Materials Included In Kit

Additional Materials Required

Prelab Preparation

Demonstrate to the students how to properly lift the white PVC pipe out of the water while lifting the tuning fork. As this technique is demonstrated, have the students listen for the change in loudness of the sound. The students should be close during the demonstration because individuals who are far away commonly have trouble hearing the change in volume.

Safety Precautions

This lab is considered to be nonhazardous. Please follow all laboratory safety guidelines.

Disposal

After the activity is complete, the water in the tube setup can be disposed of down the drain. All other materials in this activity may be dried and saved for future use.

Lab Hints

• Enough materials are provided in this kit for one group of students. This activity may be reused year after year.
• This laboratory activity can reasonably be completed in one 50-minute class period.
• If time is a factor, the lab station can be set up ahead of time and remain assembled for other students throughout the day or week.

Teacher Tips

• This activity will only work using tuning forks with a frequency of 256 Hz or higher.
• For an extension activity, this experiment can be used to explain how the length of a musical instrument is related to the sound produced by the instrument. Our data shows resonance of a high frequency tuning fork occurs in shorter air columns while resonance of a low frequency tuning fork occurs in longer air columns. This agrees with many wind instruments. Short instruments tend to produce high frequencies or pitches, while longer instruments tend to produce low frequencies or pitches.
• For further concept development, try the “Wave and Sound” Student Laboratory Kit (Catalog No. AP7014) and the “Open-Ended Resonance Tube Set” (Catalog No. AP4616) available from Flinn Scientific.

Answers to Prelab Questions

1. Can a sound wave travel in a vacuum? Explain your answer.

   Sound cannot travel in a vacuum. A vacuum is a volume of space that is empty of matter. Without matter, sound cannot travel.

2. What type of interference occurs at a node? What type of interference occurs at an antinode?

   Destructive interference occurs at a node, and constructive interference occurs at an antinode.

3. Which will produce the loudest sound, a node or an antinode?

   An antinode will produce the loudest sound because it is a point of constructive interference.

4. When you hear the sound resonate loudly in this laboratory activity, what fraction of a wavelength is present within the air-filled tube?

   In this activity, one-fourth of a wavelength is present in the air-filled tube when the sound resonates loudly.

Sample Data

Answers to Questions

1. Calculate the wavelength for the 512 Hz tuning fork. Show the formula used, substitution with units, and label the answer with the correct units.

   λ = 4 L = 4(0.162 m) = 0.648 m

2. Calculate the speed of sound for the 512 Hz tuning fork. Show the formula used, substitution with units, and label the answer with the correct units

   v = ƒλ = (512 Hz)(0.648 m) = 332 m/s

3. Observe the length and frequency of each tuning fork. What is the relationship between the length of a tuning fork and its frequency?

   Longer tuning forks have lower frequencies while shorter tuning forks have higher frequencies.

4. Look at the data in the frequency and wavelength columns of the data table. As the frequency increases, what happens to the wavelength?

   As the frequency increases, the wavelength decreases.

5. Notice the similarity between the speeds of sound calculated for each tuning fork. Using the formula v = ƒλ, and your answer from Question 2, explain how sounds of different frequencies can have similar speeds.

   According to the formula, the speed of sound depends on the frequency and wavelength of the sound. If the frequency of sound increases, the wavelength decreases. If the frequency decreases the wavelength increases. Because of this, sound of various frequencies can have the same speed.

6. The actual speed of sound in air at STP is 331 m/s. How does the average measured speed of sound in air compare to the accepted value? Calculate the percent error between the measured and accepted speed of sound.

   My average speed of sound in air is 335 m/s, which is very close to the accepted value of 331 m/s. Student answers may vary.

   {12694_Answers_Equation_3}
7. In general, do you think the speed of sound would increase or decrease if it travels through a liquid or solid? Explain the reasoning for your answer.

   The speed of sound tends to increase in liquids and solids compared to gases. The reason for this is because molecules that are close together bump into each other more easily than molecules that are far apart.

Introduction

Have you ever observed a carpenter hammering a nail from off in the distance? If you are far enough away from the carpenter, you will observe the hammer hit the nail before you hear the sound it makes. It seems as though there is a delay in the time it takes for the sound to reach your ears. Why does this happen? How fast does sound travel? Discover the speed of sound with this activity!

Concepts

• Antinode and node
• Mechanical and longitudinal waves
• Speed of sound
• Frequency and wavelength
• Standing wave

Background

Sound is a mechanical wave created by the vibrations of material objects. A mechanical wave requires a medium in order to propagate. In other words, for sound to travel, some type of substance must be present (solid, liquid or gas). A substance is needed because sound propagates by pushing molecules back and forth. If there are no molecules to move, such as in a vacuum, sound will not travel.

As a sound wave propagates, its speed will vary depending on the medium in which it travels. Sound tends to have higher speeds in solids and slower speeds in liquids and gases. This is because molecules that are close together bump into each other more frequently than molecules that are far apart. Many people wonder how the speed of sound is measured when you cannot see it. In this experiment, you will learn how this can be accomplished.

