Publication No. 13245
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!
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.
Latex sheet (rubber dam), 15 x 15 cm*
Spring, metal, (relaxed dimensions: 1 cm dia. x 28 cm long)*
Telescoping tube, cardboard*
*Materials included in kit.
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.
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.
Student Worksheet PDF
Correlation to Next Generation Science Standards (NGSS)†
Science & Engineering PracticesAsking questions and defining problems
Developing and using models
Analyzing and interpreting data
Planning and carrying out investigations
Constructing explanations and designing solutions
Disciplinary Core IdeasMS-PS2.A: Forces and Motion
MS-PS3.B: Conservation of Energy and Energy Transfer
MS-PS4.A: Wave Properties
HS-PS2.A: Forces and Motion
HS-PS3.B: Conservation of Energy and Energy Transfer
HS-PS4.A: Wave Properties
Cause and effect
Scale, proportion, and quantity
Energy and matter
MS-PS4-1. Use mathematical representations to describe a simple model for waves that includes how the amplitude of a wave is related to the energy in a wave.
Answers to Questions