First, as a class we will create a standing wave on a string, and discuss how adjusting the tension changes the wave that is formed.
- Every person grab a tuning fork! (You may grab multiple if you want).
Measure tuning fork frequency with oscilloscope.
- Connect speaker to oscilloscope. This should already be done, but follow the connections and make sense of them. The oscilloscope is measuring the oscillation of the speaker cone (vertical axis on oscilloscope), vs time (horizontal axis).
- Call me over to help explain the oscilloscope.
- Make a tone on tuning fork and hold tuning fork near speaker. This should cause a sinusoidal wave to appear on the oscilloscope. Adjust the calibration knob towards the top of the oscilloscope so the observed wave is stationary.
- The vibrations of the tuning fork cause the speaker to vibrate. What is this phenomena called?
- What are the units on the horizonal scale of the oscilloscope. Measure one complete cycle, what is this value?
Use your measurement to calculate the tuning fork’s frequency. Does this agree with your tuning fork? (Your tuning fork likely gives off multiple harmonics, and the speaker may be more sensitive to the higher harmonics so you may get a much higher frequency. By what multiple is your measurement larger than the frequency on the tuning fork?)
- Repeat this experiment with at least one more tuning fork.
- Determine speed of air using glass tube.
You are trying to determine what length of air in the glass cylinder resonates with the tuning fork. You will hold the tuning fork very near the glass tube in order to create resonance but try not to touch them together because you do not want to break the glass. By relating the frequency of the fork to the length of the glass column, you can determine the velocity of sound through air. (this should be close to 343 m/s)
- Draw the fundamental frequency associated with the glass cylinder (this is a tube with one end opened and one end closed). How does the wavelength of this fundamental frequency relate to the glass tube’s length? (write an equation, but use the variable “L” for the tube’s length instead of an actual number)
- How will the length of the fundamental wavelength change as you pour water into the cylinder?
- Slowly pour water into the cylinder. As you pour water, the ‘sound’ of the pouring water will change in pitch. When this pitch is the same as the tuning fork, the cylinder will strongly resonate when the tuning fork is held next to it. Match the resonance as best you can by adjusting the amount of water in the cylinder.
- Measure the length of the open section in the cylinder. Use this to calculate a value for the wavelength of the fundamental standing wave. Using this wavelength, and the frequency of the tuning fork, calculate the velocity of air. Also calculate what the theoretical velocity of sound in air should be considering the room’s temperature.
- Empty the cylinder for the next group.
Lab Report Title: “Calculating the speed of sound in air using resonance of a tuning fork and a cylinder filled partially with water”
Focus your lab report ONLY on things related to this title!
Be sure to fully explain the theory associated with “how this works” in your introduction section.
Your results section only needs your measurements. For this lab you are measuring the length of open air in the tube.
Your experimental section only needs to explain how you arrive at this one measurement.
Your introduction must clearly explain why this one measurement is needed to calculate the speed of sound in air.
The FIRST sentence of your conclusion MUST STATE your calculated AND theoretical speed of sound in air or else your lab report gets a 0!!! I’m serious!
Your conclusion must discuss the value you calculated and compare it to the theoretical value. Be specific in these comparisons. Give physical reasons for any discrepancies. If you do not make any SPECIFIC comparison to the theoretical value you will get at most 3 points.
Examples of bad/good sentences (with regards to scientific usefulness)
Bad: “the values are pretty close”, Good: “The calculated value is 1 m/s larger than the theoretical value”. Best: “The calculated value is 1% larger than the theoretical value”
Bad: “The values are different because of human error”, Good: “The values may be different due to error in measuring the length”, Best: “The measurement of the length of the column had an error of about +- 3 mm. This corresponds to a +- 3% error in our calculation of the speed of sound in air. Our measurement and theoretical values differ by 2%, which falls within the error expect from our length measurement.”
Bad: “We use the equation F=ma”, Good: “We use the equation F=ma to find the mass, where F is force, m is mass, and a is acceleration”, Best: “We use the equation F=ma, where F is force, m is mass, and a is acceleration. The value for a=g=9.81 m/s^2. The force will be experimentally measured using a force sensor. Following this measurement, we can calculate the mass.”
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