Labeling Compression Waves

Lesson Plan - Get It!

Audio:

Consider what you already know about the term frequency.

• How could this term relate to sound?

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?

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.

• Can you name 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.

Great job.

• 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:

^{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.

• 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. Point to where there is a compression, rarefaction, crest, and trough. As you point to each part, explain what it is in your own words.

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