Experiment with tuning fork
Materials
Figure 1. A set of microphone (by Hung Vu)
Figure 2. Tuning fork and rubberized mallet (by Hung Vu)
Description
A microphone will measure the in nearby atmospheric pressure, including static pressure from air and dynamic pressure, which is the change in pressure.
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Set up and procedure:
1) The microphone is connected to a computer to record all the data using Logger Pro.
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2) Because the microphone measure total air pressure, static pressure is zero-ed out so there is only dynamic pressure left.
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3) The microphone is put near the tuning fork, and its tip is pointed toward.
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4) Using rubberized mallet to hit the tuning fork and observe the graph.
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See the video and image on the right to have an overview of this setup.
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Figure 3. An overview of setup (by Hung Vu)
Video 1. An overview of setup (by Hung Vu)
What will happen?
Make your own prediction and watch the video below to see what happens. Try not to scroll down until you finish watching it.
Video 2. Phenomenon when tuning fork is hit (by Hung Vu)
Phenomenon:
Right after the tuning fork is hit, an audible sound is created.
There is a clear change on the graph, which shows that there are changes in atmosphere near microphone.
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Explanation
The tuning fork aggressively vibrate after being hit. The two video below can help you see this behavior of tuning fork clearer.
The motion of tuning fork is what make water splash.
The ball moves because there is a force applied on it. This force is form the prong's movement.
Video 3. Motion of tuning fork (by Hung Vu)
Video 4. Motion of tuning fork (by Hung Vu)
Because the tuning fork vibrate (moving left and right) very fast, it will temporarily change the air pressure of region around it. Duo to the change of pressure and impact from tuning fork, air molecules will start moving.
Figure 4. Local air molecules when tuning fork is at rest (by Hung Vu)
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The prongs move outward, Duo to this motion, the area between two prongs is increased but number of air molecules stays the same so pressure of that local region increase. On the other hand, area of the region outside is decreased, so its local pressure is increased since many air molecules are compressed to smaller area. Also, the prong (tine) will push those molecules away from itself; therefore, make the air molecules move.
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After reaching max position (at max amplitude). It will start moving inward.
Figure 5. Local air molecules when tuning fork is vibrating outward (by Hung Vu)
Figure 7. Changes in air pressure when tuning fork vibrating outward (by Hung Vu)
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When the prongs move inward, the area between two prongs is reduced but number of air molecules stays the same so pressure of that local region increase. On the other hand, area of the region outside is increased, so the air molecules spread out (moving to new space) so air pressure in that region decrease.
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After reaching min position (at max amplitude). It will then start moving outward again.
Figure 6. Local air molecules when tuning fork is vibrating inward (by Hung Vu)
And when it start moving inward
Then moving outward again
Figure 8. Changes in air pressure when tuning fork vibrating inward (by Hung Vu)
Position and motion of air molecules
Picture of air molecules motion / position over time (1 cycle - period includes 9 different time mark: t1 < t2 < t3 < t4 < t5 < t6 < t7 <t8 < t9)
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As we know from above, air molecules will start to move as a consequence of the tines motion.
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When air molecules move away from the tine. They will eventually touch the other molecules and make them move.
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Since almost all the collision happens at the same time, the other air molecules will start moving in the same order as what hit them.
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This patter will be repeated multiple times as the molecules will again collide with the others and push them away. The other molecules will start a new "turn" at that time.
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For those molecules colliding with the others on their way. It will bounce back to their original position right after that.
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Depended on motion of tines, air molecules near tuning fork will start all this progress over again and repeat it till the tines stop moving.
The below is animation for this motion
Figure 9. Position and motion of air molecules at t1, starting a new cycle (by Hung Vu)
Figure 10.1. Position and motion of air molecules at t2 (by Hung Vu)
Figure 10.2. Position and motion of air molecules at t3 (by Hung Vu)
Figure 10.3. Position and motion of air molecules at t4 (by Hung Vu)
Figure 10.4. Position and motion of air molecules at t5 (by Hung Vu)
Figure 10.5. Position and motion of air molecules at t6 (by Hung Vu)
Figure 11. Position and motion of air molecules at t7 (by Hung Vu)
Figure 11.1. Position and motion of air molecules at t8 (by Hung Vu)
Figure 11.2. Position and motion of air molecules at t9 (by Hung Vu)
After t9, we move to t1 of new cycle
Figure 12. Motion and position of air molecules over the time (by Physics Classroom)
1 cycle-period is the time tines need to return to their original position with the same movement direction (E.g: Starting at x= 1, moving to the left. A period of 2 seconds means the tines need 2 seconds to return to x = 1 with movement to the left).
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Duo to this motion, high/low pressure region will move/change in an interleaved sequence away from tuning fork (audio source).
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As stated above, a microphone is used to measure changes in air pressure. Since the vibration of tuning fork changes local air pressure, and as air molecules move toward microphone, the local air pressure around microphone is also changed based on how air molecules around there are hit.
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The microphone will record those changes and hence, our recorded graph start changing as shown in the video.
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Theoretically, before tuning fork is hit, air pressure should be consistent over the time. In other words, these line should be dead flat. However, we still see fluctuation here, but it is negligible as it is result of the inaccuracy of microphone in measuring changes of air pressure (limit of measurement).
Figure 12. Magnified part of graph showing the changes when tuning fork is hit (by Hung Vu)
Explanation for the changes of graph we see in video
Figure 12.1. The changes in air pressure when tuning fork is hit (by Hung Vu)
Figure 13. Graph of ambient noise-no sound (by Hung Vu)
Figure 13.1 Magnified graph of ambient noise-no sound (by Hung Vu)
The forces involve in air molecules motion
There will be two times forces appear during this motion.
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When an air molecule is pushed by tuning fork.
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When an air molecule hits another one and bounce back.
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Note: For the free body diagram, "g" is gravity, "AM" is air molecule, "TN" is tuning fork.
Figure 14. Forces appear when air molecules are pushed by tuning fork's tines
Figure 15. Forces appear when an air molecule collides with another one
How sound is created?
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We all know from the previous part that motion of tuning fork (or any vibrated object in general) will create temporary displacement of air molecules / changes in air pressure in a specific order.
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In reality, when those molecules reach our ears. Duo to the collision between our ears and air molecules, some parts inside our ears start moving in that specific order too, the same as when one molecule collides another.
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When these parts start moving in the specific order (vibrating), they will create electrical signals, which will then be transmitted to our brain.
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When our brain receive the signals, it will interpret them and the result of it is what we call "sound".
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That's how sound is created and the way we hear it!
Experiment showing how parts in our ears react when air molecules hit them​.
Set up, procedure and result
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Cover a bowl with 1 very-thin layer of clingfilm.
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Put some rice on the clingfilm surface.
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Playing a pure tone form speaker.
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Point speakers toward the clingfilm-covered bowl.
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The rice start bouncing around as a result of clingfilm's vibration, which represents parts in our ears.
Video 5. Reaction of surface when hit by molecules (by Hung Vu)
Energy transformation in the process
Figure 16. Enery bar chart of tuning fork motion
Assume that there is no lost in the process, all initial kinetic energy (K_i) of tuning fork's vibration (or any audio source) will transfer to sound energy (U_sound).