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7 Sound Transmission

Sound transmission is the movement of sound energy via longitudinal waves. In most cases, sound travels slowest in gases, faster in liquids and fastest in solids. Sounds are loudest near the source. Sound waves are produced by a vibrating object. Figure 1 shows how sound waves move out from a sound source through air. Notice that closest to the sound source, the compressed areas are very dark, but fade in color with distance from the speaker. The diagram shows that the spacing between the dark areas doesn't change. The equal spacing in the diagram illustrates that the frequency of sound doesn't change with distance, only the amplitude or energy of the wave decreases. Thus, the sound would be the loudest at position A, and softest at position C. This relationship is true with all sound waves. Also, the energy of the sound would travel farther through a solid than in air.

FIG 1

See for Yourself

Materials

 2 metal spoons

 string, 3 yards (1.8 m)

 helper

What to Do

1 Tie the string around the handle of one of the spoons so the two ends of the string are about the same length.

2 Wrap the ends of the string around your index fingers. Let the string and spoon hang in a V shape.

3 Extend your arms in front of you as shown in Figure 2 so the spoon hangs freely without touching anything.FIG 2

4 Ask your helper to use the second spoon to gently tap the hanging spoon (Figure 3). Make note of the sound produced by the spoon.FIG 3

5 Stick your index fingers in your ears and lean forward so the hanging spoon dangles freely. Check to make sure the string is not touching any object.

6 Again, ask your helper to tap the spoon as before. Compare the two sounds.

What Happened?

Everything that vibrates sends out sound waves through the air in all directions. The frequency or pitch of the sound was the same each time the spoon was tapped. But, the amplitude, or loudness, of the sound was greater with your fingers and the string pressed into your ears. This is because when traveling through air, the sound waves move slower than when they travel through a solid. Thus, more sound waves reached your ears when they traveled up the string to your ears. Also, the string provided a direct route for the sound from the vibrating spoon to your ears.