Sounds Good to Me. Engagement. Next Generation Science Standards (NGSS)

Transcription

1 Sounds Good to Me Students make a mental model of how frequencies are detected by the cochlear membrane. Using this model, students examine how cochlear implants can be used to help treat deafness. Next Generation Science Standards (NGSS) Science Practices Practice 2: Develop and use models. Disciplinary Core Ideas MS-LS1-D. Each sense receptor responds to different inputs, transmitting them as signals that travel along nerve cells to the brain; the signals are then processed in the brain, resulting in immediate behavior or memories. MS-PS4-A. A simple wave model has a repeating pattern with a specific wavelength, frequency, and amplitude, and mechanical waves need a medium to be transmitted through. This model can explain many phenomena including sound and light. Waves can transmit energy. Engagement Engagement brings students into a topic by connecting the topic to previous material and understandings while offering context. These activities may not be needed for students already very familiar with the concepts in the Exploration. Some relevant starting ideas: Sound is a wave that causes one set of molecules collides with the next set of molecules. Oscillations, the source of sound waves, are repeated motions. Frequency is the number of complete vibrations per unit of time, and is related to pitch in sound. Amplitude is the maximum displacement of the medium from the original position and is related to the loudness of a sound.

2 In these first few short activities, students will review how objects can create vibrations that relate to sound by plucking a meter stick, trying different options to get a sense of what factors control pitch and volume of sound. Activity 1: Natural Frequency Many objects produce a frequency when displaced from their initial position and allowed to vibrate freely. This is called the object s natural frequency, and it is largely independent of the amplitude that the object is struck. Materials (per group): 2 tuning forks with different pitches rubber surface like the heel of a shoe two bowls of water graphite refills (0.5mm - 0.7mm) from a mechanical pencil, paper, tape, magnifying glass and ruler 1. Show students the tuning fork. Hold the tuning fork in your hand with your hand near the single metal bar away from the tines. Strike the tuning fork against the rubber surface firmly, but not so hard as to cause the tines to collide. (If the tuning fork is hit against a hard surface, the tuning fork will also produce a high pitched clang tone. Try to avoid this situation as it confuses students about what kind of sound is produced by a tuning fork.) 2. Show the students the second tuning fork that has longer length tines. Ask students to predict how the sound will be different with the longer tines. Typical answers include that the different tunes are different lengths because they make different pitches. Most will additionally say that the longer tines make for lower frequencies and pitches. 3. Say to the students that they will repeat the experiment, except that they will hit the tuning fork with less force. Ask them to predict how the sound will change if at all.

3 4. Have them repeat the experiment. Most students will discover that the tuning fork will produce the same pitch but lower in volume. 5. Once the students get familiar with producing vibrations in the tuning fork, they can dip the tip of the tuning fork handle into the bowl of water (the water should be perfectly still before they dip the tuning fork into it). They will observe the waves produced in the water by the fork. The teacher should practice this before having students do it. The results can be too messy to see clearly and it takes a little practice to get it right. Once students have perfected their technique, they should compare the waves made with each tuning fork. If the two pitches are not sufficiently different from each other, there won t be a visible difference. The teacher should test out both tuning forks prior to the exploration. 6. Break off a piece of graphite approximately 1-2 centimeters in length, depending on the thickness of the tuning fork tines. The graphite cylinder should be taped onto one of the tines so that the lead is perpendicular to the tine (you can vary your technique and the angle the lead is attached to the tine if needed. See diagram below, originally from Helmholtz, but the results you ll get are unlikely to be as obvious). The student pings the tuning fork (but not where the lead is attached!) and then immediately, but very lightly, drags the lead across a piece of paper (in my experience, the fork is dragged rather quickly across the page but different techniques should be explored). The wave form of the tuning fork should be visible with the magnifying glass. This takes a lot of practice to get right but when it works, it s a beautiful demonstration. It can be a somewhat quantitative exploration if students are able to perfect their technique and make reproducible waves. Although the waves produced are tiny (you really need a magnifying glass to see them), the properties of the wave (amplitude, wavelength) can be measured and the waves produced by the two tuning forks can be compared.

4 Activity 2: Forced Vibration When one vibrating object is placed against another object, the first object can make the second object vibrate with the same frequency as the first object. This situation is called forced vibration. Materials (per group): 2 tuning forks with different pitches rubber surface like the heel of a shoe stiff wooden or plastic surface like a desk top 1. Explain that the students are going to strike the tuning fork against the rubber surface and then place the handle against the table top. Ask them to predict in writing how the volume of sound will change. Typical answers included the volume getting louder, softer, and staying the same. 2. Have students do the experiment. They will discover that the sound gets louder. The vibrations of the tuning fork make the handle vibrate, and placing the handle against the table makes the table vibrate. The table s vibrations added to the tuning fork s vibrations make the sound louder. 3. Some objects will diminish the sound of the tuning fork (something soft, for example). Students should predict what types of materials in their classroom might have this characteristic and test it. Why would some objects increase the loudness and some decrease the loudness? Students should use what they know about the properties of sound to infer the characteristics of the materials that increase the loudness of the tuning fork and those that decrease it. 4. Once students have had these experiences with vibrations and some of their properties, they can begin building the first draft of their model. Exploration

5 Exploration is where students use prior knowledge to investigate ideas through activities to facilitate conceptual change.

