Optional Unit V: Sound

B. Transmission of Sound

1. Production of Sound
Key Concepts

Sound is produced by vibrating sources.

The vibration causes changes in pressure near the vibrating source.

Changes in pressure produce longitudinal waves, consisting of high pressure compressions, regions of normal air pressure, and low pressure rarefactions.

Sound can not travel through a vacuum.

Sound is capable of travelling through different types of solids, liquids, and gases.

When an echo occurs, the sound travels from the source to the reflecting surface and back.

Reverberation is a repeated echo occurring in interior spaces.

Several principles can be applied to deliberately dampen sound or other types of energy:

• Transfer the energy into a medium that makes propagation more difficult.

• Increase the path length the pattern must travel in order to get from point to point.

Damping devices include such things as mufflers, log booms, shock absorbers, acoustic damping material, electrical resistors, etc.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: sound, pressure, longitudinal waves, compression, rarefaction, vacuum, echo, reverberation, damping.

2. Describe some of the ways in which sound illustrates wave behaviour.

3. Explain that sound is produced by vibration.

4. Determine some vibrating sources which produce different sounds.

5. Explain that the vibrations cause a change in pressure near the vibrating source.

6. Explain that the changes in pressure can create a series of longitudinal sound waves which are transmitted from the source.

7. State that sound can not travel in a vacuum.

8. Explain that sound can travel through different types of solids, liquids, and gasses.

9. Define an echo and reverberation and state similarities and differences between them.

10. Identify two important damping principles.

11. Give examples of different kinds of damping devices.

Teaching Suggestions, Activities and Demonstrations

1. Place a partially filled balloon in a bell jar. Tape a thumb tack to the inside of the jar. Seal the jar and evacuate the air with a vacuum pump. As the air pressure inside the jar is reduced, the volume of the balloon increases, in agreement with Boyle's Law. When the expanding balloon comes into contact with the tack, the balloon will eventually break. Except for the vibration transmitted through the tack and the walls of the jar, the sound of the balloon is not heard, illustrating that sound can not travel in a vacuum.

2. Specially designed electric bells that fit inside the bell jar can be used to illustrate a similar thing, but do not also show the change in pressure with decreased volume. Add some foam shaving cream to the bell jar before evacuating. As the air is removed from the jar, the foam bubbles expand. Be careful not to allow the foam to expand too much, otherwise it may get on the electric bell or in the vacuum pump apparatus.

3. Arrange the class into groups of four or five per group. Each group has to play a tune, using only home-made musical instruments. For example, a musical xylophone can be made by placing different amounts of water into pop bottles, or an unusual wind instrument can be made by spinning a vacuum cleaner hose in a circle. There should be at least one string, wind, and percussion instrument in each band. Some of the band members can play more than one instrument during the "concert."

   Groups can be assessed based on the originality and quality of the instruments made, the sound quality and range produced, as well as the aesthetic qualities of the musical selection played. Allow the students to have some input regarding the criteria that will be used to assess their projects.

   Musical arrangements can vary from ragtime to rock and roll to classical. The musicians can wear costumes which are appropriate to the kind of music being played (e.g., straw caps for dixieland, or overalls for bluegrass, etc.) Groups can be challenged to use selected lighting equipment in the physics lab to add atmosphere with a light show for their performance.

4. Assign groups to research musical instruments from particular cultural groups (i.e., Indian peoples, Métis, Ukrainians, etc.)

5. Invite groups of musicians to the classroom to demonstrate and discuss their instruments.

6. Brainstorm to suggest some practical difficulties which might be encountered in space because sound can not travel in a vacuum.

7. Investigate the effect that different animal skins have on the sound of a drum. See what effect stretching the skin has on the sound. Try beating the drum in different regions on the surface to see if there are differences in the qualities of the sound produced. See how changing the size and shape of the drum barrel affects the sound produced.

8. Give some examples which illustrate that sound can travel through solids, liquids, and gases. Challenge students to design demonstrations which illustrate this.

9. Inuit women invented a throat singing technique called katadjait in the Inuktituk language. Two women embrace each other and make throat sounds that are commonly heard in their environment.

   Have students research this art form. If possible, guest performers or taped recordings of throat singing could be used.