The Resonator

Introduction

Have you ever noticed a single object in your home vibrate vigorously due to the vibrations of loud music? Why is it that only certain objects vibrate and not all objects? Why do the vibrations occur only during a particular note in the song? This phenomenon is known as resonance. Amaze your students with this simple demonstration of resonance.

Concepts

• Natural frequency
• Resonance
• Vibration

Background

All objects have a natural frequency or set of natural frequencies at which they vibrate. The natural frequency of an object is dependent upon its size and what it is made of. If an object is forced to vibrate at its natural frequency, a vigorous vibration, known as resonance, will occur. An object can be forced to vibrate by a push, pull, pluck, strum or even from the vibrations of another object. When resonance occurs, it can often be seen as a dynamic vibration or even heard as a humming sound.

This kit contains two demonstrations in which resonance is visibly shown using a series of wooden dowels. Demonstration 1 illustrates how the length of a material affects its natural frequency. Four 3⁄16" diameter wooden dowels of different lengths are placed into a wooden base and moved back and forth, causing the dowels to vibrate. When the back and forth motion of the base matches the natural frequency of one of the wooden dowels, that particular dowel will vibrate vigorously, achieving resonance. By varying the frequency of the back-and-forth motion of the base, different wooden dowels will resonate at different times.

Demonstration 2 illustrates how the diameter of a material affects its natural frequency. In this arrangement, the dowel length is kept constant while the diameter is varied. Two dowels that are 18" long by 3⁄16" in diameter are placed near two dowels that are 18" long by ⅛" in diameter. When the base of the resonator is moved back and forth, dowel rods of the same diameter and length will resonate together.

Materials

Safety Precautions

Wear safety glasses when performing this demonstration. Students sitting or standing near the demonstration should also wear safety glasses. Be cautious when doing the demonstrator—the wooden dowels may break if the base is shaken harshly. Follow all laboratory safety guidelines.

Disposal

All materials in this kit should be saved for future use.

Prelab Preparation

1. Carefully poke a hole into the center of each sponge ball using a screwdriver or a nail. The sponge ball should puncture very easily. The hole should penetrate about halfway into the ball.
2. Place a sponge ball on one end of each wooden dowel included in the kit.

Procedure

Demonstration 1

1. Obtain the four 3⁄16" diameter wooden dowels of different lengths (18", 24", 30" and 36").
2. Place the dowel rods into the four aligned 3⁄16" holes in the wooden base as shown in Figure 1. If a wooden dowel does not fit into its proper hole, the wood may have swelled. Use the sand paper provided to wear down the circumference of the dowel until it will fit into the proper predrilled hole.
3. Secure the bottom of each wooden dowel to the base by placing some modeling clay around the dowel.
4. Measure the length of each wooden dowel and have students record this information on the proper section of The Resonator Worksheet.
5. Place the wooden base on a flat surface and slide the base back and forth as shown in Figure 1. Start with a low frequency and gradually increase the frequency until the 36" dowel rod begins to resonate. This will take some practice in order to find the correct timing. When the 36" dowel rod is vibrating vigorously, keep this frequency constant for 10–15 seconds and allow the students to observe. Have students record observations.
6. Gradually increase the frequency of the back-and-forth motion of the base until the 30" dowel rod begins to resonate. (The 36" dowel will stop resonating.) When this occurs, keep the frequency constant for 10–15 seconds and allow the students to observe and record their observations.
7. Again, gradually increase the frequency of the back-and-forth motion of the base until the 24" dowel rod begins to resonate. Keep the frequency constant for 10–15 seconds and allow the students to observe and record their observations.
8. Gradually increase the frequency of the back-and-forth motion of the base until the 18" dowel rod begins to resonate. Keep the frequency constant for 10–15 seconds and allow the students to observe and record their observations.
9. Remove the wooden dowels and clay from the base.

