Northern Illinois University
Physics 150
Lab 8
Speed of Sound
Compared to most objects, sound waves travel very fast. It is fast enough that measuring the
speed of sound is a technical challenge. One method you could use would be to time an echo. For
example, if you were in an open field with a large building a quarter of a kilometer away, you
could start a stop watch when a loud noise was made and stop it when you heard the echo. You
could then calculate the speed of sound.
To use the same technique over short distances, you need a faster timing system, such as a CBL
interface. In this experiment you will use this technique with a microphone connected to an
interface to determine the speed of sound at room temperature. The microphone will be placed
next to the opening of a hollow tube. When you make a sound by snapping your fingers next to
the opening, the interface will begin collecting data. After the sound reflects off the opposite end
of the tube, a graph will be displayed showing the initial sound and the echo. You will then be
able to determine the round trip time and calculate the speed of sound.
Figure 1
Objectives
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Determine the speed of sound by measuring how long it takes to travel down and back
the length of a PVC tube.
Compare the speed of sound in air to the accepted value.
Materials
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CBL interface
Vernier Microphone
Temperature Probe
tube, 1-2 meters long
book or plug to cover end of tube
meter stick
Procedure
Determining Anticipated Results:
1. Use the Temperature Probe to measure the air temperature of the classroom. To do this,
connect a Temperature Probe to the Channel 1 input on the interface.
2. Turn on the computer and start the LabPro program. The computer should detect the
temperature probe.
3. The temperature will be displayed on the computer screen. With the tip of the Temperature
Probe in open air, away from sunlight, wait until the readings become stable. Variations in the
tenths digit are not important. Record the room temperature.
4. Disconnect the Temperature Probe and connect the Vernier Microphone to the Channel 1
input on the interface. The computer should detect the microphone.
5. Measure and record the length of the tube.
6. Before proceeding, calculate (and record) the accepted value of the speed of sound under the
current atmospheric conditions in the lab according to:
Vaccepted = 331.5 m/s + (.607 m/s/°C)(Tambient)
7. Use this figure to calculate (and record) how long it will take sound to travel down and back (x
= 2 × tube length) in the tube according to:
Vaccepted = x/t
Solving for t:
texpected = x/Vaccepted
This value for texpected will be the "gap" between wave peaks that you will be looking for on the
display.
Searching for Anticpated Results:
1. Set up the computer to start taking data on the first loud sound the Microphone detects.
2. Close the end of the tube. This can be done by standing a book against the end so it is
blocked.
3. Place the Microphone as close to the end of the long tube as
possible, as shown in Figure 2. Position the Microphone so that it can
detect the initial sound and the echo coming back down the tube.
4. Begin data collection. Snap your fingers near the opening of the
tube. You may find it more effective to clap your hands. A single sound that triggers data
collection will suffice.
Figure 3
5. If you are successful, the graph will resemble the one in
Figure 3. You may not see a third reflection. In this figure each
of three highest peaks corresponds to the same point on a
waveform. The first peak is the initial sound, the second is the
first reflection, and the third is a second reflection. Use the
cursor keys to trace across the graph and locate a gap between
any two of the major peaks that corresponds to the time gap, texpected, that you previously
calculated. This is evidence of the expected speed of sound. Record these two times in your Data
Table.
6. Repeat the procedure until five "clean runs" have been obtained.
Data Table:
Trial
Time of Initial
Sound Peak (s)
Time of Second
Peak (s)
--------------------
--------------------
Time Interval (s)
1
2
3
4
5
Average
Discussion
1. From the time-pairs you recorded in the Data Table, calculate the differences to find the
time interval, and then calculate the average time interval. For the write-up, you need
only show a sample of your time interval calculation.
2. Calculate the speed of sound according to Vmeasured = x/tavg where x is the total distance
that the sound travels. Remember that your average time interval represents the time for
sound to travel down the tube and back.
3. Compare Vmeasured to Vaccepted according to the usual percent difference equation.
4. Sketch one copy of the graph displayed on the computer and indicate the peaks that you
used for your measurements.
5. What might you expect would happen to the speed of sound if the tube was full of water?
Why?