A wave is a disturbance that transports energy from one place to another without the transfer of matter. After a wave passes through a medium, there are no residual effects; the medium remains unchanged. For example, if you throw a stone in a pond, a circular wave will spread out from the point of impact. If the wave encounters an object floating in the water, the object will briefly bob up and down. However, once the wave has passed, the object, and the water that buoys it up, will be left undisturbed.
We encounter waves everyday. Some are apparent; others go unnoticed. The room in which you are sitting is being criss-crossed by all sorts of waves. These include light waves, radio waves, and sound waves. While we have receptors for light and sound, our bodies are not capable of sensing radio waves directly. Some waves, such as sound and water waves, require a material medium. On the other hand, light and other forms of electromagnetic radiation can travel through the vacuum of space.
A fascinating feature of waves is that two of them, traveling in opposite directions, can pass right through each other and emerge with their original identities. However, while the pulses overlap, the height at any point is simply the sum of the displacements due to each pulse itself. If the pulses are on the same side of the medium they add; if they are on opposite sides, they subtract. This is called interference.
The importance of wave phenomena in everyday life cannot be overstated. It is estimated that human beings receive over 90% of their information from light and sound. The experiments that follow will allow you to gain first hand experience with the properties of waves in general and sound in particular.
1. Working with Waves
Key Concept: A wave is a disturbance that travels through a medium. Waves are characterized by wavelength, frequency, and amplitude. Waves reflect when they encounter a barrier or different medium.
A rope or a long spring may be used to demonstrate many properties of waves. Hold one end of the stretched medium in your hand while your partner holds the other end. Now move one end up and down at different rates (frequencies). What happens to the wavelength as you increase the frequency? Decrease the frequency?
Does the tension in the medium have any observable effect on the speed of a wave? To find out, send a sharp pulse down the medium when the medium is under various degrees of tension. What do you observe?
Send another sharp pulse down the medium. This time watch carefully as the pulse reaches the fixed end. Does the pulse reflect? If so, is the reflected pulse the same as the incident pulse or is it upside down? Do the incident and reflected pulses travel at the same speed?
2. The Unusual Way of Waves
Key Concept: The overlapping, or superposition, of two waves produces reinforcement in some instances and cancellation in others.
You can witness wave interference on a phone cord, rope, or Slinky. After producing one pulse on the medium, generate a second shortly thereafter. Watch carefully as the two pulses meet and pass through one another. How would you describe the medium when the two pulses overlapped? Did the pulses produce a larger or smaller resultant pulse? What procedure must you follow to produce constructive interference (a larger net pulse)? Destructive interference (a smaller net pulse)?
3. Nice Nodes
Key Concept: Standing waves are formed when two sets of identical waves pass through a medium in opposite directions.
Move the end of the medium of choice (rope, phone cord, Slinky) up and down at the right frequency to create a full wave. When this has been accomplished you will notice that the center of the medium appears to stand still. This stationary point is called a node. At a node, the destructive interference of the incident and reflected waves is total. On either side of the node are regions of maximum displacement called antinodes. Have someone gently pinch the node with their fingers. What happens?
Increase the frequency of the up and down motion of your hand until two nodes appear on the medium. How many antinodes are there now? How many wavelengths do you observe? What happens when you continue to increase the frequency of the waves? Can you obtain three nodes, four nodes, etc.? Describe what happens to the wavelength as the frequency is increased.
4. Are You Chicken?
Key Concept: A surface set in motion by a vibrating string amplifies sound.
Using a toothpick, puncture a small hole in the center of the base of a paper or plastic cup. Pull a ˝ meter, or so, length of string through the hole. With the cup turned upside down, tie the string around the toothpick (see figure).
Rub a little rosin on your thumb and index finger. Using a jerking motion, pull down on the string while gently pinching it between the thumb and index finger. Describe what you hear as the string moves between the fingers.
What is the source of the sound you hear? Why is the sound so loud? To answer this question, it may be helpful to pull on the string when it is not connected to the cup.
What's it sound like?
5. The Singing Rod
Key Concept: Longitudinal standing waves may be produced in metal rods.
Hold the aluminum rod with your fingertips at its center. (Note: The center may be located by balancing the rod in your hand in the region between the thumb and index finger as in the figure above.) Place some rosin on the tips of the thumb and index finger of the other hand. Grip one end of the rod between the rosin-covered fingertips and stroke the length of the rod between the end and center. With a little practice, you should be able to produce a piercing, high-pitched sound. If you are unable to get the rod to sing, tap the end of the rod with a mallet or hammer or tap it on a hard surface.
While the rod is “singing,” bring a ping pong ball suspended on a string in contact with an end of the rod. Describe what happens. Can you explain why this occurs? What did stroking or tapping the rod do to it? Grab the singing rod at a point off center. What happens? Why?
6. The Bells of St. Weber
Key Concept: The grate’s small surface area prevents efficient energy transfer to the air at low frequencies. The strings provide a direct pathway for sound transmission at a wide range of frequencies.
Pick up the grate from a barbecue grill by the strings. ( If you don't have a clean grill grating, coat-hanger or metal utensils work too!) With the grate hanging by the strings, knock it against the side of a table. Describe the sound you hear.
How does the sound produced by the vibrating grate reach your ears?
Now, with your index fingers, place the ends of the strings on the little flap of flesh that protrudes over the opening of each ear. Allowing the grate to hang freely from the strings, again swing the grate into the side of a table. Describe the sound you hear with the strings pressed against your ears. How is the sound reaching your ears? How do you explain the difference in sound quality?