6 Activity 3: Uncooked Pasta Has a Natural Frequency The tuning fork has such a high frequency that it is difficult to see how it is moving. We can slow things down by using pasta which has a lower stiffness and so has a lower natural frequency. Materials: Thin spaghetti Marshmallow, gum drop, or clay Stopwatch 1. Have the student place a marshmallow on the end of a piece of thin spaghetti. Hold the end in one hand and pull back gently on the marshmallow and allow it vibrate. 2. Have the students sketch the motion of the spaghetti to vibrating back and forth. Have them compute the number of complete vibrations of the marshmallow in 10 seconds. 3. Have the student pull the spaghetti back a different amount (smaller amplitude). Have them predict how many vibrations will occur in 10 seconds. Most will discover that the marshmallow has the same frequency even at different amplitudes, just like the tuning fork. 4. Students remove the marshmallow from the spaghetti and tear it in half. They return it to the spaghetti. 5. They are asked to predict in writing what will happen to the frequency of the marshmallow when the spaghetti is pulled back again. Most will predict no change in frequency since a change in amplitude didn t change the frequency. 6. The students perform the experiment and discover that the frequency has increased. Students break the spaghetti so that the stick is shorter. They shake pluck the end again, and discover that the shorter pasta also has a higher frequency Activity 4: That Resonates with Me If an object is forced to vibrate at its natural frequency, its amplitude will increase.

7 Materials: Thin spaghetti Marshmallow, gum drop, or clay 1. Have students place a marshmallow on the end of a piece of spaghetti, and have them hold the spaghetti at the bottom and shake the spaghetti back and forth a small distance. Change frequencies of their hands vibrations until they find the same frequency of the spaghetti. They will see the spaghetti move back and forth an increasing distance. This situation is called, resonance. They may even break the spaghetti. 2. Give the students another piece of spaghetti and have them place a marshmallow on the end. Have them break the spaghetti so that this piece is approximately two-thirds the length of the other piece. 3. Have the students hold both pieces of spaghetti in the same hand. Have them predict in writing if it is possible for them make both pieces of spaghetti resonate at the same time. Many students will answer yes, but it will be impossible. They will be able to make one length resonate or the other, but not both. 4. At this point, students models should include factors that affect the frequency, wavelength and amplitude of mechanical waves. Students can now add the concept of resonance to their model. They should consider what kinds of constraints are placed on a system based on the natural resonance frequencies of its components. What benefits could they confer? Explanation This is where students put together their ideas to create a final mental model of how a system works.

8 1. Students are asked to explain what factors determine natural frequency. Typical answers include more mass reduces natural frequency, increased stiffness increases natural frequency. 2. Students are asked to explain the situations in which resonance will occur. Typical answers include when an object is being forced to vibrate at its natural frequency. 3. Limitation in this model includes objects that have multiple natural frequencies due to multiple modes of vibration and objects that dissipate energy so effectively that vibrations can t build up. Elaboration Students extend their understanding to new systems. 1. Students should watch the video. 2. The teacher should pause the video at key locations to emphasize the ideas with the students and help them become active viewers. 3. After watching the video, the teacher should lead a discussion where the students determine and speculate: a. Why does the scientist/engineer want to make a better implant? b. How does making smaller wires improve the implant? c. Did the scientist/engineer only make one new implant or is this from years of work and iterations? 4. Students should be introduced to the cochlea, the structure in the ear that converts mechanical motion conducted from the tympanum (eardrum) to an electrochemical signal in nerves that is transmitted to the brain. At the most basic level, inside the cochlea is long membrane called the basilar membrane. Its stiffness and size vary along its length where it is most stiff and has the smallest fibers near the entrance and longer less stiff fibers near the end. More about how the cochlea works and drawings can be found here. A good video summarizing the mechanics of the ear and cochlea can be found here.

9 5. Now students are ready to explain how their model explains how the basilar membrane and nerve cells can distinguish individual frequencies (pitches). 6. If there is a failure in the cochlea occurs, a cochlear implant can be used to replace some of the cochlea s functionality. Have the students explain what a cochlear implant needs to do and how smaller leads might improve its functionality. More information on cochlear implants can be found at the National Institutes of Health. Do note: cochlear implants improve hearing in most people who get them, but they don t cure deafness or make a person using one able to hear as well as a person with normal hearing. Many people with cochlear implants still need to lip read or use sign language as an aid to communication. Evaluation Teachers and students determine if they understand the material. In this case, we will apply their understanding of making models to a novel situation. 1. Some pasta is thicker and stiffer than others. If you have two pieces of pasta that are the same length with the same marshmallow on the end, but one is made of thicker spaghetti, how will that change the natural frequency and resonance of that pasta? 2. Computers can do some of the same work as the implant. Generally they are called spectrum analyzers. You can find a free Windows Version here. An Android version can be found here. 3. Students can play different frequencies by using a tuning fork, playing an instrument, or singing into a microphone and see the histogram produced by the spectrum analyzer. They will be able to see how sounds are distributed over a range of frequencies. Students can then be asked how well one could distinguish a sound by reducing the resolution of the graph. That is, bundling areas together. They should see that it would become harder to distinguish between different pitches, which is the typical limitation of cochlear implants. Patents Patents are a way for inventors to create property rights in their inventions. Patents provide inventors the right to exclude others from making, using, offering for sale, or

10 selling their inventions in the United States or importing the invention into the United States. Please see What Are Patents, Trademarks, Service Marks, and Copyrights? In exchange for this right, inventors must disclose to the public how to make and use the inventions in their patent applications. This information often can help other inventors make improvements, as well as spur on new inventions. Reading a patent introduces students to technical language and gives them familiarity with the way inventors describe their work. The language can sometimes be difficult for students; however, it can also be instructive to see what claims the inventor has made, and to learn more about how the device or process works. Drawings can also be helpful in understanding some of the key design elements of an invention. Please look at The Anatomy of a U.S. Utility Patent and then at the following two patents related to microfabrication of cochlear implants. US Class 174/255 (Electricity: Conductors and Insulators) AU 2847 US Class 174/255 (Electricity: Conductors and Insulators) AU 2847 Spaghetti as a model of resonance is based on a lesson originally developed by John Lahr.