Bouncing Sound

INTRODUCTION

Sounds travel at different speeds through solids, liquids, and gases, and they do not pass very well from one substance to another. So, if you shout at a brick wall, little of the sound energy will come out into the air on the other side. Instead, most of the sound waves bounce back off the wall. You will probably have noticed this when you hear an echo in a tunnel or an empty room. An echo is a reflected sound. Echoes can be useful or they can be a nuisance. Architects use their knowledge of how sound waves are reflected when they design buildings. A noisy restaurant can be made quieter by covering the floor, walls, and ceiling with soft or bumpy materials that soak up the sounds. But the walls and ceiling of a concert hall will be built from materials that reflect the sounds of the orchestra toward the audience.

Even sound waves we cannot hear produce echoes that give us important information. Some vibrations with very high frequencies produce ultrasound. Ultrasound can travel through some solid objects and be reflected by others. An example is the use of ultrasound to "see" an unborn baby inside its mother’s womb. Another example is sonar, which stands for "Sound Navigation and Ranging" and is used by ships to study the seafloor.

MATERIALS

• 1 clock or watch with a quiet tick
• 3 or 4 similar cardboard tubes
• Sound-reflecting and sound-absorbing materials (i.e. stiff cardboard, plastic, aluminum foil, ceramic tiles, cork, egg cartons, modeling clay, scissors)

WHAT TO DO

1. Arrange two cardboard tubes on a table so that they are at right angles to each other.
2. Place the clock or watch at the end of one tube and put your ear close to the end of the other. Can you hear the tick of the clock?
3. Now cut out a square of stiff cardboard and stand it upright in modeling clay supports. Place the cardboard tube where the open ends of the tubes come close together.
4. Listen again for the tick of the clock. How does the loudness of the tick compare to the first test?
5. Repeat the test with squares of the other materials.
6. Next, try reflecting the sound of the ticking clock by using some of the other cardboard tubes and squares of the best sound reflecting material. How many times can you reflect the sound?

QUESTIONS

1. Which material is the best sound reflector?
2. What makes some materials good sound reflectors and other materials poor sound reflectors?
3. Think of some times when echoes may be useful and other times when echoes may be harmful.

SUMMARY

When you talk into the cup, you cause it to vibrate. This in turn causes the string to vibrate, which causes the cup on the other end of the string to vibrate. Whoever is listening hears the vibrations caused by your voice after they have traveled through the cups and the string.

SOURCE

"SOUND: Science Projects." Simon de Pinna, Raintree Steck-Vaughn Publishers: Austin, 1998, p. 8-9.

© S. Olesik, WOW Project, Ohio State University, 2000.