibration originates
in the solid matter of the container itself. The glass vibrates
and transfers this movement to the surrounding air. Again,
we detect the vibrations as sound.
When water is added to the container, it reacts as if it were
part of the glass. When the glass is struck, both the solid
container and the liquid water vibrate. The pitch of the sound
reflects this additional matter.
you add more water to the container, it takes on a greater
vibrating mass. Like the thicker strings of a bass, the larger
mass produces sounds of lower frequency. That's why the pitch
goes down when you add water to a glass that you are striking
with a spoon.
In this activity, you'll observe how structures can be "tuned"
to respond to different frequencies.
activity page will offer:
an overview of cochlear implants
activity in sound making and sound transfer
opportunity to "tune" a vibrating receiver
- Six 1-liter plastic bottles
- Work with a partner. Clean and dry all plastic bottles.
- Blow into the neck of a 1-liter bottle to produce a sustained
- What will happen to this note if water is added to the
bottle? Make a prediction and then find out by filling the
bottle one-third full with tap water. When you blow into
the bottle, is the note higher or lower than when the bottle
- Add more water and observe the differences in the sound
- What happens to the sound quality as more water is added
to the bottle?
- What happens to the amount of air within the bottle space
as water is added to the container?
- How does the mass of a vibrating object affect its pitch?
- Why does adding water to the bottle affect the pitch?
2 - Tuned Whistles
- Obtain three 1-liter bottles. Fill a 1-liter bottle 1/3
full with water. Fill a second bottle 2/3 full of water.
Position these two water-filled bottles along with an empty
bottle side-by-side on a desk.
- Select a fourth 1-liter bottle as the "master" tone maker.
- Have a partner put his/her ear close to the mouths of
the three stationary bottles. From a distance of a few feet,
blow into the master bottle to produce a steady tone. Then
- Have your partner note in which stationary bottle the
sound continues to ring loudest. 5. Fill the "master" 1/3
full of water. Repeat step 3. Note in which bottle the sound
continues to ring loudest. 6. Repeat step 3 using a master
bottle that is 2/3 full of water. Note in which bottle the
sound continues to ring loudest.
- In what form was energy transferred from the master tone
maker to the other containers?
- Which container continues to ring after the empty master
- Which container continues to ring after the 1/3-full container
- What relationship can you infer from these observations?
It All Together
Review how the cochlea separates sounds according to frequency.
Then, have students discuss the concept of a tuned "reed."
Apply this understanding to the transferring of whistle tones
between bottles. You might wish to present the following sequence
of scripted questions:
- What causes the central filament within the cochlea to
- How is the basilar membrane adapted to distinguish pitches?
- What caused the target container to produce the sound
of the master tone maker?
- Why didn't all of the containers respond to the master
- How is the tuning of the basilar membrane and the water-filled
a piano or portable keyboard as a guide for creating a set
of plastic bottles that will correspond to the notes of the
musical scale. Work in a group, using the set of plastic bottles
to produce a musical composition that includes all of the
When you gently tap on a glass with a metal utensil, you also
produce a musical note. Think about it. As you add water to
this glass, the sounded note drops in pitch. Can you explain
the Ear's Energy Transfer
Sound energy moves through the surrounding air as compression
waves. When these waves strike the eardrum, the wave energy
is changed into vibratory motion in the drum. This vibration
is passed mechanically through the middle ear bones. This
mechanical energy is then transferred to the liquid within
the inner ear. Nerve cells that line the cochlea detect these
movements and generate an electric impulse. This impulse moves
through the neuron, eventually reaching the tip of the cell's
axon. Here, the electric impulse is turned into a chemical
signal. The chemical signal is released into the gap between
neighboring cells. A chemical change stimulates the next neuron.
Once again the impulse becomes electrical. It continues to
flip-flop between an electrical and chemical signal as it
races to the brain.
this transfer sequence with students. Then, challenge teams
to construct a large mural that depicts these transfers. If
applicable, supply them with materials and have students assemble
an alternate pathway (from sound to brain) using Rube Goldberg-type
Build a model of the ear. Use print and electronic resources
to uncover information about ear anatomy. If practical, include
working parts such as a vibrating eardrum and moving middle
Virtual Tour of the Ear
Introduction to Cochlear Implants
Fact Sheet on Cochlear Implants
more Web links on this topic - visit our Resources
activities in this guide were contributed by Michael DiSpezio,
a Massachusetts-based science writer and author of "Critical
Thinking Puzzles" and "Awesome Experiments in Light & Sound"
(Sterling Publishing Co., NY).
Academic Advisors for this Guide:
Corrine Lowen, Science Department, Wayland Public Schools,
Suzanne Panico, Science Teacher Mentor, Cambridge Public Schools,
Anne E. Jones, Science Department, Wayland Middle School,