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Measuring the Speed of Sound (Moving Tube) Essay

1.Set Up the Experiment:
Place the open ended tube into the water, and verify that it can produce a resonance tube length within the limits set by your instructor. Confirm that you can hear the sound produced by the constant frequency source. If using a tuning fork, place it as shown in the picture, with the tuning for directly over the tube but not so close that it can vibrate and damage the tube. If using a different sound source, adapt your instructions below to your situation.

2.Find the Resonance Condition:
Starting with a very short air tube length, strike the tuning fork, place it near the tube, and increase the air column listening for a length that has a louder than normal sound. Place your ear near the top of the tube but off to the side slightly. (If you place your ear directly over the top of the tube, you will be able to hear the “sea shell” effect and your ear will tend to make the open end a partially closed end, which will affect your results.) Continue striking the tuning fork and raising the tube slowly until a marked increase in sound intensity is heard. Continue to refine your location until you have located the point of maximum sound intensity.

This is the resonance point. Measure the distance from the top of the water to the top of the glass tube. This is the length of the resonance length and corresponds roughly to ¼ wavelength, as shown in the diagram. Other resonance conditions may be possible if your tuning fork is matched appropriately with the tube. Measure the temperature of the room in 0C and the inside diameter of the resonance tube.

3.Record Data: Analyze the Experiment:
Record your measurement on the Lab Report Sheet, perform calculations, and answer questions. Submit one report for each lab group.
Measuring the Speed of Sound (Moving Tube) KEY
Lab Report

The ¼ wavelength resonance tube length (L) measured is____.775 cm = .0775 m____. The frequency of the tuning fork (f) used is_____1000____ Hz. The temperature of the room (T) is __25___0C.
The inside diameter of the resonance tube (D) is ____2.5 cm = .025 m_____.
It has been discovered that the open ended resonance condition partially exists outside the tube and thus, the effective length of the resonance tube is longer than the measured length by a factor approximately equal to 0.4 times inside diameter of the tube. Calculate the effective resonance tube length L’.

L’ = L + (0.4)(D) = __.0775 + .4 x .025 = .0875 m__

The wavelength λ is equal to four times the effective length. Calculate the wavelength.

λ = 4L’ = ____.35 m____

The speed of sound measured is given by the equation, speed = (frequency)(wavelength). Calculate the speed of sound in air.

vmeasured = f λ = ___1000 x .35 = 350 m/s___

In order to compare your measurement to the accepted speed of sound, you must have a value for comparison. Since the speed of sound increases slightly with increased temperature, increasing approximately 0.6 m/s for each degree Celsius above 00C, where the speed is 332 m/s, the temperature of the room must be taken into account. Calculate the standard value of the speed of sound using the following formula:

vstandard = 332 + 0.6T = ___332 + .6 x 25 = 347___ m/s

Calculate the error in your value of v: % error = [(350-347)/347] x 100 = .1% This is much too accurate for the experiment. Expect 5–10% error.

Your results including your major sources of error. Is this a reasonable error? These results are too good. The primary sources of error will be hearing the resonance point and making that measurement.

How might the measurement be improved to obtain a more accurate measurement? Using the next resonance location, or different frequencies, would be useful. A lower frequency would give a larger length and thus, a better chance for good results.

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