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# Waves of Sound (page 2)

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Author: Janice VanCleave

1. Sound is energy and thus is able to do work. Design an experiment to show that sound can do work by moving an object a distance. One way is to cut the bottom from a 9-ounce (270-ml) paper cup (see Figure 28.2). Use a fine-point permanent marker to draw a grid with 1-cm (or 1/4-inch) squares on a 6-inch (15-cm)–square piece of waxed paper. Cover the top of the cup with the waxed paper, and secure the paper with a rubber band. Cut away any excess waxed paper. Mark an X in the center square. Lay a radio on its back so that its speaker side is up. (If speakers are separate, lay one speaker on its back.) Set the open end of the cup on the speaker. Turn the radio on to a low volume and tune it so that static is heard. You want a continuous sound with the same frequency. Determine where you should position the volume dial for low, medium, and loud volume. Turn the radio off and place one grain of rice in the center square drawn on the waxed paper (square with an X). Turn the radio onto a low volume and observe any movement of the rice grain away from the center square. Repeat the procedure using medium and loud volume. Determine the relationship between volume and sound energy.
2.
1. Sound travels through all phases of matter—solid, liquid, and gas. Design an experiment to compare the efficiency of sound traveling through a gas and a solid. A ticking watch can be used to do this. Hold the watch at arm's length from your ear. Slowly bring the watch toward your ear until the first faint ticking sound can be heard. Measure the distance from the watch to your ear. Press your ear to a table and place the watch on the table an arm's length from your ear. Again, listen for the watch's ticking sound. If the ticking sound can be heard, ask a helper to slowly move the watch farther from your ear until the ticking sound is faint. If the ticking sound cannot be heard at arm's length, then slowly move the watch toward your ear until it can be heard. Measure the distance from the watch to your ear and compare it to the distance at which you could just hear the watch ticking through the air.
2. Design an experiment to test the efficiency of sound in water. One way would be to place the same ticking watch from experiment 2a in a sealable bag. Tie a string to the bag and lower it into an aquarium filled with water. With the watch in the center of the water at one end of the aquarium, place your ear against the aquarium at the opposite end. If the ticking can be heard, measure this distance. If not, ask a helper to move the watch toward you until the ticking can be heard, then measure this distance. Compare this distance to the distances measured in experiment 2a. For more information about sound travel in different materials, see P. Erik Gundersen, The Handy Physics Answer Book (Detroit: Visible Ink, 1999), pp. 213–217.

### Get the Facts

1. As the temperature of air increases, its molecular motion increases. How does each degree Fahrenheit (Celsius) affect the speed of sound in air? For information, see Sir James Jeans, Science & Music (New York: Dover, 1968), p. 119.
2. A microphone changes sound energy into electrical energy. For information on how this is done, see Mary Jones, Physics (New York: Cambridge University Press, 1997), pp. 110–111.
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