02.03 - Sound Stations (rev.10-13-06)
Background:
Now that you have determined the speed of sound and some of the variables which can effect your ability to find the speed of sound, we will move on to some of the other properties of sound. You and your lab group will go to seven(7) stations set up around the room and outside the room on the lana'i. It should be clear, that with the topic being sound, we need to listen carefully and keep the noise in the room down, so that each group can have a fair chance at each station. There are 3 or 4 stations where the sound spillover into the classroom is at a minimum. There are other stations where we may need to help coordinate the sound in the room. If you are having a problem, let us know as soon as possible. You will have only 5 minutes at each station.
The questions from each station must be answered so that we can have a good class discussion of the findings. Each student in each group should answer the questions and be ready to hand them in on the day after going through the stations. Please, save all the data you are asked to collect at each station. We will use your findings to help learn about the properties of sound and they relate to each other. Be good observers and good data collectors. We will demonstrate how each station works before you begin the activity.
Station 1 - Coffee can telephone
Take the coffee can telephone outside in front of the classroom. One person will be the speaker and one the listener. One person checks the tension in the monofilament line. Not too tight, not too loose, just right. The line is about 10m in length, be careful not to tangle the line. Speak into the can and see if the listener can hear you. It helps to have the listener plug his/her open ear. Have the listener now become the speaker, and repeat what he or she heard from the original speaker. Repeat the above sequence until each person in the group has had a chance to be a speaker and a listener.
Questions:
1. What happens if someone pinches the monofilament line while someone is speaking?
2. What happens as the line, between the coffee cans, gets slack in it?
3. Why do you think the monofilament line is tied to paperclips inside the cans?
Station 2 - Wave machine
The wave machine shown in the illustration below, is a device to see what you can learn about wave motion. There is a potentiometer (device with knob) that can change the speed of rotation of the two small motors. You can also change the tension in the string by carefully pulling apart the arms holding the motors. If you pull too hard, the dowels that are pushed on to the motor shafts will slip off. If this happens, disconnect one wire to the battery holder and reconnect the dowel to the motor shaft.
Questions:
1. How many nodes (crossover points between high and low wave travel) can you get?
2. What did you need to do to the tension and motor speed to get multiple peaks and troughs in the string?
3. Can you get just one half-wave (one egg-shaped loop in the string)?
4. If you were able to get one loop, what did you do to the tension and motor speed?
Station 3 - Sound bite
The device is a pencil with a small electric motor (similar to the ones used on the wave machine) shaft pushed into the metal jacket around the eraser. The motor has a headphone jack wired to its two connection points. The headphone jack is plugged into the headphone receptacle in a radio. Tune in a station (FM or AM) before plugging in the jack. Put a piece of plastic wrap around the pencil and gently bite down on the pencil. Adjust volume as necessary. It may helps to plug your ears with your fingers. Try it with your hears plugged and unplugged. Which is better? Each person in the group will try the activity and using a clean piece of plastic wrap. Please, throw away the used plastic wrap in the trash receptacle.
Please make sure to use a new piece of plastic wrap for each student.
Questions:
1. Can you hear a radio station through your teeth?
2. How do suppose the sound travels to your ears, especially, with your ears plugged?
3. Does the DC motor act like a speaker? Explain.
4. What do you hear, if anything, when you put the DC motor to your ear?
Station 4 - Laser voice vibration transmitter
This device is composed of a coffee container with a balloon stretched across an open end. The coffee container has both ends removed. A small mirror is fastened to the stretched balloon. A laser is aimed at the mirror and the laser reflection can be seen on the white screen mounted over the blackboard in the front of the room. Speak into the open end of the can. What do you see on the screen? Change the pitch and loudness of your voice. What differences do you see in the patterns formed on the screen?
Questions:
1. What is causing the wave patterns on the screen?
2. What effect does raising the volume of your voice have on the laser pattern?
3. Could the mirror be taped to your throat near your voice box, Adam's apple(or for women, the Eve's Apple) to get the same pattern? Explain?
