HPP Activity 48v3
What frequencies does the larynx produce?
Exploration
Open up the DataStudio file with the microphone setup: SoundBasic.ds.
Make the oscilloscope view active.
Press “Start” and hum an “ahhh” sound into the microphone.
Change the “time/div” setting until you can get several cycles of the signal to display.
GE 1.
1. Do you think the sound wave you produce with your humming has a
definite frequency associated with it? What evidence do you have for your
opinion?
2. Now display the signal generator output on the oscilloscope. You may have
to change the time/div setting to get several cycles to display. The signal
generator should be producing a pure sine wave. How does the sine wave
shape compare with the humming sound wave shape?
A mathematical procedure called the Fourier transform can give information about the
frequencies present in a signal. You can get the computer to perform a Fourier transform on your
data by setting up a “Fast Fourier Transform”, or FFT, display.
Close the oscilloscope display.
Create an FFT display of the microphone voltage.
Set the sample rate to 10,000 samples/s on the FFT display.
Click the Start button.
Start humming.
When you have a stable signal click the Stop button.
GE 2.
1. What frequencies are strong in your "ahhh" sound?
2. Obtain a Fast Fourier Transform of the sine wave signal. What frequencies
are present in this signal?
Activity Guide
 2010 The Humanized Physics Project
Supported in part by NSF-CCLI Program under grants DUE #00-88712 and DUE #00-88780
HPP Activity 48v3
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3. Start humming the "ahhh" sound again and form the FFT of this signal.
Without changing the pitch of your voice, change the sound from "ahhh" to
"eee". What happens to the frequencies shown in the FFT?
4. Perform an FFT analysis of the sound produced by the larynx model. Does
it produce one or multiple frequencies?
5. What might you do to the larynx model to change the intensities of the
frequencies it produces without changing the actual frequencies? Would
changing the tension of the latex strips work? Can you think of anything else?
6. Using available materials, try your ideas. Describe the results here.
We have two problems to study related to the experiments you just did. The first problem is to
understand why the vocal folds produced a series of almost equally spaced frequencies. The
second problem is to understand why many of the produced frrequencies disappear or have
reduced intensities once the sound wave interacts with the oral/nasal cavity.
The two problems are related and can be at least partially understood in terms of a concept called
standing waves. We will develop this idea now.
Obtain a slinky and "fix" one of the ends so it cannot move. You can do this by either placing it
in a clamp or having someone hold it. Stretch your whole Slinky to its full length so it hangs
freely in the air, fixed at one end and in your hand at the other end.
Shake the non-fixed end of the slinky to create a standing wave. Practice until you are able to
produce standing waves with two nodes (at the two ends), three nodes (two at the ends and one in
the middle), and four nodes (two at the ends and two between the ends). There are some sample
pictures below to guide your work.
Nodes at the ends
Nodes in the middle & at the ends
GE 3.
1. Determine the frequency of the side to side (or up and down) oscillations
necessary to create each of the following three standing waves:
Activity Guide
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HPP Activity 48v3
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a. 2 nodes (one at each end); find frequency = f0
b. 3 nodes (at the ends and one in the middle); find frequency = f1
c. 4 nodes (at the ends and two between the ends); find frequency = f2
Sketch the relevant standing wave patterns and record your frequency values.
2. Write the mathematical relationship between the frequencies you found
above. What would you expect for the frequency of fn ?
3. Stretch half of your Slinky about the same length as the whole Slinky
above. What is now different from the activity you did above? How would you
expect that to change the behavior of the system? Explain.
4. After having stretched your half Slinky to the appropriate length and
keeping one end fixed, determine and record the frequency of the oscillations
necessary to create a standing wave with:
a. 2 nodes (one at each end); find frequency = f0
b. 3 nodes (at the ends and one in the middle); find frequency = f1
5. Write the mathematical relationship between the frequencies you found in
question #4. What would you expect for the frequency of fn ?
6. Compare your answers to questions 2 and 5.
Invention
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 48v3
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Let's try to find a relationship between the length of Slinky L and the allowed wavelengths.
GE 4.
1. Given that the endpoints of the Slinky must remain still (nodes), why is it
not possible to place a wave with any arbitrary wavelength on the string?
2. What is the simplest standing wave pattern that could be fit on the Slinky?
3. What is the relationship between the Slinky length L and the wavelength 
for this case?
4. What is the next simplest standing wave pattern that can be fit on the
Slinky?
5. What is the relationship between the Slinky length L and the wavelength 
in this case?
6. Try to develop a general formula for the allowed wavelengths for standing
waves on the Slinky with both ends fixed. Check your result with the
instructor.
7. Use your result from question 6 to write down a formula relating the
allowed frequences for a standing wave and the length for a Slinky with both
ends fixed.
Application
GE 5.
Let’s try to relate standing waves on the Slinky and string to the sound
frequencies produced by the vocal folds.
1. Do you think the vocal folds can vibrate with any frequency? Explain your
reasoning or evidence.
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 48v3
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2. In what ways might the vocal folds be like the Slinky used in the previous
experiment?
3. Develop a hypothesis to explain the frequency pattern observed when you
made the “ahhh” sound.
Application
GE 7. Standing Sound Waves and The Speed of Sound.
Obtain a
long glass tube with water reservoir
tuning fork
rubber mallet
Get the instructor to show you how to set up a standing wave in an air column.
1. Do you think that the open end of the tube is a node or an antinode?
2. Do you think that the water surface is a nod or an antinode?
3. The standing sound wave can be represented as a pressure versus position
graph. Draw the simplest possible standing wave pattern that could fit in the
air column.
4. What is the relationship between wavelength and tube length L for this case.
5. What is the next simplest standing wave pattern that can fit it the tube?
6. What is the relationship between wavelength and tube length L for this
case?
Activity Guide
 2010 The Humanized Physics Project
HPP Activity 48v3
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7. Develop a general formula for the the relationship between wavelength and
tube length for standing waves with one end fixed and one end free. Check
you result with the instructor.
8. Use the result from question 7 to find a formula relating allowed
frequencies and the length of the tube.
9. Devise an experiment for measuring the speed of sound in air using
standing wave observations. Describe your plan here.
10. Record your data.
11. Analyse your data. What is your estimate of the speed of sound in air?
Activity Guide
 2010 The Humanized Physics Project