SOUND Introduction In Unit 3 you learned about energy and the many forms that energy can take. In Unit 4 you will learn how energy moves from one object to another object, or from one system to another system. One way that energy can move from one place to another is through sound. Below you will find an overview of the concepts and properties of sound that are relevant to the needs of an elementary school teacher. Following this overview you will be given an opportunity to put together your own activities or demonstrations that illustrate concepts and properties associated with sound. A Model for Sound Sound is just one of the many ways in which energy can move from one place to another. Sound energy can travel through gases, liquids, or solids, but it is perhaps easiest to consider sound moving in a gas. Sound in a gas is created when a moving object bumps into some of the gas particles Some of the kinetic energy of the moving object is given to the gas particles, and the gas particles begin to move in the same direction as the object was moving. These gas particles continue to move in this direction until they collide with something else (usually more gas molecules). This next set of gas molecules then also begins to move in the same direction because they have receive some of the original energy of the moving object. As this process continues on (gas molecules bumping into more gas molecules), energy is moving through the gas. This moving energy is what we call sound! The next several diagrams illustrate the creation and movement of sound energy in a gas. The first sketch below shows a rectangular solid surrounded by a gas. In order to make it easier to distinguish between the solid and the gas, the individual atoms making up the solid have not been shown. Do keep in mind, however, that the solid is made of particles, just as the gas is. At the outset, the gas particles are moving about randomly. IV-1 CALVIN COLLEGE
Imagine now that the solid suddenly moves a short distance to the right. The gas particles near the right-hand side of the solid will suddenly get pushed to the right when the moving solid hits them. The result is that many of the gas particles will get pushed closer together than usual, and they will begin moving to the right. Before the gas particles were hit, they were moving about randomly due to their thermal energy. After they are hit, they are still moving randomly, but they also acquire a general motion to the right. In terms of particle motion, this is what distinguishes thermal energy from sound energy. The gas particles that have been hit will now move towards the right until they, too, collide with other particles. This second group of particles will begin moving to the right as the first group of gas particles rebounds back. In this way, the sound energy continues to move to the right. UNIT 4 ENERGY AND INTERACTIONS IV-2 CALVIN COLLEGE
Sound energy in a liquid or a solid is created and moves similarly to how it is created and moves in a gas. In solids and liquids, however, the particles are virtually in contact with each other before the sound is created. Therefore, these particles do not move as far as the gas particles do before they collide with other particles, but they do move slightly. But, no matter whether sound is traveling in a gas, a liquid, or a solid, there is never any overall movement of the particles making up the medium. That is to say, although the particles carrying the sound energy do move until they collide with other particles, they do not move very far before this happens, and soon after they collide, they bounce back and end up about where they started from. Therefore, when sound energy moves through a medium, there is no overall transfer of matter. A human hears sound when gas particles inside the ear canal are suddenly pushed against the eardrum by the moving sound energy. Not surprisingly, sound carrying a lot of energy tends to be loud to our ears, and low energy sound seems soft to our ears. A human s ability to detect sound depends on a number of criteria, and an accurate measure of sound energy can only be made with special instruments called decibel meters. Vibrations The description of the creation and propagation of sound given up to this point is correct in regards to general details, but it is slightly misleading in one way. It is very hard, in fact almost impossible in most cases, to make a solid suddenly move in just one direction in order to create sound. Generally, the object creating the sound is vibrating back and forth. When this occurs, there is not a just single pulse of sound energy, but rather there are a whole series of energy pulses that are created. These pulses are called sound waves. The frequency of a sound wave refers to how rapidly the object creating the sound is vibrating. A rapidly vibrating object will create sound of high frequency, and the traveling pulses of energy will be very close together. A slowly vibrating object will create sound waves of low frequency. The speed of the sound depends only on the properties of the medium the sound is traveling through. It does not depend on the frequency of the sound. Pitch refers to the frequency of sound detected by the ear. High pitches correspond to high frequency vibrations, and low pitches correspond to low frequency vibrations. When the frequency of sound becomes very high (above about 20,000 vibrations per second = 20,000 hertz) or very low (below about 20 hertz), humans are no longer able to hear the sound. However, there are many applications for sound energy that have frequencies too low or too high to hear. One example is the medical use of ultrasound, which uses frequencies form about 1 to 20 million hertz An echo occurs when a sound wave hits an object that causes the particles carrying the energy to bounce off and start moving in the opposite direction. Echoes are most likely to occur when a sound wave traveling in one medium hits a second medium that is very different from the medium it is already in. A familiar example of this is when a sound wave traveling in the air strikes the side of a large building or a large rock face. There are many ways to demonstrate that vibrating objects produce sound. For example, a ruler that is partially extended over the side of a table and plucked will vibrate and create a sound. The frequency of the vibration is determined by the amount of the ruler that is extended UNIT 4 ENERGY AND INTERACTIONS IV-3 CALVIN COLLEGE
over the edge. It is easy to hear and see that the pitch of the sound created corresponds to the frequency of the vibration of the ruler. You can feel the vibrations creating the sound of your voice by placing your finger on your neck or on your lips as you hum or create a tone with your vocal cords and lips. See if you can detect the change in frequency as you raise or lower the pitch. The vibrations can be amplified if you speak into a balloon pressed against another person's ear. UNIT 4 ENERGY AND INTERACTIONS IV-4 CALVIN COLLEGE
Below are some concepts appropriately taught in elementary school: 1. Sound is produced by vibrations (of a source). 2. Sound waves can produce vibrations in objects. 3. Sound can be described by properties of pitch and loudness. 4. Pitch is varied by changing the rate of vibration The following concepts related to sound may be taught in middle school: 5. Sound can travel through various media--but it does require a medium to travel in. 6. The speed of sound depends on the medium. 7. Sound can reflect to cause echoes. Other ideas and concepts not specifically mentioned in the national standards for science teaching, but which you may wish to cover include: 8. Human perception of loudness and the energy of sound at the ear are related, but they are not the same thing. 9. Mechanics of how the human ear works. 10. Pitch and intensity range of human ear; comparison with other creatures. 11. Sounds can also be described in terms of tone. Find some activities related to sound in elementary school textbooks or in trade books and present them to the rest of the class. Make sure the activities cover many of the concepts listed above. UNIT 4 ENERGY AND INTERACTIONS IV-5 CALVIN COLLEGE
Sound Homework 1. If two people stand at either end of a long narrow tunnel, they can hear each other talking quite easily. If these same people were the same distance apart in an open field, they could not hear each other nearly so well. Explain why this is so by referring to the reflective property of sound. 2. In general, sound travels faster in solids than it does in liquids, and it travels faster in liquids than it does in gases. Use your particle model of matter to try and explain why this is so. 3. Use your particle model of matter to explain why tightening a guitar string causes the pitch to go up. 4. Children often enjoy talking on a telephone made from two metal cans and a string. The children can hear each other well even if they whisper when they are far apart. Explain how this device works. 5. a. Why do you normally see lightening before you hear the thunder? b. If the speed of sound in air is 1100 ft/sec, about how long after a lightning flash will you hear the thunder if the lightning occurred 1 mile away? 6. Why is it noisier to have a classroom with bare floors and no drapes than it is to have one with carpet and drapes? 7. Why is it quieter after a snowfall? 8. Why is there no sound in outer space? UNIT 4 ENERGY AND INTERACTIONS IV-6 CALVIN COLLEGE