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| Essential questions | | What happens when there is more than one wave at a time? | |
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Virtually all waves in the environment are composed of many frequencies at the same time, including sound waves and light waves. In physics, the term interference describes the addition of multiple waves that interfere with each other to produce a complex, multifrequency wave that is the sum of many single-frequency waves. 
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Part 1: Constructive and destructive interference
- Create a wave that has a frequency of 50 Hz and adjust the amplitude and the time axis until you can see around 5 full cycles.
- Add a second wave of the same frequency and set the display to show the sum (interference) of the two waves.
- Adjust the phase and amplitude of the individual waves until they interfere to make the largest possible or the smallest possible sum.
- In your own words, describe what the phase and amplitude variables do.
- Describe how you can create constructive interference (or a sum that is larger than either component wave).
- Describe how you can create destructive interference (or a sum that is smaller than either component wave) by varying either the amplitude of the second (red) wave or its phase.
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Installing the Physics of Sound application
The application Physics of Sound will allow you to visualize sine waves and add them together.
Install and run the application under MS Windows OS: - Download the LabVIEW runtime environment for Windows (zipfile). Choose to save this file to disk.
- Unzip the downloaded file on your computer by double-clicking on it. (The file may be located in your browser’s downloads folder.)
- Click on the folder “PhysicsOfSound_Windows”.
- Run the LabVIEW installer (the application marked “Setup” in the unzipped folder). Follow the prompts.
- The installer will create a shortcut “Ergopedia—Physics of Sound” on your computer’s desktop.
- Double-click on the shortcut to launch the application.
Install and run the application under Mac OS X: - Download the LabVIEW runtime environment for Mac OS X (zipfile). Choose to save this file to disk.
- Unzip the downloaded file on your computer. (It may be located in your Downloads folder.)
- Run the LabVIEW installer (the file marked “LabVIEW2013RuntimeEngine.dmg” in the unzipped folder).
- Double-click the upper-left icon titled “LabVIEW 2013 Runtime Engine.” Follow the prompts.
- In the downloaded folder named “PhysicsOfSoundApplication”, run the application by double-clicking on file titled, “Ergopedia—Physics of Sound.”
Once you have launched the Physics of Sound application, click on the tab marked “Analyze Waveform” to begin the investigation. 
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Part 2: Superposition principle
- Create a wave that has a frequency of 50 Hz and adjust the amplitude and the time axis until you can see around 5 full cycles.
- Add a second wave of a different frequency and set the display to show the sum (interference) of both waves.
- Vary the frequency difference between the two waves and observe the waveform on both a short time scale (3–5 periods) and a long time scale (1 s or more).
- In your own words, describe the difference in the appearance of a wave with a single frequency compared to a wave with more than one frequency.
- Describe how the amplitude changes over time when two waves interfere that differ in frequency by a small amount, such as 2 Hz out of 100 Hz.
- In your own words, what does the superposition principle mean?
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