To understand how sound travels and to discover the difference between transverse and longitudinal waves.
Sound is an important concept in science. Understanding how sound travels, why some sounds are louder than others, how different kinds of instruments produce sound, and learning why it is important to protect our ears are just a few of the reasons why studying this topic is important. In addition, our knowledge of sound waves has resulted in medical advances, such as ultrasound machines that are used to monitor pregnancies and check for heart irregularities. Ultrasound also has been used to determine how animals such as dolphins and bats communicate.
In the 19th century, scientists developed wave models to visualize how sound waves moved. They understood that sound is a series of compressed air, characterized by sound molecules close together, followed by rarefied air, in which the molecules are farther apart. Waves that follow this pattern are called longitudinal waves.
But throughout the 19th century, scientists persisted in depicting sound waves as transverse waves, or those that move energy across a medium, such as air, while causing the particles to move up and down. Other waves being studied at the time, such as light, are transverse.
During the lesson, students will research characteristics of waves and explain, in both words and drawings, the differences between longitudinal and transverse waves. They will begin the lesson by watching a short video about acoustics. This video will give students an overview of what acoustics is and how different instruments from long ago illustrate different principles about sound.
During the Development phase of the lesson, students will focus on the wave models developed in the 19th century. Before beginning the activity, students should watch a second video about wave models. Working in pairs, students will research a series of questions about longitudinal and transverse waves. The goal is for students to understand the similarities and differences and to explain them in words and drawings. Students will record their ideas on their student sheets.
Following their research, students will participate in a demonstration illustrating the difference between longitudinal and transverse waves. After completing the activity, the class will come back together and discuss what they learned.
Sound is a difficult topic for many students to understand, and they often have misconceptions about it. This activity will address the following misconceptions, as identified on Children’s Misconceptions about Science:
- Sound moves between particles of matter (in empty space) rather than matter.
- As waves move, matter moves along with them.
When introducing the topic, try to ask questions that will help you determine if your students have these misconceptions.
To prepare for this lesson, watch the two videos from NMAH:
The second video makes the point that sound waves are longitudinal waves but were usually depicted as transverse waves. But the video does not define these two waves or explain the differences. Understanding these similarities and differences is the goal of this lesson.
By the end of the lesson, students should understand these concepts:
- Longitudinal waves are a series of compressed air, where the sound molecules are close together, followed by rarefied air, where the sound molecules are far apart.
- Transverse waves move in an up-and-down fashion, characterized by high points called crests and low points called troughs.
- The areas of compressed air of longitudinal waves correspond to the crests of transverse waves, while the areas of rarefied air (in longitudinal waves) correspond to troughs (transverse waves).
After students have watched this video, bring them back together as a class and make sure you have their attention. Then clap your hands, ring a bell, or do something else to make a sound.
Then ask students this question: “How does sound, such as music, get to your ears, so you can hear it? Write and draw your response.” Ask students to draw their ideas on the Longitudinal and Transverse Waves student sheet. Then hold a class discussion about the students’ ideas.
In this part of the lesson, students will learn more about sound waves by studying some online resources and conducting a hands-on activity. Begin by having students use their student esheet to go to and watch the Introduction to Wave Models video. Once students have finished watching the video, discuss the questions on the Longitudinal and Transverse Waves student sheet. You can find answers to the questions on the Longitudinal and Transverse Waves teacher sheet.
After your discussion, tell students that they will investigate how sound travels and the difference between longitudinal and transverse waves. Divide students into pairs and tell them that they should work together to conduct research so that they can answer the guiding questions on their Sound Waves Research student sheet. Students can make use of these websites to help them get started on their research:
- Longitudinal and Transverse Wave Motion
- What is the Difference between Longitudinal and Transverse Waves?
- How Sound Waves Work
- Sound is a Mechanical Wave
Once students have completed their research, hold a class discussion where you go over what they have discovered through their research. You can go over the questions on their student sheet:
- What do you already know about sound waves?
(Answers will vary. Encourage students to explain their answers.)
- What would you like to learn about sound waves?
(Answers will vary. Encourage students to explain their answers.)
- How do sound waves work?
(They work as variations of pressure in a medium such as air. They are created by the vibration of an object, which causes the air surrounding it to vibrate. The vibrating air then causes the human eardrum to vibrate, which the brain interprets as sound.)
- What are the characteristics of sound waves?
(Some characteristics of sound waves are: 1. it is a mechanical wave because it is a disturbance that is carried through a medium via the mechanism of particle-to-particle interaction; 2. it is produced by an oscillating object; 3. it cannot travel through a vacuum; and 4. it is capable of existing with a variety of frequencies.)
- How is sound created and detected?
(Sound is created by vibrating objects and propagated through a medium from one location to another. It is detected by our ear drums.)
After students complete their research, have them participate in the demonstration described below. Students can take turns moving the slinky and the jump rope to demonstrate longitudinal and transverse waves. This lesson was adapted from an online lesson (Schmidt-Jones, Catherine. "Transverse and Longitudinal Waves." Connexions. October 1, 2011. http://cnx.org/content/m12378/1.9/).
- Load the slinky onto the broomstick and stretch it out a bit. Ask two students to hold the broomstick horizontally at waist level, as steadily as possible, or secure the ends of the broomstick on desks or chairs.
- Holding one end of the slinky still, one student should jerk the other end of the slinky forward and back along the broomstick as quickly as possible. Ask students: “What kind of wave is being created?” (This is a longitudinal wave, illustrated by the fact that it is traveling down the slinky to the other end.)
- Now ask a student to hold one end of the jump rope very still at waist height. Another student should stretch the jump rope out taut, horizontally.
- One of the students should suddenly jerk the end of the rope up and down again. Ask students: “What kind of wave is being demonstrated?” (This is a transverse wave, illustrated by its up-and-down motion.)
- To sum up, ask students: “What is the difference between longitudinal and transverse waves?” (Make sure they understand that transverse waves have an up-and-down motion, while longitudinal waves move side-to-side.)
Bring the class back together and review the concepts in this lesson by asking questions like these:
- What kinds of waves are sound waves?
(They are longitudinal waves, characterized by compressed air molecules followed by rarefied molecules, those that are farther apart.)
- In the 19th century, sound waves were often depicted as transverse waves. How can this be done?
(As long as the areas of compressed air matches the crests of transverse waves and the areas of rarefied air match the troughs, then it is possible to do so.)
- What is the most important difference between longitudinal and transverse waves?
(Longitudinal waves move side to side, while transverse move up and down.)
- In the beginning of the lesson, you drew a picture showing how sound travels. How would you draw that picture now?
(The important idea here is that students depict sound as a wave in some way.)
Have students go to the Science Teaching Collection: Acoustics and find additional videos that demonstrate other properties of sound. Have students present their choice to the class.
To expand student understanding of waves, they can check out What is Sound? a video from the National Institute on Deafness and Other Communication Disorders.