Core Unit II: Wave Motion
A. Properties of Waves

3. Principle of Superposition

Key Concepts

Wave interference occurs when two or more waves act simultaneously on a medium.

Whenever two or more waves pass through each other, the resulting disturbance at a given point in the medium may usually be found by adding the individual displacements that each wave would have caused. (Principle of Superposition)

Constructive interference occurs when the resulting disturbance is larger than the individual disturbances which created it.

When the resultant displacement is smaller than the individual displacements, destructive interference has occurred.

Learning Outcomes

Students will increase their abilities to:

1. Define the following terms: interference, constructive interference, destructive interference.

2. State the Principle of Superposition.

3. Predict the resulting wave pattern when two waves travelling in opposite directions meet at a given point.

4. Illustrate constructive and destructive interference using diagrams, models, or computers.

Teaching Suggestions, Activities and Demonstrations

1. Design a computer program to illustrate graphically the superposition of waves.

2. Design a computer program to illustrate the effects of changes in frequency and amplitude on transverse waves.

3. Input an audio signal to an oscilloscope to examine the patterns produced by various different types of sound and noise.

4. Draw interfering wave pulses on graph paper and predict the pattern when the interfering waves are in a specific position.

5. Illustrate the interference pattern produced by two point sources vibrating in phase or out of phase in a ripple tank.

6. Pose the following question for students to consider. "What happens to the energy associated with waves when destructive interference occurs?"

7. An interesting application of destructive interference of sound waves is a new design in automobile mufflers. These new types of mufflers use a digital sensor to detect the sound wave frequency generated by the exhaust system. Another sound wave pattern is then generated to interfere destructively with it, thus reducing the sound intensity of the exhaust system. This type of muffler should be on the market soon.

   Also, some types of headers are "tuned" by changing their length, producing a region of destructive interference at the point of sound emission from the vehicle. The principle of an open-ended resonating air column is used to control the sound intensity.