Lesson 5

Standing Waves and Sound Interference

IMPORTANT Teacher Notes:

To set up the speakers, first charge them. Recommended time for charging is three full hours before you use them for the first time. After the first time, the approximate time to fully charge them is 2 hours. See the section below on how to set up the Bluetooth speakers.

Review - Using PhET

Navigate to PhET's Wave on a String Simulation. https://phet.colorado.edu/sims/html/wave-on-a-string/latest/wave-on-a-string_en.html
On the right side of the screen, select "No End."
Ask students for input. Try making wave disturbances manually. Try making them with wave pulses. Try oscillations. Play with the settings for about three minutes.
Reset settings, still choosing the "No End". Start a pulse. "Notice the green circles. What is the direction of motion for the 3rd green dot as the wave passes?
Use the step forward button to slowly step forward.
Feel free to improvise more questions.

Standing Waves

PhET_Standing_Waves.mp4

Once again, use PhET's Wave on a String Simulation. https://phet.colorado.edu/sims/html/wave-on-a-string/latest/wave-on-a-string_en.html
Select "Fixed End" use settings as seen in the video (Amplitude: 0.03 cm, Damping: None, Tension: High)
Select "Oscillate" to send oscillating waves through the string.
Feel free to improvise more questions.
To get the standing wave phenomena, set Frequency to 0.83 Hz. You will have to wait a bit as you can see in the video for the standing wave to be really visible. Feel free to have it going in the background as you continue to go through the slides.
Investigate other orders of standing waves. Feel free to let students guess frequencies. More frequencies that work well with high tension and no damping are 0.41 Hz, 1.25 Hz, 1.67 Hz, and 2.08 Hz.

Drawing Standing Waves

Show students https://www.acs.psu.edu/drussell/Demos/StandingWaves/string-standing-2.gif (seen on the left as well). The top row is how we draw/model a standing wave on a piece of paper.

Show students how the nodes and antinodes are represented in the drawing.

Discuss the wavelengths of the harmonics. The distance between two nodes is half of a wavelength.

Make sure you DRAW your own on the student-facing slide.

Lab Investigations

Tuning Fork Interference

Students should strike the tuning fork with something soft like the bottom of their shoe.

Students should not touch the tuning fork to any part of their body.

Students should rotate the tuning fork near one ear to hear the loud and quiet spots.

Lab Materials per pair

Each pair of students will need:

1 Tuning fork (any frequency is okay)

Two external speakers are connected to the computer via Bluetooth and set apart. For best results, have students plug one ear as they listen for loud and quiet spots from the interference patterns. Sound must be coming from both speakers at the same time.

TWO SPEAKERS AND AN OSCILLATOR

Materials

Lab Materials (for whole class)

Teacher sets up

Two External Speakers (both speakers must be working at the same time)
Bluetooth Adapter

Set up the Wireless speakers

To set up the speakers, first charge them. Recommended time for charging is three full hours before you use them for the first time. After the first time, the approximate time to fully charge them is 2 hours. The image on the left shows the speakers charging via the classroom computer.

Pair the speakers to the classroom computer

Plug your bluetooth adapter into the USB port on the classroom computer.
Be sure the speakers are in the off-mode. Place the speakers next to the computer.
Press and hold the power button for three seconds to enter pairing mode.
Activate wireless on the computer and select "My Speaker F019." Enter "0000" if prompted for a password.

If speakers don't connect to the computer...

You have two options:

If you are comfortable, this lab works easily with a smart phone. Pair the Bluetooth speakers to a smart phone. Run the online tone generator from your phone and have students proceed with the lab.
Use the PhET simulation for the classroom as seen in the section below.

Use the website https://www.szynalski.com/tone-generator/ to play a sound. Test the speakers with a low frequency between 100-200 Hz. Make sure the sound is coming from the external speakers and is not too loud or uncomfortable for students or yourself.

You can press the PLAY button or use the space bar to play the sound from the website. You can use the slider or the arrow keys to change the frequency.

While the 200 Hz sound plays continuously, direct students to move around the room and listen to the interference produced.

It is suggested that the students place a finger in one of their ears to better hear the effect or try cupping around one ear to suppress sound on one side. (An individual’s two ears may be far enough apart so at least one would be out of the center of the node hence the large decrease in sound intensity would not be noticed.) The sound moves three-dimensionally so people at different heights will hear different parts of the interference patterns.

PhET Simulation (if speakers don't work)

Simulation

Teacher Tips

PhETWaveInterference.mp4

Play with the simulation at https://phet.colorado.edu/sims/html/wave-interference/latest/wave-interference_all.html

As seen in the video to the left, click on "Interference." You'll be taken to the water drop interference simulation at first. On the right side of the screen you'll see three little images of a water faucet, a speaker, and a laser. Click on the speaker for the sound interference simulation.

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