Assignment B

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Lab: Sound Waves PHYS 242

Sound Waves In this lab we will investigate the properties of sound waves, using a simulation from the PhET team. The simulation is available at the following link: https://phet.colorado.edu/sims/html/waves-intro/latest/waves-intro_en.html

The simulation can be run in a browser. If you have issues with the simulation, try using another browser. If you are unable to run the simulation, your TA will provide you with remote assistance. When you run the simulation, choose the “Sound” option.

NOTE: For this activity, you do not need to write a full lab report. Instead, answer the questions that are included in this lab. Please type up your answers, if possible. For the sketches, you can take pictures and include the images in your write up. Submit your answers to these questions on iLearn as a PDF. If you need help with this process, please ask your TA for assistance.

Part I: An Intro to Sound Waves

When you begin the lab, be sure to select the “Sound” option. Note that there are other options, for “Water” and “Light.” If you select the wrong option, you can return to the “Sound” version at the bottom of the screen.

This lab simulates sound waves that are emerging from a speaker (on the left) into a volume (the gray box). To turn the speaker on, press the green button on the left. You will then see the speaker creating pressure waves that propagate through the box. The buttons beneath the speaker dictate whether the waves are ongoing (the top button) or whether the speaker produces a single pulse (the bottom button). You can control the frequency and amplitude of the waves on the right side of the screen. As with previous simulations, you can pause and slow down the simulation with controls at the bottom of the screen.

A. Predict what will happen to the tone that you hear as you change the frequency and amplitude of the sound wave.

B. Check your result by selecting the “Play Tone” option on the right, and listen to the tone that is generated by the speaker. Be sure to take care that you have your volume set to a safe level. Describe what happens to the tone as you change the frequency and amplitude of the wave.

Lab: Sound Waves PHYS 242

Part II: Properties of Sound Waves

Restore the simulation to its default mode by clicking the circular arrow in the lower right corner of the screen. Now start a wave going by pressing the green button on the speaker; let several cycles go by, so that the grey box is filled with the wave. Notice that there are helpful tools in the upper right corner. The measuring tape will help you measure lengths in the grey box. The timer will help you to record how long something takes.

A. Measure the frequency and wavelength of this wave. Be careful to explain your rationale, provide all units, and provide drawings, if necessary.

B. Using your frequency and wavelength, find the speed of the sound wave. Show your calculation.

C. How does the speed you calculated compare with the expected speed of sound?

D. Increase the frequency to the maximum value and measure the new wavelength. How has the wavelength changed? Calculate the speed. Has the speed changed? Explain.

Lab: Sound Waves PHYS 242

Part III: Pressure Waves

Sound waves are pressure waves.

A. Predict how the pressure varies with distance from the speaker.

B. To see how the pressure varies, click the graph button on the right. This will display the pressure as a function of distance from the speaker. Pause the simulation. Provide a sketch of this graph (not a screen shot) and explain its shape.

C. Is the amplitude of this wave constant with distance from the speaker? Is the frequency? Explain.

D. How does your measurement in Part B compare with your prediction from Part A? Explain any discrepancies.

Lab: Sound Waves PHYS 242

E. Imagine a sensor located at a single point in the box. Predict how the pressure will change over time at that point.

F. Select the graphing tool from the upper right corner. This tool will allow you to select two points in the volume and plot the pressure at those points as a function of time. Place the two probes on two peaks in the paused simulation. Try to ensure that they are in line with the center of the speaker (i.e., on the dashed line shown with the graph option). Then resume the simulation. Sketch a plot of pressure vs. time at the locations of the two probes (not a screen shot). Describe the shape of these curves. How does your measurement compare with your prediction in Part D?

G. Predict how the two graphs (pressure vs. distance from the speaker and pressure vs. time at a given point) will change if you:

a. increase the frequency?

b. increase the amplitude?

Lab: Sound Waves PHYS 242

H. How do the above two graphs (pressure vs. distance from the speaker and pressure vs. time at a given point) will change if you:

a. increase the frequency?

b. increase the amplitude?