J. Josˇ Bonner
Sound is invisible. One can make it visible by dipping a tuning fork into water, or putting pebbles on a drum or speaker, but these examples don't convey the whole story. These things vibrate, but students often see these as exceptions to the general rule that sound is invisible. The challenge is to make the connection between sound-as-vibration and all of the media through which sound can travel.
What follows is written as a discussion, almost as a type of script you can use while presenting this to your students. Of course, the precise phrasing is not as important as conveying the concepts, and will diverge from what is written here as students discuss the material.
First, build a string telephone.
Get a long string, and push one end through a hole in the bottom of a tin can or plastic cup. On the inside of the can or cup, tie a knot in the string so it can't be pulled through the hole. Then, tie another can or cup onto the other end of the string in the same way. Now, two people should be able to stand on opposite sides of the room, holding the string taut, and have a conversation. One person speaks into her cup, and the other person listens to hers. Have the students try it, and then figure out how it works.
1. How does the sound get from one end of the string to the other?
What do students think is likely? Let 'em kick around ideas.
Can they test their ideas?
To test the idea that vibrations travel from one end to the other, we can block the vibrations by bending the string around a corner, or over the top of a chair. This solid object won't vibrate enough and will stop the sound from traveling down the string.
To test the idea of vibrations making sound at all, have them consider a guitar (and, perhaps, in collaboration with the music teacher, work with a guitar to see that shorter strings produce higher-pitched sounds.)
If we could see the vibrations what would it look like? We might see vibrations of louder and softer sounds moving down the string, maybe from left to right, like this:
2. What do the vibrations do when they get to the other can?
Again, students can kick around ideas. Perhaps they will think of a drum (and, perhaps, the classic experiment of putting sand on a drum or speaker and watching them dance as sound is produced).
Again, what would this look like if we could draw it? Maybe something like this?
3. How, then, do the vibrations get from the vibrating can-bottom to the ear of the listener?
Use the same discussion/test/draw strategy. Here, the test might be to ask how you feel when a car with a loud subwoofer drives by. They tend to make you rattle--you can feel the air waves bashing into you.
But...how does air vibrate like this? What are the air particles doing? [Note: if you refer to air particles, having discussed them during the unit on matter and mass, they should understand the concept. If you refer to particles at first, and then sometimes say "molecules" instead of particles, and then switch to using "molecules" automatically, they will learn that "molecule" refers to these tiny particles. In fact, the word molecule is Latin, and means, literally, "tiny mass."]
How can we model air molecules bumping into one another to transmit a "bump" from one place to another? What about those desktop toys that have 5 balls hanging on strings, and that bump each other?
In air, of course, there are bazillions of particles, but they still bump into each other and transmit the vibrations. When the can bottom (or speaker or drum head) bumps against air molecules, they bump against more air molecules, which bump against more. This transmits the first bump through the air. Sound moves similarly through water or solids--by transmission of molecular bumps. When the molecules are actually bumping each other, they are close together--compressed, so to speak. Therefore, this wave of molecular bumping is called a compression wave.
Our ears respond to the air vibrations by converting them into nerve impulses that we recognize as sound.
4. How does the speaker's voice get to the can and the string?
By the same process, but in reverse.
Do vocal cords vibrate? Hum, and feel them. Yep. They vibrate.
The rest is bumping air molecules, air molecules bumping into the can bottom and making it vibrate, and this making the string vibrate.
The Rationale Behind the Strategy
"Science" is not simply learning facts; it is building understanding from the evidence. The evidence need not be complicated; here, we ask students to think about a number of observations that they have undoubtedly made many times before. This is evidence from which they can build their understanding. To work with evidence, it is necessary to build explanations that can explain the observations. It is these explanations that we call Scientific Knowledge.
The strategy here is to provide students with an intriguing observation--the string telephone--and ask them to kick around ideas about how it might work. It may be necessary to demonstrate some of the basic principles to help them along--such as guitar strings, or dipping a tuning fork into water to produce waves, or watching pebbles dance on the surface of a drum. Once the basic notion of sound-as-vibration is in place, it becomes a matter of reasoning how different materials can vibrate. The telephone provides an intriguing tool to think about many different media.
Will students be able to think up their own explanations? Probably, but they may be shy about it until they are comfortable with this method of teaching science. If they become stumped, think out loud to "model" how you think your way through it.
A large part of thinking through this kind of problem is visualizing what's happening. The drawings are very important. Students need to become comfortable with the skill of visualizing processes in their minds.
Note that it is tremendously important to ask students to construct their own understanding from the evidence (even if it requires some verbal thinking by the teacher). This is essential to students' long-term learning, and to their development of skills in problem-solving and scientific reasoning. (If we simply tell them the answers, they may be able to answer test questions in a few days, but they are unlikely to retain the learning for a significant length of time. Nor will they have the opportunity to practice the thinking skills that they will eventually need in the world they will enter as adults.) However, we do not leave their knowledge-construction to chance. We designed the learning experience to reach a goal; we need to ensure that it gets there.
The most helpful strategy here might be to give students a new sound-maker, and ask them to draw a picture of how they visualize this thing producing sound. A tuning fork would be a good starting point--the lesson above doesn't use such a picture, though it may involve using a tuning fork to show vibrations. Another might be hands clapping, or a single clap. Perhaps, with a video clip of an old Western movie, one could show a person listening to the ground to determine whether horsemen are approaching, and ask students to draw how the rock and soil might transmit sound. [A modern analog is listening to railroad tracks, but the potential for mishap suggests that we not mention this.] To turn this exercise into a reinforcement strategy, we can ask the students to explain what their drawings show--or perhaps ask the teacher to interpret the drawings, thinking out loud about each of those she chooses to describe to the class.
As with verbalization, the use of artwork is limited by students' facility with the medium. They may know what they want to articulate, but have difficulty expressing it in text or pictures. One should be very cautious about considering any of these to be "wrong," unless, of course, it illustrates a very clear misconception.
What else is it important to know about sound? Probably the musically-useful part, for which guitars, recorders, tuning forks, etc are useful. The shorter the vibrations, the higher the sound. Short strings (or long strings pressed at a fret near the bridge) produce a high pitch. A short air column in a recorder (the distance from the whistle at the top to the first open hole) produces a high pitch. There are other things that are more complicated, like that little hole on the back of the recorder that gives us a higher octave. That might make a fun science project.