4. What is the source of the sound vibrations seen on the screen?
Station 5- Slinky and long spring
These two simple devices can be used to simulate wave motion. One is a slinky and the other a long, rather thin in diameter, spring. These devices can both be used to demonstrate two types of waves, longitudinal and transverse. To get a transverse wave, have one student hold on to one end of the spring or slinky, while the other student gently stretches out the spring. Pulling too hard on either device can ruin it for future use! One student will hold an end stationary while the other shakes the device up and down to form a wave that looks like an ocean wave. This also can be done on the floor with one person holding the device while the other moves the other end side to side to side. Both cases are examples of transverse waves; like ocean waves.
These same two devices can be used to demonstrate the other type of wave, longitudinal. This type of wave does not go up and down perpendicular to a flat surface, but goes along parallel in a straight line from end to end. To get this effect with either device, pinch together several coils of the spring or slinky while the device is gently stretched between two students (this is probably best done on the floor or a table top). Pinch these coils together and release them and watch what happens to your packet of loops as it moves from one end of the device to the other.
Questions:
1. What happens when you speed up the shaking when making a transverse wave?
2. What happens when the person shaking a device moves his or her hand up and down (standing-up) or side to side (on the floor or table top) in a greater high to low range?
3. What happens in a longitudinal wave when you pinch a greater or lesser number of coils?
4. What happens in a longitudinal wave when the device is stretched apart quite tightly or relaxed?
5. Which device worked best for longitudinal waves and which device worked best for transverse waves?
Station 6 - Speakers out of phase
You will have four (4) speakers mounted on a board. These speakers will be hooked to a tone generator. You can change the pitch of the tone by changing the knob setting on the tone generator. Have each group member take a turn moving from A to D, with his or her head at the same level of the speakers, about 1m from the speakers. The speakers will be marked: A, B, C and D.
Four speakers wired out of phase. Tone generator
Move your head slowly back and forth from A to D
Questions:
1. Write down what you hear at: [A], between A and B, [B] and between B and C, [C] and between C and D and finally, [D]
2. Predict what you think might be causing this effect.
3. Do stereo speakers have to be wired a certain way? Why is this true?
4. Does the effect you heard happen, if you change the sound pitch from the generator? Try It!
Station - 7 - Singing bottles
You will find four (4) plastic water bottles labeled A, B, C, and D. Three of the bottles are identical. You are to hold the bottle A by the neck between your thumb and index finger, close to, but not against, an ear. You are to pick up bottles B, C, and D in turn the same way and blow across the mouth each one. Listen to see if you hear something in bottle A each time you blow across the mouth of the each of the other bottles.
1. What do you hear in bottle A as you blow across the top of bottle B?
2. What do you hear in bottle A as you blow across the top of bottle C?
3. What do you hear in bottle A as you blow across the top of bottle D?
4. Explain the cause of the effects that you noticed in each of the trials above.
02.03 - Sound Stations - Teacher Section
Goals
- Students should learn that in each station, sound is a result of a vibration in a medium.
- Students should begin to see that waves have properties.
- Students should learn that waves can reinforce each other or cancel each other out.
- Students should learn that sound waves are compression or longitudinal waves.
Area / Concepts / Skills / HCPS III standards
Physics / Sound is a vibration in a medium / Students will be able to recognize wave properties / PS.6.5
PS.6.6
Physics / Sound waves have mathematical formulas to quantify them / Students will learn to apply = to sound waves
Warm-up Questions:
1. How can you tell if a sound wave is vibrating at a high frequency? A low frequency?
(Students have probably heard the word frequency used before, but chances are they don't understand its relationship to sound. This question is just to get them thinking)
2. How can you tell if your stereo speakers are hooked-up properly?
3. How do vibrations cause sound in a medium? (Really tough question, but again, something to get them thinking about what sound is and how it is created.)
Presentation:
The main goal of these stations is to get students to see that sound is created by a vibration in a medium. In most of the stations the medium is air, just like it was for the outside and classroom determinations of the speed of sound. It is true that sound travels much faster in metals and water. In air the speed of sound is in the 340's m/s range, in aluminum about 5100m/s, in glass about 4500m/s, in water (fresh) about 1440m/s and finally, in helium about 1005m/s. Now we know why our voices get so weird when we breath in helium from a balloon. Even in concrete sound travels at 3000m/s.
Station 1-Coffee can telephone.
