Experiment 12
The Speed of Sound
The oscilloscope is used to determine the speed of sound in air. A sound pulse triggers
the sweep of the oscilloscope, and the time of travel of the sound through a plastic tube will be
determined
Theory
Pulses of sound emanating from a speaker are sent from one end of a plastic tube to the
other end of the tube where it is picked up by a microphone. If the length of the tube and the
time for sound to travel the length of the tube is known, then the speed of sound can be
determined from the expression
V 
L
,
t
(1)
where L is the length of the tube (from speaker to microphone) and t is the time it takes the sound
to travel the length of the tube.
In the experiment, a square wave signal produced by a signal generator is fed to a
speaker and also to the external trigger input of the oscilloscope. (Refer to Figure 1.) The signal
produces the pulses of sound sent down the tube and also triggers the sweep of the electron beam
across the face of the oscilloscope. The microphone is connected to the vertical input terminals
of the oscilloscope and the signal received by it is registered on the oscilloscope. (Refer to Figure
2.)
Figure 1. The experimental arrangement.
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Figure 2. The shape of the signal seen on the oscilloscope screen.
In order to find the time taken by the signal to travel from the speaker to the microphone,
the distance from the "start of the triggered signal" to the "beginning of signal received by the
“microphone" is measured. When this distance is multiplied by the sweep speed, the time is
determined.
Apparatus
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plastic tube, approximately one meter long
speaker
oscilloscope
microphone
signal generator
meter stick
0-50ºC thermometer
Procedure
1) Record the temperature of the air in the tube.
2) Measure the length from the base of the speaker cone to the center of the microphone.
Record this value as L.
3) Connect the circuit as shown in Figure 1.
4) Turn on the signal generator and adjust it to give a square wave at a frequency of 20Hz.
5) Turn on the oscilloscope and ensure that the horizontal sweep is in the calibrate position.
Engage the SOURCE button to select the external (EXT) signal to start the sweep, then
adjust the TRIGGER level knob to get the type of signal as shown is figure 2. The
frequency of the signal generator may need to be adjusted to lower frequencies in order to
obtain a reasonably good signal.
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Note: Adjusting the trigger level knob allows the beam of electrons to sweep across
The screen only if the input signal is some minimum value. The signal coming
From the signal generator provides the signal for the trigger.
6) Record the distance x on the oscilloscope screen from the "start of the triggered signal” to
the “beginning of signal received by the microphone.”
Analysis
The speed of sound in air as a function of temperature is
u = 331.4 + 0.6T ,
2)
where u is the velocity of sound in meters per second and T is the temperature of the air in the
tube in Celsius degrees.
Calculate the speed of sound in air from (1) and find the percentage error between the
experimental value (1) and the theoretical value (2). Report both values of the speed of sound
and the percentage error in a results table.
Conclusions
What are the sources of error that contributed most to the error in the experimental value
of the speed of sound? Why do you feel they produced large errors?
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