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Notes
Sound
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Physics Tool box



Frequency: is the number of occurrences of a repeating event per unit time.
Wavelength: the distance over which the wave's shape repeats
Speed of Sound in Air: 322 m/s.

Speed of Sound in various Temperatures: v  332
m 
m/s
  0.59
t
s 
C 
Sound waves are generated by rapidly move objects, which in turn push against air molecules which in turn bump
against their neighbours. This creates a compression which moves along the air, when the molecules move back they
create a region of emptiness (a rarefaction).
Each molecule moves back and forwards only a tiny distance, but it is enough to cause the air particles to bump into each
other. This creates areas where there are many molecules pushed close together --compression; and areas where
molecules are spread far apart --rarefactions. These compressions and rarefactions move outwards away from the
sound source in circles. A sound wave is created when a series of these pressure changes/waves move through the air.
What is the frequency of sound waves?
When we draw a sound wave, the wave peaks and valleys are close together or far apart. Sound waves vibrate at different
rates or "frequencies" as they move through the air. Frequency is measured in cycles per second, or Hertz, after the
German physicist who experimented with sound in the 19th century. The faster an object vibrates, i.e. the higher the
frequency, then the higher the pitch of the sound. For example, a tuning fork for A above middle C will vibrate 440 times per
second and has a frequency of 440 Hertz.
What is the wavelength of a sound wave?
When a wave is created, the distance between one compression and the next compression is called the wavelength. The
faster the sound waves pass a given point, the shorter the wavelength and the higher the frequency. Sounds of all
frequencies travel at the same rate in the same medium. (Sound in dry air at 0 C travels at the rate of 1200 kilometres per
hour, or 331.6 m/sec; in a solid medium the sound waves travel faster.)
What is the amplitude of sound waves?
The vibrations can also "squeeze" the air molecules together very hard or very gently. This squeezing is called "amplitude"
and is represented on the top half of the diagram below. The bottom half of the diagram is a representation of the pressure
of the air during a sound wave. The horizontal line represents normal air pressure.
The more we push an object to make it vibrate, the larger the vibrations and the louder the sound, or the greater the
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amplitude. Sound waves with the same frequency can have different amplitudes.
Example
Sound from a trumpet travels at a speed of 3.5  102
m
in the air. If a note is played with the frequency of
s
2.0  102 Hz , what is the wavelength of the sound wave?
Solution:

v
f
m
s

2
2.0  10 Hz
 1.75m
3.5  102
The Speed of Sound
Like any wave, the speed of a sound wave refers to how fast the disturbance is passed from particle to particle.
While frequency refers to the number of vibrations that an individual particle makes per unit of time, speed
refers to the distance that the disturbance travels per unit of time. Always be cautious to distinguish between
the two often-confused quantities of speed (how fast...) and frequency (how often...).
Accurate measurements of the speed of sound in the air have been made at varian air pressures and at
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Sound
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various air temperatures. At the normal atmospheric pressure with the air temperature bing 0oC, if has been
determined that the speed of sound is 332
m
.
s
At other temperatures, the following equation can be used when you know the temperature in degrees
Celsius:
v  332
m 
m/s
  0.59
t
s 
C 
Example
Determine the speed of sound in air when the temperature is 5 degrees Celsius?
Solution:
v  332
m 
m/s
  0.59
t
s 
C 
m 
m/s
  0.59
  5C 
s 
C 
m
 335
s
 332
Example
A 250 Hz tuning fork is struck in a room where the air temperature is 30oC
a) What is the speed of sound in the room?
b) What is the wavelength of the sound in the room from the turning fork?
Solution:
a) v  332
m 
m/s
  0.59
t
s 
C 
m 
m/s
  0.59
  30C 
s 
C 
m
 339.7
s
 332
b) v   f

v
f
m
s

250 Hz
 1.36m
339.7
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Sound
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Sound
Extra Notes and Comments
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