Module 8—Mechanical Waves

Lesson 4—Resonating Air Columns

 

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A photograph shows six long-neck glass bottles: one clear, one red, one blue, and three green.

© Quayside/shutterstock

What would happen if you were to blow across the top of one of the bottles in this photo? If you did it right, a sound would be produced. Find an empty bottle, and try it now.

 

What do you think would happen if you poured some water into your bottle? Would the tone change? Yes, it would; but how it changes would depend on how much water you put into the bottle. The tone produced by each bottle depends on the length of the air column in the bottle itself. When you add water, you effectively change the length of the air column in the bottle. From your studies in Module 8: Lesson 2, this implies that the sound wave produced by the airflow at the top of the bottle must enter the bottle, reflect off the bottom, and return to the top. This means that wave interference must occur inside the bottle.

 

As you observed from the rope exercise from Lesson 3, a standing wave is always produced at a resonant frequency specific to the length of the medium that the wave travels. In this case, the medium is the air inside each bottle.

 

There are many examples of air columns that carry sound waves. Pipe organs and wind instruments, such as the flute and trumpet, are all resonating air columns. How is mechanical resonance applied to produce desired tones in such instruments? How do the principles of superposition, interference, and reflection apply to air columns that are closed on one end, such as a bottle, or ones that are open at both ends, such as an organ pipe?

 

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

Module 8: Lesson 4 Assignments

 

Your teacher-marked Module 8: Lesson 4 Assignment requires you to submit a response to the following:

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.