Activity 2Sounds in Vibrating Strings

Goals

In this activity you will:

  • Observe the effect of string length, amplitude and tension upon pitch produced.
  • Control the variables of tension, amplitude and length.
  • Summarize experimental results.
  • Apply your new knowledge of how sound works to play songs on instruments you made.

What Do You Think?

When the ancient Greeks made stringed musical instruments, they discovered that cutting the length of the string by half or two-thirds produced other pleasing sounds.

  • How do guitarists or violinists today make different sounds? ______

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  • How does someone tune a string instrument? ______

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  • How does sound get from an instrument or radio to your ear? ______

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  • Describe, to the best of your abilities, the physics behind how instruments make sound.

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Physics Words(review these words before you move onto the activity)

transverse wave: ______

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compressional (longitudinal) wave: ______

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high pitch: ______

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low pitch: ______

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tension: ______

medium: ______

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wavelength: ______

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frequency: ______

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amplitude: ______

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For You To Do

1.Your instructor will have one setup available for you to see.

  • Carefully mount a pulley over one end of a table.
  • Securely clamp one end of a string to the other end of the table.
  • Tie the other end of the string around a mass hanger. Lay the string over the pulley or table.
  • Place a cup under the string near the clamp, so the string can vibrate without hitting the table.
  • You can adjust the length of the vibrating string by sliding the box (or cup) back and forth.

2.Hang one 500-g mass on the mass hanger. Pluck the string, listen to the sound, and observe the string vibrate. Use a finger to feel the vibrations in the string.

3.Shorten the length of the vibrating string by sliding the cup.

a)Pluck the string again and compare the new sound to the sound made by a longer string.

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b)Try making sound with many different length strings. Write down how you make a higher pitch sound and a lower pitch sound.

  1. To make a higher pitch sound ______
  2. To make a lower pitch sound ______

4.To investigate the effect of tightening the string, increasing tension, add a 200-g more to the mass hanger. Make the length of the vibrating string as long as possible. Pluck the string again, observe the vibration, and listen to the pitch of the sound.

a)With the string as long as possible pluck the string with ONLY ONE 500-g mass on it. After you have done that add the additional 200 grams onto the hanger. Record how the two sound compare. ______

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b)Continue adding weights and observing the sound until the total mass is about 1500 g. What happens to the sound as the tension increases? ______

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c)Why do you think the sound changes this way, you may guess if you do not know? _____

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d)As the mass increases, the string becomes tighter. We say that its “tension” increases. Make a general statement about what happens to the pitch you hear as you change the tension on the string. ______

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5. With the vibrating string long and a mass of 500 grams, pluck the string and take a close look at it.

a)Where do you see nodes and where do you see the anti-node?

  1. The nodes are ______.
  2. The anti-nodes are ______.

b)Draw what the wave looks like, be as accurate as possible. Label the nodes, where you measure amplitude, crest and trough.

6.Now lets see how the amplitude affects the pitch.

a)With a long wavelength change the amplitude of the wave by pulling the string in the center.

  1. What happens to the sound when you have the amplitude of the vibrating string small? ______
  2. What happens to the sound when you have the amplitude of the vibrating string large? ______

b)The amplitude of a vibrating string relates to how ______the sound is.

c)Using this new knowledge of what amplitude is, how do speakers get blown out, be thorough. ______

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7. Teach yourself how to play “Take Me Out to the Ball Game” by changing the tension. First group to play it correctly for the teacher gets a bonus.

8. Do the same thing as in #7 but by changing the wavelength.

FOR YOU TO READ

Changing the Pitch

Sound comes from vibration. You observed the vibration of the string as it produced sound. You investigated two of the variables that affect the sound of a vibrating string. When you moved the cup, the length of the string that was vibrating became shorter. Shortening the string increased the pitch (resulted in a higher pitch). In the same way, a guitarist or violinist pushes the string against the instrument to shorten the length that vibrates and increases the pitch.

When you hung weights on the end of the string, that increased the pitch too. These weights tightened the string, so they created more tension in it. As the string tension increased, the pitch of the sound also increased. In tuning a guitar or violin, the performer changes the string tension by turning a peg attached to one end of a string. As the peg pulls the string tighter, the pitch goes up.

Combining these two results into one expression, you can say that increasing the tension or decreasing the length of the string will increase the pitch.

Physics To Go

1.a) Explain how you can change the tension of a vibrating string.

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b) Tell how changing the tension changes the pitch; use the last activity to answer this.

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2.a) Explain how you can change the length of a vibrating string on a string instrument. .

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b) Tell how changing the length changes the sound produced by the string;use the last activity to answer this.

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3.How would you change both the tension and the length and keep the pitch the same?

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4.Suppose you increased the length and decrease the tension of the string at the same time. What would happen to the sound? ______

Why? ______

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5.a) For the guitar and the piano, tell how a performer plays different notes.

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b) For the guitar and the piano, tell how a performer

(or tuner) changes the pitch of the strings to tune the instrument.

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6.a) Look at a guitar. Find the tuning knobs (at the end of the neck). Why does a guitar need tuning knobs? ______

b)What is the purpose of the frets on a guitar? ______

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c)Does a violin or a cello have frets? ______

d)Why do a violinist and a cellist require more accuracy in playing than a guitarist?

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Read in your CP book 6.1, 6.2, 6.3.