This was a classic, which was really popular with kids before cell phones. The monofilament can be stretched tightly so that the sound vibration made in the coffee can from a person's voice, can be sent to the other can, where a student can hear a message. We use these cans instead of the classic soup cans because a person can put his or her mouth inside the opening of the large coffee can. The listener cannot hear the message through the surrounding air or by reading lips. We use about 10m of monofilament for this station. The paperclip or button acts as an attenuator. The vibration comes through the monofilament to the metal paperclip which in turn vibrates against the metal coffee can. A cheap amplifier! The pinch of the line stops the vibration from going down the monofilament to the other can. A slack line also doesn't vibrate very well and the sound vibration doesn't make it to the other can. It should later be stressed that the vibration of the monofilament wasn't just any old vibration, but a vibration that was in the frequency range of human hearing from 20Hz to 20,000Hz. We'll work on frequency and its unit, Hertz, later.
Station 2 - Wave machine
The wave machine is a device that students can use to see nodes, peaks, and valleys in waves. This, of course, is an example of a transverse wave such as an ocean wave or a light wave. All waves, whether transverse or longitudinal are graphically represented the same. The problem is that sound waves are compression (longitudinal) waves and are generated in the form of compressions and rarefactions that are parallel to their line of generation and not perpendicular, like their transverse cousins. It is easier for students to recognize: frequency, wavelength, nodes, anti-nodes, and amplitude in transverse waves. At this point we want them to see how many nodes they can get and the circumstances of rotation speed and tension that caused them. The wooden dowels may pull off the motor shafts, but they are easily put back on, after stopping the motors. They may not immediately see what is going on and know all the wave vocabulary, but as they learn the properties of waves, teacher and students can refer to patterns seen on the wave machine. They should learn that a big half-wave is achieved by slowing the motors and releasing the tension on the string. To get lots of nodes and half-waves, the string tension needs to become tighter and the motors sped up. The big half-wave has a large amplitude or wave height and the smaller, faster multiple half-waves have smaller amplitudes. The concept of wave length can also be studied on the wave machine. This station is just a first step in understanding wave properties.
Station 3 - Sound bite
Every student has used a headset not really knowing how the sound was passed from a radio, iPod or CD player. It is definitely a vibration through a medium. In this case the little 3v motor vibrates at the frequencies sent out by the radio to the headphone jack and sets the motor(magnet with wire coils) into vibration with the frequency of the radio signal. The motor shaft is pushed into a pencil and when a person bites down, gently, on the pencil the sound is sent through the jaw to the ear. If the students plug their ears, they will learn that the vibration is not just traveling through the air to their ears, but through the bones in their jaws and heads. Again, this is a vibration through a medium, but this time, does not use air as the medium. If it wasn't so weird, the students could actually bite down on the motor and get the same effect. The pencil lead helps to transmit the sound vibration from the motor.
Station 4 - Laser voice vibration transmitter
This simple device shows the vibration of the vocal cords that create the human voice. As the person speaks or makes sounds in the can the tightly stretched membrane sends the pulses of sound vibrations to the mirror that also vibrates. As the light from the laser reflects off the mirror to the screen, the vibrations can be seen. This is fun and students can see some strange patterns as they change the sounds they make in the can. The concept that sound is a vibration in a medium is reinforced. The students see that they are the source of the vibration.
Station 5 - Slinky and long spring
The slinky and the long spring help students to model wave behavior. They can see how a wave can travel either parallel to the line of generation or perpendicular to it. By stretching the slinky a bit and then pinching together a few coils of the slinky and releasing them, causes a pulse to travel through the slinky. These pulses model the compressions and rarefactions of sound waves; longitudinal waves. The students can see that the pulses are parallel to the floor or table upon which the activity is performed. The long spring when stretched a bit can be shaken up and down to create a wave pattern which is perpendicular to the floor. The spring can be held between to students or tied to a cabinet or door. This pulse looks more like a wave on the ocean; a transverse wave. When these stations are posted out on the front board, drawings can be made and labeled, showing amplitude (wave height), wave length and frequency. The appropriate symbols can be introduced. Wavelength ( ) and frequency () . The unit for frequency is Hertz (Hz) and for wave length, usually, nanometers (nm). Angstrom units can also be used for wavelength. These units of measure are, of course, adjusted, in magnitude, as waves become very fast or slow and very long or very short. A wavelength of visible light would be in the nm range, where as an ocean wave can be measured in m.