1. Lab 5: Previously “Touch Tone” now “Interactive Sound Lab”
PHY111: Summer
Lesson 09: Waves & Periodicity
Wave vocabulary
Anatomy of a Wave
Sound waves
Light waves
Standing Wave patterns
Superposition
Revision Note: I am sure no one will believe it, but I forgot my Vernier Microphone at home, today!
Since one of our labs hinged on this piece of equipment, I am revising that lab for tonight’s class:
Lab 5: Previously “Touch Tone” now “Interactive Sound Lab”
1/15

2. Notes / Discussion: Types of Waves (transverse and longitudinal)
Examples from everyday life for both of these
What IS “Sound” and what IS “Light”? (great examples)
Anatomy of a Wave (intensity vs. time and vs. distance graphs)
Constructive vs. Destructive interference (Superposition)
Relationship between frequency and Period
- Units for both?
Wave-speed (determining from average-speed equation)
- “Why is this use of average-speed equation VALID”?
- “What determines wave-speed”?
Speed of sound (through water/air/vacuum??/&tc)
Speed of light (through water/air/vacuum??/&tc)
Doppler Effect (“What is it”?)
2/15

3. Simulations: INTERACTIVE PHYSICS SIMULATIONS: - 11.1 11.2 11.3 11.5
Doppler Effects (sic) Frequency
Roller Coaster
3/15

4. Practicum: Speed of Sound
Let us attempt to measure the speed of sound through air at our room’s temperature:
Normally, I would have a microphone setup for this. However, in the interest of time, let us do the following:
Pass around tubes and put ear near one end.
*snap* at that same end – do you hear an echo?
I will draw out what is happening on the board (standing wave pattern due to reflection).
Then I will show you a recording of these and we can analyze.
4/15

5. Some Wave Calculation Examples:
Find wavelength of sound wave produced by tuning forks with differing frequencies (say 250 Hz and 500 Hz)
Find wave speed using both average speed equation and then with wave speed equation (say there are 3 complete waves in a distance of 10 meters…and it takes the crest of one wave 4 seconds to cover that distance)
Find Period from frequency and verify expected wavelength from frequency (using common red LASER-light values…wavelength ~633 nm, frequency ~4.7E14 Hz)
5/15

6. Lab 5: Interactive Sound Lab
Complete the following questions from these java applets at:
Part 1. Beats: Mix 2 frequencies (imagine two tuning forks sounding off near each other or two people singing slightly off-key next to each other). You must explore in order to answer these two questions:
Q1. “How does the difference between frequencies relate to the number of ‘beats’ produced”?
Q2. “Is this the case no matter which frequency combinations might be chosen”?
Part 2. Doppler Effect: Go through the 4-step process and review the “Other Features.” Then experiment with the applet to answer the following three questions:
Q3. “How does the distance between wave-fronts (the lines representing the waves) differ in the front of the plane to those in the back of the plane”?
Q4. “What happens when the plane goes fast enough for the wave-fronts to ‘pile up’ on each other”?
Q5. “At what point in the plane’s motion does the person hear the sound produced by the plane as a lower pitch”?
Part 3. Wave: For the Wave Applet, combine two waves (vary the combinations – even try adding the same types of waves). Now explore by adding more than two types together. This ‘adding together of waves’ is called “Superposition.”
Q6. “What happens when you add two waves of the same type”?
Q7. “What type of interference would this be”?
Q8. For question #8, do the following: “Draw two waves that would result in completely destructive interference with each other (i.e., cancellation!)”
6/15

7. Check your understanding
What would happen if pulse B were -1?
-2?
+2?
What happens to the pulses after they have interacted?
7/15

8. Notes on Standing Waves
View Tacoma Narrows Bridge video clip:
Standing Wave Patterns
What is necessary for a standing wave pattern in terms of wavelength?
What is a node?
What is an antinode?
8/15

9. Check your understanding
Where are the nodes? The antinodes?
How many full wave cycles are present in this standing wave pattern?
How long is the wavelength produced by this tuning fork?
Where is the string vibrating the most violently? (multiple places).
9/15

10. Check your understanding
Consider a situation where a speaker is positioned over the opening of a tube with the opposite end closed off. The frequency output by the speaker is changed such that we cut into the antinode of four different standing wave patterns.
Why are these fractions of the corresponding wavelengths chosen?
What would happen if the frequency output were changed
such that L = λ5/2 ?
10/15

11. Lab 6: Standing Wave Patterns
Do a write-up with your group mates to be turned in for this lab grade.
Fill your apparatus with water (to a reasonable level…away from computers and near the sinks). You can change the level of the water by raising and lowering the “cup” which is attached.
For your tuning fork, calculate the wavelength of the sound wave produced by striking it (in centimeters, for this lab).
Strike the tuning fork, place it at the top of the column of water (near the lip of the spoon) and begin lowering the level of the water until you hear the maximum sound resonation. (What is the height of the “air column” that produces this sound?)
Keep going down (careful with the water!) until you reach the next maximum.
Ask your instructor (when you are done all of this and can show me your answers to #’s 2, 3 & 4) why this result might make sense. (Record the results of our discussion!)
We will do this last one as a WHOLE CLASS.
We will choose the Mode of your length measurements to drive this discussion.
11/15

12. Lab 7: Superposition of Waves
In small groups, follow the lab instructions to learn how to manipulate waves. As waves superimpose (“add together”), different effects are seen.
The lab files are available at the following online addresses:
mrlafazia.com/labs/SUPERPOS.XLS (case sensitive)
mrlafazia.com/labs/Superposition.DOC (case sensitive)
I have printed out 4 copies for our lab groups, this evening.
Similar to our last Excel lab, I will guide you through these steps. Your group needs to complete the steps of the lab exercise along with me. Then, the very last Challenge will be completed on your own and I must sign off that your group completed it before the end of class.
12/15

13. Grades/Assignments:
Labs 5, 6, and 7 should be completed and turned in (where applicable) by the end of the class.
13/15

14. Some Practice Problems (for fun!)
1) Clarification of what we saw in the Superposition Lab results
2) Practice with Period and Frequency relationship
3) A bit of fun with Wave Pulses (superposition)
4) Some final online practice
Assuming this site will work for us (since it has been acting up lately!), you may use these worksheets as practice.
14/15

15. Looking Ahead:
Lesson 10 will take place online. It will be Test II (which will include questions from content in the lessons on the atomic nature of matter, electricity & magnetism, and waves & periodicity).
Reminder: We DO NOT have a lesson for July 4th – go celebrate our Nation’s Independence! Lesson 11 (July 9th) will be an online introduction to Linear Mechanics. Lesson 11 heralds in our final Unit for the course.
REMEMBER that when Blackboard goes down you will be accessing the files for our lessons (and any announcements) here:
15/15