Module 8 Lesson 5

Module 8—Mechanical Waves

Lesson 5—Two-Point Interference Patterns

Get Focused

A photograph shows a chair set in front of an entertainment centre. The entertainment centre includes a television set. On either side of the TV are two speakers.

The home entertainment system delivers both a visual and audio experience for the user. Usually, the visual part of the system is set up directly in front of the user. Typically, the audio is delivered via a set of speakers, ranging from two to dozens of speakers found in various locations relative to the user. In this photo, the speakers are placed on either side of the screen. In this kind of setup, there will be certain spots in the room where the sound quality is excellent; in other spots, it will be poor. Why is this? How could you predict where the good listening spots are going to be?

The two speakers can be thought of as two point sources of sound waves. If the speakers produce identical, constant tones, and you were to slowly move around the room, you would notice that the volume of the sound changes.

You may have already guessed that interference between the identical sound waves produced by each speaker will produce a pattern of interference throughout the room. As in any interference pattern, there will be points of maximum constructive interference and points of maximum destructive interference. These points will be observed with high and low volumes of sound when the speakers are emitting identical sound waves. How can you predict where these points will be? Do you need to consider the interference pattern when designing a home entertainment room? Is it possible to reduce the amount of sound interference in the entertainment room? If so, how is it possible?

As you work through this lesson, keep the following questions in mind:

What is path length and path difference?
What is the relationship between path difference and constructive/destructive interference?
What are nodal and antinodal lines?

Module 8: Lesson 5 Assignments

Your teacher-marked Module 8: Lesson 5 Assignment requires you to submit a response to the following:

Try This—TR 1, TR 2, and TR 3
Lab—LAB 1

You must decide what to do with the questions that are not marked by the teacher.

Remember that these questions provide you with the practice and feedback that you need to successfully complete this course. You should respond to all the questions and place those answers in your course folder.