Part 1-Sounding Off

QUESTIONS

1. What happens to the sound quality as more water is added to the bottle?
(The pitch goes up.)
2. What happens to the amount of air within the bottle space as water is added to the container?
(It decreases because the air is forced out of the bottle.)
3. How does the mass of a vibrating object affect its pitch?
(The greater the mass, the lower the pitch.)
4. Why does adding water to the bottle affect the pitch?
(As water is added, the pitch goes up because the size of the vibrating air mass has decreased.)

Part 2 - Tuned Whistles

QUESTIONS

1. In what form was energy transferred from the master tone maker to the other containers?
(The energy was transferred as sound waves that traveled through the surrounding air.)
2. Which container continues to ring after the empty master is sounded?
(The empty test container.)
3. Which container continues to ring after the 1/3-full container is sounded?
(The 1/3-full container.)
4. What relationship can you infer from these observations?
(Containers will vibrate "in-sync" if they contain the same amount of water as the master tone maker.)

Bringing 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:

1. What causes the central filament within the cochlea to vibrate?
(Vibrations within the surrounding fluid.)
2. How is the basilar membrane adapted to distinguish pitches?
(It has a series of fibers with varying lengths. Each length responses to a specific frequency.)
3. What caused the target container to produce the sound of the master tone maker?
(Vibrations traveled through the air and struck the container causing it to vibrate.)
4. Why didn't all of the containers respond to the master sound tone?
(Only the containers that were "tuned" to that frequency responded.)
5. How is the tuning of the basilar membrane and the water-filled containers similar?
(Objects or regions will vibrate if they are "in tune" to the surrounding waves. If they are not in tune, they won't respond.)

EXTENSIONS

Critical Thought
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 this result?
(In this case it is the container, not the volume of trapped air that is vibrating. As water is added to the glass, it becomes part of the "container." When a greater mass vibrates, it produces a tone of lower pitch.)

The 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).