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Sound Waves

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Sound Waves!

What  type  of   wave  is  this?

Longitudinal Wave Example

Components of a Sound Wave

Components of a Sound Wave

• A:  Amplitude     • 𝜆:  wavelength   •  f:  frequency

𝑣=𝜆f

•  Pitch  (high/low)   •  Volume  (loud/soM)

How fast does sound travel?

/v B ρ=

Bulk  modulus

Density

•  B  depends  on  pressure  and  volume   •  But  we  can  re-­‐write  this  in  terms  of  only  one

variable

The  speed  of  sound  in  air  at  a  constant  temperature   is  a  constant.

C331 1 273 T

v = +

v  at  room  temperature  (20°𝐶)  is  about  343  m/s

Lab: Sound Waves PHYS 242

© 2020 SFSU. DO NOT DISTRIBUTE

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.

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

© 2020 SFSU. DO NOT DISTRIBUTE

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

© 2020 SFSU. DO NOT DISTRIBUTE

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

© 2020 SFSU. DO NOT DISTRIBUTE

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

© 2020 SFSU. DO NOT DISTRIBUTE

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?

Lab: Sound Waves Post-Lab Knowledge Check PHYS 242

Copyright 2020 San Francisco State University

Sound Waves Suppose that you are listening to a musician who is playing the cello (also known as a cellist). They begin by playing a sound at a given frequency. They then switch to a note with a lower pitch. The sound is similar to the beginning of the Jaws theme, as demonstrated by this child on YouTube: https://www.youtube.com/watch?v=uR689WR4fZM

A. When the cellist changes between these two notes, what happens to: a. the frequency of the sound wave?

b. the wavelength of the sound wave?

c. the speed of the sound wave?

For each question, explain how you know.

B. Suppose the musician decreases the amplitude of the sound wave. What happens to the resulting sound that you hear?

C. Imagine that you are standing in front of the cellist.1 Plot the pressure changes at your position as a function of time.

1 Of course you are at least 6 feet away from the cellist to maintain social distancing.

Sound: Summary Answer the following questions and submit your responses as a PDF. 1. Write down one major conclusion you can draw from this week’s laboratory. Please explain. 2. Describe the experimental evidence that supports your conclusion. Please explain. 3. Give one example of applications/situations for the finding(s) you described above in your everyday life outside of physics lab.

4.  What  did  you  like  and  dislike  about  this  week  lab  ?

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