This activity explores the frequency and amplitude of sound waves.

How can we describe sounds?

• microphone
• Sound Grapher. If you do not have the Sound Grapher on your desktop, go to this link and download the Sound Grapher.
• piece of stiff paper to make a sound tube

This activity addresses NSES standards for physical science (for transfer of energy) and inquiry at grades 5-8
(http://books.nap.edu/readingroom/books/nses/6d.html#ps).

Do not make loud noises close to other people’s ears. Eardrum damage may result.

• What is the loudest sound you have ever heard, and how was it made?
• What is the softest sound you have ever heard, and how was it made?
• What is the highest pitched sound you have ever heard, and how was it made?
• What is the lowest pitched sound you have ever heard, and how was it made?
• Explain how you think sounds are physically produced. Give several examples of sound producers that can vary both loudness and pitch. What are different ways of making a sound’s pitch go up or down? What are different ways of increasing or decreasing its loudness?

Here are some words to help you describe sounds:

Sounds are actually very fast variations in pressure in the air; these periodic variations are called vibrations or waves.

One complete vibration is called a cycle.

Wavelength is the length of a cycle, for instance, the distance between two peaks.

Frequency is the number of cycles per second of the pressure variation. Most sounds have a mix of frequencies. The ear usually perceives one as dominant, and this dominant frequency is called its pitch.

Amplitude is the size of the pressure variations, seen as the height of the waves.

Intensity is the amount of energy in the sound, and loudness is what the ear perceives.

1. Make sure your computer has either a plug-in or a built-in microphone. Refer to Technical Hints to connect the microphone.
2. Turn up the input volume for the microphone.
3. Start the Sound Grapher. Refer to this link to download the Sound Grapher. It will open in a separate window.
4. The Sound Grapher shows two graphs. Click on the graph you want to use. Click on the Start button to start the Sound Grapher.
5. As you make noise, wavy lines should appear on the screen. When you click on the Stop button, the graph “freezes” the sound picture at that moment. Here is an example:
   
6. You can then switch to the other graph to look at another sound. The space bar and the Return key also control Start/Stop functions.
7. A single frequency appears as a simple sine wave. When more than one frequency is present in the sound, the waveform is a combination of several sine waves and looks more complex. It is difficult to tell what is happening from the waveform alone, so an additional analysis tool is provided. Click on the “Waves” button, and change the drop-down menu to “Frequencies.” The screen now displays the distribution of frequencies that are present in the sound, with frequency plotted on the x-axis. Here is an example. The first plot shows a sound with five major frequencies; the second sound has two major frequencies.
   

1. Make sure your computer has either a plug-in or a built-in microphone. Refer to Technical Hints to connect the microphone.
2. Turn up the input volume for the microphone.
3. Start the Sound Grapher. Refer to this link to download the Sound Grapher. It will open in a separate window.
4. Hum into the microphone for 10 seconds. Try changing how loudly you hum. What feature in the graph shows you how loud the sound is?
5. Start the Sound Grapher. Hum close to the microphone, then farther away. Don’t change the loudness of your hum. What changes do you notice in the graph? Does the loudness of the sound at the microphone decrease as you move farther away? Why do you think this happens? Be prepared to share your thoughts with the class.
6. Make a tube by rolling up a piece of paper. Refer to the picture below. Put one end of the tube near the microphone and hum into the other end. Note the height of the pattern. Now take away the tube and hum with exactly the same loudness. Note the height of the pattern now. Why does the tube make a difference in loudness at the microphone?
   
7. Hum a low pitch (frequency), making a “uuu” sound. Keep the picture of the sound by clicking on the Stop button.
8. Now click in the bottom graph. Hum a high pitch, making exactly the same “uuu” sound. Capture the picture of the sound by clicking on the Stop button.
9. Find the pattern that repeats for each pitch. Draw a picture of the pattern for each pitch. Draw just the part that repeats and not the whole screen. This represents one cycle.
10. Count the total number of cycles that show on each graph. Which has more repeats, the higher pitch or the lower pitch? What can you say about the relationship of the length of a cycle and the frequency of the sound?
11. Click on the top window of the Sound Grapher. Have someone make a “uuuu” sound. Click on the Stop button to capture the graph. Click on the bottom window of the Sound Grapher. Have another person whistle. If they can’t whistle, find someone in the class who can. Click on the Stop button to capture the graph. Which sound has the higher frequency? How can you tell?
12. Click on the top window and start recording. Have one student make the uuuu sound while another student whistles into the microphone at the same time. Keep the picture of the sound by clicking on the Stop button. Can you see the patterns for both sounds at the same time in the Sound Grapher? Draw the pattern of the combined whistling and humming. Label one cycle of whistling and one cycle of humming.
13. Try making the highest frequency sound you can, as quietly as possible. Remember that high pitched does not mean loud! How many cycles are visible on the graph?
14. Try making the lowest frequency sound you can. How many cycles are visible on the graph?
15. Change the Sound Grapher from “wave” mode to “frequency” mode. Compare humming and whistling. How are different frequencies displayed on this type of graph? Try making sounds that have a very low and a very high frequency, using the frequency mode to test them.

1. Look at the graph of a combined hum and whistle. The graph represents 30 milliseconds (0.030 seconds) of time. Count how many cycles occur in that time for each pitch. Calculate the time for one cycle for each pitch. This is called the period, measured in seconds.
2. If you know the time for one cycle, calculate how many cycles occur in one second. Explain how you made this calculation. This is the frequency, measured in cycles per second.
3. The speed of sound in air is about 340 m/s. How far does the sound travel in 30 ms?
4. For each pitch, how far does the sound travel during one cycle (in meters)? This is the wavelength of that sound. If this were a water wave, it would be the distance from one peak to the next.
5. For each pitch, use the formula (wavelength) * (frequency) = (speed) to calculate sound. How does it compare to the accepted value of 340 m/s?

1. Draw two sound patterns that have the same frequency but different amplitude.
2. Draw two sound patterns that have the same amplitude but different volume.
3. On your drawings, label the amplitude and the period of the wave.

Of two sounds that have the same amplitude, which is easier to hear, a low-frequency or a high-frequency sound? Test this using the Sound Grapher.