University of Virginia
Physics Department

Wavelength and Pitch - Musical Instruments

A Physical Science Activity

2003 Virginia SOLs

• PS.1
• PS.8

Objectives

Students will

• understand the correspondence between length and pitch;
• understand how musical instruments work.

Motivation for Learning

Waterphone Demonstration

The teacher should have the "waterphone" set up and ready when the students come in.

Materials
• 3 or more identical glass soda bottles
• Mallet
• Water

Procedure

1. Fill the soda bottles with varying amounts of water. Tap the bottles with the mallet to insure that they produce distinctly different pitches.

2. Have the students predict the order from lowest to highest pitch that each bottle will generate if you blow across the tops of the bottles.
3. Blow across the tops of the bottles and see if the students are correct. The highest pitch should be the bottle with the most water.
4. Ask the students if the order would be different if the bottles were struck with a mallet.
5. Using a mallet, strike the bottles below the water line. They should be in the opposite order.

• Were the orders of the pitch the same when the bottles were hit compared to when they were blown over? No.
• What was vibrating when you blew across the top of the bottles? In this case, the air.

Background Information

An air column will vibrate with a wavelength dependent on the length of the tube, and, therefore, a certain pitch. The wavelength cannot be seen of course, but it can be seen that the longer the air column, the lower the pitch. When you blow across a bottle with water in it, the vibrating medium is the column of air whose length ends at the top of the water. Therefore, the more water that is in the bottle, the shorter the column of air, and the higher the pitch. When the same bottle is struck with a mallet, the vibrating medium is the glass and the water. The more water that is in the bottle, the more vibrating material there is (longer column), and the lower the pitch when struck with a mallet.

Student Activity

Materials

• Straw
• Scissors
• Pipe of different lengths

Procedure

 Flatten the end of a drinking straw, and cut the flattened end to a point to simulate a double reed such as an oboe. You would like the two tips to be almost touching each other. Place the "reed" end inside your mouth so your lips or tongue don't interfere with it, and blow. This takes a little practice. Cut straws to different lengths to get different pitches. If straws of different widths are available, place one inside the other to make a slide trombone. Try to cut holes in the straw and play it like a real instrument. Note how the pitch changes with length. Now hit a piece of pipe on the end with the palm of your hand. Notice the pitch of the sound. Do the same to other pieces of pipe of different lengths. How does the pitch correspond to length?

Explanation

When you blow on the end of the straw, the two pieces of the tip vibrate together. This makes a vibration, which is necessary to make sound. The tips vibrate at a specific frequency and wavelength that is determined by the length of the straw. The vibration travels down the straw and reflects from the end. This sets up a wave in the air in the straw; the vibration will bounce back and forth between the two ends. It is this vibration that you are hearing. Changing the length of the straw (by clipping it off, or by making a straw trombone) changes the wavelength and frequency, and so changes the pitch. Making a hole in the straw so it is like a real instrument lets the vibration bounce off from where the hole is, which will also change the pitch. Similarly, hitting the pipe causes it to vibrate, and the frequency and wavelength depend on the length.

Extensions

Have the students compare straws and pipes of different widths. Have them try different designs for the straw's tip. Which one works best? Does it make a difference how hard you blow into the straw? Tune the instruments to simple notes, and have the students perform an easy song, such as "Mary Had a Little Lamb." You could also explain how vocal chords use the same principle to make different pitches.

Students with Special Needs

Many students may not be able to get a noise out of the straw oboes. These students can cut the straws while the other students are blowing through them. They can also adjust the "trombone slide" as another student is playing the oboe.

Click here for further information on laboratories with students with special needs.