MUSC 1325 – Acoustics
Reflection of Waves
Leader: ______Reporter: ______
Skeptic: ______
Materials
Slinky
Laptop
Ring stand rod, no base
Introduction
In this activity we will examine reflection of waves at a boundary. Whenever a wave encounters a boundary part of the wave will be reflected and part of the wave will be transmitted. A useful idea – borrowed from electricity – is to describe the ability of a sound wave to be transmitted through a medium by a number called the acoustic impedance. The amount of the wave that is reflected then depends on the difference of the impedances, or the impedance mismatch, at the boundary.
In parts 1 and 2 of this activity, we will examine waves at ideal boundaries where 100% of the wave is reflected. In part 3 we will look at waves where a portion of the wave is reflected at the boundary.
Part 1 – Reflection of Waves at Ideal Boundaries
Procedure
1. Start the simulation
Start up internet explorer. Go to the URL http://phet.colorado.edu Click on the Play with sims… button. Click on the Sound and Waves link. Click on the Waves on a String icon. Click on the Run Now button.
Once the Java applet has loaded move the slider labeled damping to 0. Click on the Pulse radio button.
2. Investigate Reflections
The simulation is set up so that at the boundary, in this case shown by the C-clamp, does not allow the string to move at all. This is called a fixed boundary. In terms of impedance the incident wave is reflected from a medium with infinite impedance.
Click on the pulse button to launch a single transverse pulse.
Q1) Compare the incident and reflected waves. How does the appearance of the wave reflected from the boundary compare to the incident wave? Draw a picture to illustrate your answer.
Click on the Reset button to stop the wave. Click on the Loose End radio button. Now the wave pulse will reflect from a boundary at which the string is allowed to move. This is called a free boundary. In terms of impedance the incident wave is reflected from a medium with zero impedance.
Q2) Compare the incident and reflected waves. How does the appearance of the wave reflected from the boundary compare to the incident wave? Draw a picture to illustrate your answer.
Q3) Compare the two situations you just observed. What is similar about a wave hitting a fixed boundary and a free boundary? What is different?
Now that we are armed with easy to see pictures in computer simulations, we will explore the more difficult to observe phenomena with wave pulses traveling on slinkies.
Part 2 – Reflection of Waves on a Slinky
Stretch the slinky about 2 m. Hold the slinky at both ends. Launch a single transverse pulse from one send and carefully observe how it appears before and after it is reflected at the opposite end.
Q4) Sketch a diagram showing how the wave pulse looked before and after the reflection.
Q5) Was the boundary from which the slinky reflected a fixed boundary or a free boundary in this case?
Q6) Does your observation of the slinky reflecting from this boundary agree with the simulation you ran before? Explain.
Now hook the last few coils one end of the slinky over a metal rod so that the slinky is free to slide horizontally on the metal rod. Launch a single transverse pulse and carefully observe how it appears before and after it is reflected at the metal rod. Note be patient with this step. It can be difficult to see at first.
Q7) Sketch a diagram showing how the wave pulse looked before and after the reflection.
Q8) Was the boundary from which the slinky reflected a fixed boundary or a free boundary in this case?
Q9) Does your observation of the slinky reflecting from this boundary agree with the simulation you ran before? Explain.
Q10) Was the reflected wave on the same or opposite side of the slinky as the incident wave in Q4).
Q11) Was the reflected wave on the same or opposite side of the slinky as the incident wave in Q7).
There are two distinct ways that a wave can reflect at a boundary. One way is that the reflected wave is on the same side as the incoming wave. In this case, the reflected wave is in phase with the incident wave.
Q12) Did this occur for question Q4) or Q7)?
The second way is that the reflected wave is on the opposite side as the incoming wave. In this case, the reflected wave is 180° out of phase with the incident wave.
Q13) Did this occur for question Q4) or Q7)?
In question Q4) we held the position of the slinky fixed. This is sometimes referred to as a fixed boundary condition.
Q14) Describe the phase of the reflected wave compared to the incident wave at a fixed boundary condition.
In question Q7) we allowed the end of the slinky at the reflected end to move freely. This is sometimes referred to as a free boundary condition.
Q15) Describe the phase of the reflected wave compared to the incident wave at a free boundary condition.
Part 3 – Reflection at Non-ideal Boundaries
In this part we will examine reflection at a boundary that both reflects and transmits. The Simulation we will use is actually a simulation for light, but the same principles apply to sound waves.
1. Start the simulation
Enter the URL webphysics.davidson.edu/Applets/Reflection/WaveComponenets.html
Once the Java applet loads (it may take a minute or so) click on the Air-Glass link. Scroll down and click on the link Hide total wave. Now click on the link Show right traveling wave. This simulation shows any situation where a wave is incident from a medium with a smaller impedance reflects at a medium with a larger impedance.
Q16) There are a few obvious difference between the wave incident from the lower impedance and the wave transmitted in the larger impedance medium. Describe them.
Now click on the link Show left traveling Wave. You may need to scroll back up so that you can see the entire simulation.
Q17) What is the phase difference between the incident and reflected wave at the boundary? Clicking on the pause button may make this easier to see.
Q18) Is the phase difference here the same as what you saw for a free or a fixed boundary?
Q19) A wave incident from a lower impedance medium onto a higher impedance medium will reflect with a ______phase shift.
Click on the Glass-Air-Glass link. Scroll down and click on the link Hide total wave. Now click on the link Show right traveling wave. This left boundary of this simulation shows any situation where a wave is incident from a medium with a larger impedance reflects at a medium with a smaller impedance.
Now click on the link Show left traveling Wave. You may need to scroll back up so that you can see the entire simulation.
Q20) What is the phase difference between the incident and reflected wave at the boundary? Clicking on the pause button may make this easier to see.
Q21) Is the phase difference here the same as what you saw for a free or a fixed boundary?
Q22) A wave incident from a lower impedance medium onto a higher impedance medium will reflect with a ______phase shift.