Measuring the Velocity of Sound

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Department of Physics
Measuring the Velocity of Sound
Li Xianglian
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Contents

• Objectives
• Pre-lab Questions
• Introduction Apparatus
• Procedure Experiment
• Problems

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Objectives

• Learn how to measure the speed of sound.

• Practice using an oscilloscope.

• Learn to manipulate the evenly spaced data with
  the method of successive subtraction.

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Pre-lab Questions

• Which determines the speed of sound, the source
  frequency or the medium?
• The distance between successive nodes or
  antinodes in a standing wave is a measure of what?

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3. Introduction Apparatus
Sound wave as a longitudinal wave
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In this experiment, we use the so-called
resonance method to measure the wavelength of the
sound wave.
If a sound wave propagates in a limited
region, it will be reflected at the border, and
the reflected wave would interfere with the
original one, producing a standing superposed
wave.
Demonstration of a standing sound wave
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One of the characteristics of a standing wave is
that there are successive nodes or antinodes at
intervals of half of the wave-length of the
traveling waves.
Distribution of nodes (N) and antinodes (A) in
standing waves
Therefore, if we get the distance of two
adjacent anti-nodes, we can have the wavelength
.
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The apparatus to be used is shown as follows
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4. Procedure Experiment
4.1. Connect the wires as in the following
figure.
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4.2. Set the oscilloscope to show the signal from
S2. 4.3. Check to be sure that the plates of the
two transducers are parallel. 4.4. Set the
signal generator to produce a sinusoidal signal
and choose an appro-priate frequency of the
signal such that the crystal transducers
resonate. Record the frequency value f on your
notebook.
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4.5. Move S1 slowly and unidirectionally away
from S2, and observe the received signal on the
screen. Record the location one by one (as shown
in the following table) where the antinodes of
the standing wave appear.
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4.6. Calculate the sound speed in air. At first,
calculate the mean intervals of the adjacent
antinodes using the method of successive
subtraction as follows
Then calculate the sound speed in air according
to .
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4.7. Measure the speed of sound in water. Use the
lower pair of transducers instead of the upper
ones (you need reconnect some of the wires). Fill
the tank half full with water and place the
transducers into the water.
4.8. Repeat steps 4.3-4.5 and calculate the sound
speed in water the same way as that
in step 4.6.
4.9. Calculate the theoretical value of the sound
speed at room temperature in air,

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using
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Problems

• What determines the pitch of a sound wave? The
  source, or the medium?

• Where does sound travel faster? In air, or in
  water?

End