Thundering Tube

Introduction

When a tree falls in a forest and no one is around to hear it, does it make a sound? The same question can be asked of a vibrating spring. Does a vibrating spring produce sounds even though they cannot be heard? Use the Thundering Tube amplifier to find out!

Concepts

• Sound pitch
• Resonance
• Amplification

Background

Many musical instruments work because air is vibrated in an air column and then the length of the air column is varied to change the sound produced. The length of the air column determines the pitch of the sound that is heard from the vibrating air. A mixture of different frequencies and the resonation of air columns on a particular set of frequencies can turn noise into music. The sound produced is the loudest when the air column is in resonance (in tune) with the vibrational source.

How does resonance occur? A vibrating source produces a sound wave. This wave of alternating high- and low-pressure variations moves through the air column. Sound waves are often depicted as a sine wave as shown in Figure 1. The sound wave is ultimately reflected back toward the vibrational source. It is either reflected back off a closed end of the column or as a low-pressure reflection off the open end of the column. If the reflected wave reaches the vibrational source at the same moment another wave is produced, then the leaving and returning waves reinforce each other. This reinforcement, known as resonance, produces a special wave—a standing wave. A standing wave is a wave 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, half-way between two nodes, where the largest amplitude occurs.

The Thundering Tube acts as an open-closed-end air column. One end is open while the other end is covered by a latex sheet. Therefore, the sound frequencies that resonate inside the column are equal to nv/4L, where n is an odd harmonic number (i.e., 1, 3, 5, etc.), v is the speed of sound in the air column, and L is the length of the column. As the length of the air column increases, the sound frequency, also known as the pitch, that resonates within the column decreases. An example of an instrument that generates small sound frequencies, or low-pitched sound, is a tuba. High frequency sound is generated by a flute. A tuba is composed of a very long air column, whereas a flute is very short in comparison.

The vibrating spring, by itself, produces very faint sounds. However, when one end of the spring is attached to a flexible latex sheet (a “drum”) that is placed over the end of an air column, the vibrating sound frequencies of the spring become amplified. The sound frequencies that become the loudest are the ones that are in resonance (in tune) with the air column. The long tube will resonate the low-frequency sound and a short tube will resonate the high-frequency sound. It is important to note that all the original sound frequencies of the vibrating spring are still present in the air column. However, only the sound frequencies that are in tune with the air column become amplified. As the length of the Thundering Tube changes, different sound frequencies resonate and the pitch changes.

The thunder-like sound is produced because more than one sound frequency resonates in the air column. Numerous frequency harmonics (n = 1, 3, 5, 7, etc.) resonate inside the air column at once and the overall sound that is heard is the combination of all these frequencies as they reach the ear drum.

Materials

Safety Precautions

The materials for this demonstration are considered non-hazardous. Do not pull too hard on the spring. This will prevent the spring from snapping together quickly when it is released, which could result in tearing the latex or cause the spring to “launch” across the room like a projectile.

Disposal

The materials may be saved for future demonstrations. To store the Thundering Tube, remove the rubber band from the latex sheeting. Separate the latex sheeting from the tube and the spring and store the latex sheeting flat. This will prevent the latex sheeting from tearing or weakening over time, so that it may be used for future demonstrations. To prevent the spring from becoming tangled, place the spring into a large envelope where it can remain fully extended and not bunched up.

Prelab Preparation

1. Securely tape one end of the spring to the center of the 15 cm x 15 cm latex sheet. Use needle-nose pliers, if necessary, to straighten or flatten out a portion of the end of the spring so that it lies flat on the latex (see Figure 2).
2. Once the spring is securely attached to the latex, insert the spring into the wider cardboard tube (see Figure 3).
3. Stretch the latex slightly, and pull it down around the end of the wider of the two tubes. The spring should hang through the center of the tube.
4. Holding the latex stretched around the end of the tube, slide the rubber band over the latex to secure it to the tube (see Figure 4). Note: This may require a second pair of hands.
5. Hold the tube vertically so the spring hangs down.
6. Pull down on the spring approximately 2 cm, and then release it. (A thunder-like noise should be produced.)
7. Inspect the latex and spring to make sure they are still securely taped together.
8. Insert the thinner tube into the wider tube at the end opposite the latex. The spring should hang through the center of both tubes (see Figure 5).
   {13245_Preparation_Figure_2}

Procedure

1. Remove the inner tube.
2. Place a hand over the latex sheet to prevent it from vibrating. Pull down on the spring a few centimeters and release it. (The spring will vibrate and produce slightly audible noises.)
3. Reinsert the inner tube into the wider tube.
4. Hold the Thundering Tube vertically so the spring hangs down. It will be necessary to hold, with one hand, the bottom of the wide tube and the top of the thin tube as shown in Figure 6.
5. Pull down on the spring a few centimeters and then release it. A rumbling sound—similar to that of thunder—should be heard.
6. Pull the spring down farther and release—the rumble or “thunder” will be louder.
7. ull the telescoping tube out to make it longer and repeats steps 5 and 6.
8. Push the telescoping tube together to make it shorter and repeat steps 5 and 6.
9. Reposition the telescoping tube to make a “medium-size” Thundering Tube.
10. Pull the spring down several centimeters and release. As the rumbling sound resonates in the tube, quickly adjust the telescoping tube to make the tube longer and/or shorter.
11. Repeat step 11 as often as necessary so students can hear the change in pitch as the spring continues to vibrate and the length of the Thundering Tube changes.
12. Another way to maintain the “rumbling” is to gently shake the Thundering Tube.
13. Discuss resonance and the functions of the parts of the Thundering Tube with students.

Student Worksheet PDF

13245_Student.pdf

Teacher Tips

• Use several pieces of tape to secure the spring to the latex sheet. However, make sure the pieces are small so they do not affect the flexibility of the latex sheet significantly. Transparent tape works better than masking tape when taping to the latex.
• Tape the spring to the outside of the latex sheet and “play” the Thundering Tube in the same manner. Does it produce the same sound? Is it louder or quieter?
• Various springs may be experimented with using the Thundering Tube.
• Resonance may also be achieved using a tuning fork.
• Use this fun demonstration to begin (or end) discussions about resonance tubes and wave properties.
• Additional latex sheets may be purchased from Flinn Scientific (Catalog No. AP4573). The latex from a latex glove can also be used as replacement material.

Answers to Questions

1. Describe the sight and sound of the lone spring after it is stretched and then released.

   The spring moves and vibrates. A faint metallic sound is heard.

2. Describe the sight and sound of the Thundering Tube after the spring is stretched and then released.

   The spring again moves and vibrates. A loud rumble is heard, similar to the sound of thunder.

3. How does the pitch (the frequency of the sound) change when the Thundering Tube is lengthened?

   As the Thundering Tube length increases, the thundering sound becomes deeper and a little louder, like that of a bass instrument.

4. How does the pitch change when the Thundering Tube is shortened?

   As the length of the Thundering Tube decreases, a higher pitch is heard. It sounds more muffled, like that of the “ocean” heard by placing your ear inside a cup or a Conch shell.

5. As different sound frequencies resonate inside the Thundering Tube (depending on its length), what happens to the other sound frequencies that are heard when the tube is at a different length. Do the sound waves disappear?

   The sound frequencies produced by the vibrating spring are always present. However, they only frequencies that are amplified by the Thundering Tube are ones that match the length of the tube and generate a resonating or standing wave. The sound frequencies that are not “in tune” are concealed by the much louder resonating frequencies.