1. Obtain two 18" long wooden dowels that are 3⁄16" in diameter and two 18" long wooden dowels that are ⅛" in diameter.
2. Place the dowel rods into the proper holes in the wooden base as shown in Figure 2.
3. Secure the bottom of each wooden dowel to the base with some modeling clay.
4. Measure the diameter of each wooden dowel and have students record this information on the worksheet.
5. Place the wooden base on a flat surface and slide the base back and forth as shown in Figure 2. Start with a low frequency and gradually increase the frequency until the ⅛" diameter dowel rods begin to resonate. This will take some practice in order to find the correct timing. When the ⅛" diameter dowel rods are vibrating vigorously, keep the frequency of the base constant for 10–15 seconds. Allow the students to observe and record their observations.
6. Gradually increase the frequency of the back-and-forth motion of the base until the 3⁄16" diameter dowel rods begin to resonate. When the 3⁄16" diameter dowel rods are vibrating vigorously, keep the frequency of the base constant for 10–15 seconds. Allow the students to observe and record their observations.

Student Worksheet PDF

12698_Student1.pdf

Teacher Tips

• This kit contains enough materials to perform the demonstration indefinitely. All the materials are completely reusable.
• A wooden dowel will only resonate when the frequency of the back-and-forth motion of the base matches the natural frequency of the dowel, therefore proper timing is very important. Practice this demonstration before performing it in front of the class. The shortest dowel requires a rapid back-and-forth motion to achieve resonance.
• For further concept development, try vibrating various combinations of wooden dowels. For example place the two 18" long, ⅛" diameter dowels in the base along with the 30" and 24" long 3⁄16" diameter dowels. This will show that dowels of different lengths can resonate together, but the diameter must vary in order to do this.
• If a wooden dowel breaks, replacement dowels are easily available at most arts and crafts stores.
• Do not glue the wooden dowels into the holes of the base. The wooden dowels used in the first demonstration must be removed in order to perform the second demonstration.

Answers to Questions

Demonstration 1

1. Record the length of each wooden dowel used in Demonstration 1.
   

   36", 30", 24" and 18"

2. Which dowel resonated first (at the lowest frequency)?
   

   36" (longest)

3. Which dowel resonated last (at the highest frequency)?
   

   18" (shortest)

4. As a wooden dowel achieved resonance, what did you observe about the other dowels?
   

   As a wooden dowel achieved resonance, the other wooden dowels were either vibrating slightly or not moving at all.

5. At any point during the demonstration, did two or more dowels resonate at the same time?
   

   No, two or more dowels did not resonate at the same time. As one dowel resonated, the others were motionless or vibrating slightly.

6. Other than the length of the dowel, what variable affects the resonance of each dowel?
   

   The frequency of the back-and-forth motion determines which dowel will resonate.

Demonstration 2

1. Record the diameter and length of the wooden dowels used in Demonstration 2.
   

   All dowels in this demonstration are 18" long. The diameters of the thin dowels are ⅛", and the diameters of the thick dowels are 3⁄16".

2. Which dowel(s) resonated first (at the lowest frequency)?
   

   The ⅛"-thin dowels resonated first when the instructor started moving the base.

3. Which dowel(s) resonated last (at the highest frequency)?
   

   The 3⁄16" diameter dowels resonated last.

4. At any point during this demonstration did two or more dowels resonate at the same time? Explain your observation.
   

   Yes, the wooden dowels that have the same diameter resonate together. Therefore they have the same natural frequency.

5. What caused the different dowels to resonate at different times?
   

   The instructor changed the frequency of the back-and-forth motion of the base in order to resonate different dowels.

Post-Lab Analysis

1. Based on your observations in Demonstration 1, do any of the dowels share the same natural frequency? Explain your answer.
   

   No, the dowels do not share the same natural frequency. They all resonate at different frequencies.

2. Based on your observations in Demonstration 2, do any of the dowels share the same natural frequency? Explain your answer.
   

   Yes, wooden dowels that have the same diameter and length resonate together. Therefore they have the same natural frequency.

3. What characteristics are necessary in order for two dowel rods to resonate at the same time?

Wooden dowels must have the same length and diameter in order to resonate together.

4. Using the same dowels provided in this demonstration kit, describe how an experiment could be set up to test if dowels of different lengths and different diameters share the same natural frequency.

Individually test each of the 3⁄16" diameter wooden dowels next to the 18" long ⅛" diameter dowel. Observe if any of the 3⁄16" dowels resonate at the same time as the ⅛" dowel when the base is moved back and forth. If resonance occurs, the two dowels share the same natural frequency.

5. Describe the relationship between the length of an object and the frequency causing the object to resonate.

The longer the object, the lower the frequency needed to cause resonance.

References

www.exploratorium.edu (accessed January 2008).