Tones, Vowels and Telephones

In this experiment, you will analyze various common sounds. You will use a Microphone connected to a computer. Logger Pro will display the waveform of each sound, and will perform a Fast Fourier Transform (or FFT) of the waveform. The FFT tells you the amplitudes and frequencies of a collection of sine waves that, when added together, would sound the same as the original waveform.

In the first part of the experiment, you will study the sound of a tuning fork which produces a tone composed mainly of a single frequency. Next, you will observe the production of overtones on a tuning fork. Overtones whose frequencies are multiples of the fundamental are called harmonic; other overtones are called inharmonic. You will also analyze the sound produced when you say two vowels. An FFT graph will reveal that your voice is composed of a large number of individual frequencies.

In the last part of the experiment, you will be given a problem to solve. How does the telephone company know what numbers you dial? Do those tones that you hear when you press the buttons on the telephone contain some kind of code? You can solve this problem and crack the code using the Microphone, the FFT analysis, and an inexpensive tone dialer.


·   Use a Microphone to analyze the frequency components of a tuning fork and your voice.

·   Record overtones produced with a tuning fork.

·   Examine how a touch-tone phone works.



Power Macintosh or Windows PC

Vernier Microphone

LabPro or Universal Lab Interface

2 tuning forks (one ~256 Hz)

Logger Pro

tone dialer


Preliminary questions

1.   Strike one of the tuning forks with a rubber mallet or the bottom of a rubber-soled shoe. Do not hit the tuning fork on a hard surface; doing so may damage the tuning fork. Listen to the sound. Now press a button on the tone dialer. Which sound do you think is more complex? Explain your answer.

2.   Compared to the tuning fork and the tone dialer, is the sound of a person humming a simple or a complex sound?

3.   Try pressing the buttons on the telephone dialer. Can you observe any order to the tones? Does the pitch get higher or lower for larger numbers, or does it seem to be random?

4.   How do you think the phone company recognizes the numbers that you dial?


Part I  Pure Tone

1.   Connect the Vernier Microphone to Channel 1 of the LabPro or Universal Lab Interface.

2.   Open the file in the Experiment 22 folder of Physics with Computers. The display will include both a graph and an FFT window. The horizontal axis of the graph has time scaled from 0 to 0.05 s and the vertical axis corresponds to the variation in air pressure; the units are arbitrary. The FFT display has frequency on the horizontal axis scaled from 0 to 2000 Hz.

3.   Gently strike a tuning fork with a rubber mallet and hold it near the Microphone. Click  to begin data collection. If you strike the fork too hard, it will create overtones, or a blend of higher frequencies in addition to the main frequency.

4.   Print or sketch the wave that you observe.

5.   Click the Examine button, , and scan across your data to determine the average time interval between adjacent peaks, or one complete cycle. Record this value in your data table.

6.   Calculate the frequency and record it in the data table.

7.   As you move the mouse across the FFT graph, record the predominant frequency as displayed.

8.   Repeat Steps 3 – 7 with the second tuning fork.

Part II  Overtones on a Tuning Fork

9.   In this step, you will make the 256-Hz tuning fork produce an overtone. This time, strike the tuning fork on your knuckle and listen to the sound. Describe the difference.

10.    Strike the tuning fork on your knuckle and hold it near the Microphone. Click  to begin data collection.

11.    Compare the waveform and the FFT to the ones produced in Part I. Click the Examine button, . Move the mouse cursor across the FFT graph and determine the fundamental frequency and the first overtone. Record these values in the data table.

Part III  FFT of Two Vowels

12.    Hold the Microphone near your mouth, say the vowel “e” and hold it while you click the  button. Print or sketch copies of the Graph Window and FFT Graph.

13.    Repeat Step 12 and this time say “o”.

Part IV  Telephone Dialer Frequencies

14.    In this part of the experiment, you will analyze the sound made by a telephone tone dialer. Hold the speaker of the dialer near the Microphone, press the “1” button, then Click  to begin data collection. Using the FFT display, record the two predominant frequencies for this sound.

15.    Repeat Step 14 for numbers “2” through “9”.

Data Table

Part I  Pure Tone



Tuning fork 1

Tuning fork 2

Frequency stamped on tuning fork




Period from waveform




Frequency from waveform




Frequency from FFT graph





Part II  Overtones on a Tuning Fork

Frequency stamped on tuning fork



Fundamental frequency




Overtone frequency





Part IV  Telephone Dialer Frequencies





Low frequency (Hz)




High frequency (Hz)








Low frequency (Hz)




High frequency (Hz)








Low frequency (Hz)




High frequency (Hz)






1.   For each tuning fork, compare the frequency calculated from the waveform and the FFT to the value stamped on the tuning fork.

2.   Describe the difference in the frequency structure between the two vowels examined in Part III.

3.   Examine the data for the tone dialer. What pattern do you observe?

4.   Which frequency is higher, the row frequency or the column frequency?

5.   What is the row frequency for the 4, 5, and 6 buttons?

6.   What is the column frequency for the 2, 5, and 8 buttons?

7.   Summarize how the telephone company tells what numbers you pressed.


1.   Use the Microphone to examine the waveforms of notes from some musical instruments. Which instruments produce the purest tones? Is a C note on one instrument the same frequency as a C note on another? Does the waveform shape change as the loudness of the sound changes? Does the waveform shape change as the frequency of the sound changes?

2.   If an electronic keyboard is available, use the sound sensor or reference material to determine which keys or musical notes are closest to the frequencies that make up your home telephone number. Write the music for your phone number. If no one in the lab group can read music, seek help from a music student or music teacher. Play your phone number on the keyboard.

3.   Play some common musical tunes using the tone dialer.

4.   Compose and play an original tune using only the notes from your phone number. You can use the tones in any order; however, at some point in the tune play the 7 tones in the order of your phone number.

5.   Extend your analysis of vowel sounds to the remaining three (a, i, u). Try to develop a means of producing the different vowels with combinations of overtones, much like when generating an artificial voice.

6.   Extend your investigation to singers in your school choir. Have them make vowel sounds while trying for “rounded” or “pear-shaped” sounds. Note the differences that a single singer can make and investigate how they go about making these differences.

7.   Extend your investigation of the phone dialer to include the bottom row: *, 0 and #. Does the row pattern seen earlier apply here, too?