Module 8 Lesson 5

Module 8—Mechanical Waves

Reflect and Connect

A photograph shows two speakers set outdoors under a blue sky with clouds. One speaker is set between rocks. The other speaker is placed underneath a poplar tree.

When two speakers are located near one another, and they each continually emit identical sound waves that are in phase, the principle of superposition will apply wherever the wave fronts meet. This produces an interference pattern with nodal lines and antinodal lines. The nodal lines are areas of destructive interference, where the medium appears undisturbed. Along these lines, the volume of sound will be very low because the waves are out of phase, causing destructive interference. Along the antinodal lines, the volume of sound will be very high because the waves are in phase, causing constructive interference. Determining the difference in path length between the observer and each speaker, in terms of the wavelength being emitted, would allow you to predict what kind of interference will occur at the observer’s position. These interference effects can be applied in the design and layout of the sound system in an entertainment room.

Discuss

Twenty-three vehicle speakers are arranged in a design that includes grid lines in this art work.

Interference effects are generally considered to have a negative impact on the quality of sound produced by speakers. For example, both the Jubilee Auditoriums in Calgary and in Edmonton were renovated to reduce the interference at several “hot” and “cold” spots in the seating areas. This involved moving sound-reflecting surfaces, sound-absorbing surfaces, or the location of the seats.

If it’s possible to physically accommodate interference, could it not be used to eliminate unwanted sounds? For example, could the principle of superposition be used to destroy undesirable sound waves? Consider the interior of a car with a known configuration of speakers, such as those shown in the photo.

When the car is being driven, it produces sounds from the moving tires and vibrations from the engine and other moving components. Research the idea of using speaker placement in a car to minimize background noise. In the discussion forum, propose a method in one paragraph to eliminate background noise in a vehicle by using the vehicle’s sound speakers.

Reflect on the Big Picture

Each of the Reflect on the Big Picture sections in this module deal with waves and transferring energy. To help reinforce your learning from this lesson, complete at least one of the following reflection activities:

Rogue waves are relatively large, spontaneous waves that occur in the ocean. They were once considered to exist only in legends, but scientists have proof that they are a naturally occurring phenomenon. There are numerous possible causes of rogue waves that are currently being researched, and one theory involves constructive interference. Explain how this could cause rogue waves. Research two other possible causes of rogue waves.
Imagine that you work in a store that rents sound equipment for outdoor events, such as concerts and block parties. You want to be able to clearly explain to your customers how to best set up the equipment to minimize interference patterns. Create an analogy for wave interference using a sport to explain appropriate equipment placement. Try to use your favourite sport, but if you don’t have one, hockey should work.
Think about what you would consider an ideal surround-sound system. Describe your system, and explain how you would minimize destructive interference in setting it up.

Store your completed reflection in your Physics 20 course folder.

Module 8: Lesson 5 Assignment

Remember to submit the Module 8: Lesson 5 Assignment to your teacher.