Labeling Compression Waves

Contributor: Meghan Vestal. Lesson ID: 11474

Have you studied sound waves until it Hertz? Has your knowledge of sound waves reached its crest? Probably not, so you can get out your Slinky and learn about frequency and the parts of sound waves!



learning style
personality style
Grade Level
Middle School (6-8)
Lesson Type
Quick Query

Lesson Plan - Get It!


sound wave

  • What is the frequency of the sound wave pictured above?
  • Do you need more information?

Read on!

Consider what you already know about the term frequency.

  • How could this term relate to sound?

Discuss your ideas with your teacher or parent, and make a prediction about the frequency of the sound wave. You will find out if you are correct at the end of this section.

In the previous Related Lesson, you learned that sound waves have a specific name.

  • What do you call a sound wave?

Tell your teacher or parent.

If you missed, or need to review, the previous Related Lessons in our All About Sound series, check them out in the right-hand sidebar.

Sound waves are also referred to as compression or longitudinal waves. Tell your teacher or parent at least three facts about sound waves.

You may have mentioned:

  • Sound waves are made of vibrating molecules.
  • Some molecules vibrate close together and other molecules vibrate farther apart, creating waves.
  • Compression waves are also called longitudinal waves because the molecules vibrate parallel to the direction the wave is traveling.
  • Sound waves travel at the rate of 760 miles per hour.
  • Those nearest to the source of the sound will hear the sound produced before those sitting a few feet away.

Who knew there was so much to know about sound waves? You aren't even finished learning about them yet!

In this lesson, you will discover the parts of a sound wave, and practice labeling these different parts. When looking at a sound wave, you should be able to identify four parts:

  • Compression the part of a sound wave where the molecules are closest together
  • Rarefaction the part of a sound wave where the molecules are farthest apart
  • Crest the highest point on a wave
  • Trough the lowest point on a wave

This image shows what the molecules look like in a sound wave:

sound wave diagram

Image by Pluke [all labels added], via Wikimedia Commons, was releases into the public domain.

In addition to knowing the four parts of a compression wave, you also need to be able to describe its frequency and wavelength.

Wavelength describes the distance between two compressions or two rarefactions, also called a cycle. A compression can be measured from any point on a wave, as long as it is measured to the same point on the next wave. Look at the image of the molecules above. Point to where the wavelength is labeled.

When measuring wavelengths, use the metric units millimeters and centimeters, unless another unit is specified. You have to know what a wavelength is in order to be able to find the frequency.

Frequency is the number of wavelengths in a given unit of time, usually measured in cycles per second, or Hertz. Look back at the picture of the sound wave from the beginning of the lesson.

  • What did you say was the frequency of that sound wave?

Now, count the frequency. You need to know that the time measured is one second. Tell the answer to your teacher or parent.

  • Did you say four cycles per second?

That's correct! The sound wave pictured has a frequency of four because there are four wavelengths shown.

Continue looking at the sound wave at the beginning of the lesson. Show your teacher or parent where there is a compression, rarefaction, crest, and trough. As you point to each part, explain to your teacher or parent what it is.

Then, move on to the Got It? section to practice labeling more sound waves.

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