The speed of sound can be calculated using Equation 1 below. According to this formula, if the frequency (ƒ) and wavelength (λ) of a wave are known, the speed (v) can be calculated by multiplying the two values together. In this activity you will be using various tuning forks to determine the speed (v) of sound in air. When a tuning fork is set into motion, the sound produced will have a specific frequency and wavelength. The frequency of a tuning fork is usually printed on it. The wavelength, on the other hand, must be determined in order to calculate the speed of sound.

v = speed (m/s)
ƒ = frequency (Hz)
λ = wavelength (m)

In this experiment, a piece of white PVC tubing placed in water. The water is used to close off the tubing at one end (see Figure 1). A tuning fork is used to generate a sound wave over the open end of the white PVC tube. The length of the white PVC tube is altered by slowly lifting it out of the water. As the PVC tube is lifted, the length of its air-filled portion increases. The sound wave travels through the air in the tube and reflects off the water at the closed end. At the appropriate length (this length varies for tuning forks of different frequencies), the reflected wave interferes with the incident waves generated by the source (the tuning fork), and a standing wave forms. A standing wave is a pattern that results when two waves of the same frequency, wavelength, and amplitude travel in opposite directions and interfere with each other. A node is a point in a standing wave that always undergoes complete destructive interference and therefore is stationary. An antinode is a point in the standing wave, halfway between two nodes, at which the largest amplitude occurs due to constructive interference (see Figure 2). Because the amplitude is largest at an antinode, the sound will be the loudest at this point. Figure 3 represents various standing waves that can be created in a close-ended column of air. When an antinode is present at the open end, the sound will resonate or hum loudly. The Procedure section of this lab lists the steps necessary to create a standing wave having only one node and one antinode, as shown in the top section of Figure 3. Creating a standing wave with one node and one anti-node will mean that only ¼ of a complete wavelength is present inside the air-filled PVC tube. If the length (L) of the air-filled portion of the PVC tube is measured in meters, this will be the length of ¼ of one complete wavelength. In order to calculate one complete wavelength, the tube length (L) must be multiplied by 4 (Equation 2).

λ = wavelength (m)
L = tube length (m)

Experiment Overview

The purpose of this lab activity is to determine the speed of sound in air.

Materials

Prelab Questions

1. Can a sound wave travel in a vacuum? Explain your answer.
2. What type of interference occurs at a node? What type of interference occurs at an antinode?
3. Which will produce the loudest sound, a node or an antinode? Explain your answer
4. When you hear the sound resonate loudly in this activity, what fraction of a wavelength will be present within the airfilled tube?

Safety Precautions

This lab is considered to be nonhazardous. Please follow all laboratory safety guidelines.

Procedure

1. Set up a support stand and attach one universal extension clamp to the top of the rod, and a second universal extension clamp to the bottom of the rod.
2. Place a rubber stopper in the bottom of the clear plastic tube.
3. Attach the clear plastic tube to the support stand using the universal extension clamps (see Figure 4). The rubber stopper should be resting on the base of the support stand.
   {12694_Procedure_Figure_4}
4. Place the white PVC tube inside of the clear plastic tube.
5. Fill a large graduated cylinder with 200 mL of water.
6. Make sure the end of the clear plastic tube is completely sealed by pouring a small amount of water into the tube and watching for any leaks. Petroleum jelly may be put around the edge of the stopper if leaking does occur.
7. Pour the rest of the water from the graduated cylinder into the sealed plastic tube. The water should be near the top of the tube but not overflowing.
8. Obtain a tuning fork that matches a frequency listed in the data table on the Discovering the Speed of Sound Worksheet.
9. Hit the tuning fork on a tuning fork activator.
10. Hold the tuning fork over the tube setup. Hold the white PVC tube with your free hand and slowly lift the tube out of the water while at the same time lifting the tuning fork. Make sure the tuning fork remains over the opening of the white PVC pipe (see Figure 1). Keep moving the tube and the tuning fork upward until a very loud humming noise is heard. There may be a faint hum the entire time, but at a certain point it will get very loud.
11. Repeat step #10 to until you have found the precise position where the sound is the loudest.
12. Hold the tube in place at the position where the sound resonates the loudest. Using a metric ruler, measure the length of the white PVC tube that is above the surface of the water. Measure the length (L) in centimeters, and record the length in both cm and meters in the data table.
13. Repeat steps 8–12 with the remaining tuning forks.
14. Using Equation 2 from the Background section, calculate the wavelength of sound for each tuning fork.
15. Using Equation 1 from the Background section, calculate the speed of sound in air for each tuning fork.
16. Using the speed of sound for all tuning forks, calculate an average speed of sound and record this value in your data table.
17. Consult your instructor for appropriate disposal procedures.

Student Worksheet PDF

12694_Student1.pdf