Unit 4: Science and Materials in Construction and the Built Environment
Sound
8.1
Origin of Sound
Sound is a variation in the pressure of the air of a type which has an
effect on our ears and brain. These pressure variations transfer energy
from a source of vibration that can be naturally-occurring, such as by the
wind or produced by humans such as by speech. Sound in the air can be
caused by a variety of vibrations, such as the following.

Moving objects: examples include loudspeakers, guitar strings,
vibrating walls and human vocal chords.
Chapter 8: Sound
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Unit 4: Science and Materials in Construction and the Built Environment

8.2
Moving air: examples include horns, organ pipes, wood and brass
instruments, mechanical fans and jet engines.
Wave motion
The mechanical vibrations of sound move forward, using wave motion.
This means that, although the individual particles of material such as air
molecules return to their original position, the sound energy obviously
travels forward. The front of the wave spreads out equally in all directions
unless it is affected by an object or by another material in its path.
The waves are longitudinal in type because the particles of the medium
carrying the wave vibrate in the same direction as the travel of the wave,
as shown in following figures. The sound waves can travel through solids,
liquids and gases, but not through a vacuum.
Chapter 8: Sound
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Unit 4: Science and Materials in Construction and the Built Environment
Sound Waves in Air
A single-frequency sound wave traveling through air will cause a sinusoidal pressure
variation in the air. The air motion which accompanies the passage of the sound wave will be
back and forth in the direction of the propagation of the sound, a characteristic
of longitudinal waves.
Physics professor Clint Sprott of the University of Wisconsin shows one way to visualize
these longitudinal pressure waves in his "Wonders of Physics" demonstration show. A
loudspeaker is driven by a tone generator to produce single frequency sounds in a pipe which
is filled with natural gas (methane). A series of holes is drilled in the pipe to release a small
amount of gas. Igniting the gas produces flames for which the height increases with the
pressure in the pipe. The pattern of the flames shows the pressure variation and can be used to
roughly measure the wavelength of the pressure wave in the pipe.
Low frequency
High frequency
Shown below is more detail on the attachment of the loudspeaker to the pipe. The
loudspeaker is driven by the amplified output of a tunable oscillator.
A series of small holes were drilled at regular intervals
in the pipe. They appeared to be about 8 mm apart.
This article was taken from: http://hyperphysics.phy-astr.gsu.edu/hbase/sound/tralon.html
Chapter 8: Sound
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Unit 4: Science and Materials in Construction and the Built Environment
8.3
Sound Calculations
The velocity of sound is influenced by three factors:



Wavelength () is the distance between any two repeating points
on a wave. Unit: metre (m)
Frequency (f ) is the number of cycles of vibration per second.
Unit: hertz (Hz)
Velocity (v) is the distance moved per second in a fixed direction.
Unit: metres per second (m/s)
For every vibration of the sound source the wave moves forward by one
wavelength. The number of vibrations per second therefore indicates the
total length moved in 1 second; which is the same as velocity. This
relationship is true for all wave motions and can be written as the
following formula.
Where
v = velocity in m/s
f = frequency in Hz
 = wavelength in m
Practical Example 1
A particular sound wave has a frequency of 440 Hz and a velocity of
340m/s. Calculate the wavelength of this sound.
Answer
v = 340 m/s
f = 440 Hz
=?m
So
So wavelength = 0.7727 m
Chapter 8: Sound
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Unit 4: Science and Materials in Construction and the Built Environment
8.4
Velocity of sound
A sound wave travels away from its source with a speed of 344 m/s (770
miles per hour) when measure in dry air at 20oC. This is a respectable
speed within a room but slow enough over the ground for us to notice the
delay between seeing a source of sound, such as a distant firework, and
later hearing the explosion.
The velocity of sound is independent of the rate at which the sound
vibrations occur, which means that the frequency of a sound does not
affect its speed. The velocity is also unaffected by variations in
atmospheric pressure such as those caused by the weather.
But the velocity of sound is affected by the properties of the material
through which it is travelling, and table 8.1 gives an indication of the
velocities of sound in different materials. The velocity of sound in gases
decreases with increasing density since the molecules are heavier. Moist
air contains a greater number of light molecules and therefore sound
travels slightly faster in moist humid air.
Sound travels faster in liquids and solids than it does in air because of the
effect of density and elasticity of those materials. The particles of such
materials respond to vibrations more quickly and so convey the pressure
vibrations at a faster rate. For example, steel is very elastic and sound
travels through steel about 14 times faster than it does through air.
Table 8.1 Velocity of sound
Material
Air (0oC)
Air (20oC)
Water (25oC)
Pine
Glass
Steel
Granite
Chapter 8: Sound
Typical velocity (m/s)
331
344
1498
3300
5000
5000
6000
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Unit 4: Science and Materials in Construction and the Built Environment
8.5
Frequency of sound
If an object that produces sound waves vibrates 100 times a second, for
example, then the frequency of that sound wave will be 100Hz. The
human ear hears this as sound of a certain pitch.

Pitch is the frequency of a sound as perceived by human hearing.
Low-pitched notes are caused by high-frequency sound waves and highpitched notes are caused by high-frequency waves. The pitch of a note
determines its position in the musical scale. The frequency range to which
the human ear responds is approximately 20 to 20 000 Hz and
frequencies of some typical sounds are shown in the following figures.
Chapter 8: Sound
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Unit 4: Science and Materials in Construction and the Built Environment
8.6
Cancellation of sound
The nature of a sound wave means that the vibration of the wave has
alternate changes in amplitude called phases. If a wave vibration in one
direction meets an equal and opposite vibration, then they will cancel.
The effect of this phase inversion in sound waves is to produce little or no
sound and gives the possibility of ‘cancelling’ noise.
Chapter 8: Sound
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Unit 4: Science and Materials in Construction and the Built Environment
8.7
Nature of hearing
The ear is divided into two parts: the outer and the inner ear. The outer
ear contains the cartilage that forms the external ears and a tube that
directs sound into the inner ear. The eardrum is at the bottom of this tube
which vibrates when sound reaches it. Connected to the eardrum is a
series of bones called ossicles.
A mechanism passes the sound as vibration via these bones into the inner
ear. Here a liquid vibrates which contains tiny hairs which move with the
sound vibration. When they move this is transmitted to the brain in the
form of an electrical signal which is interpreted as sound. Having two ears
gives humans the ability to detect from where sound is coming in stereo.
Go to the following site for more information about sound.
http://www.ndt-ed.org/EducationResources/HighSchool/Sound/introsound.htm
Chapter 8: Sound
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Unit 4: Science and Materials in Construction and the Built Environment
Exercise 8.1
1. Calculate the wavelength of sound in air at:
i)
20 Hz
ii)
50 Hz
The velocity of sound in air is 340 m/s
2. The frequency of the note produced by a particular tuning fork is
320 Hz. Calculate the wavelength of the wave produced in the
surrounding air if the speed of sound in air is 340 m/s.
3. If the sound from the tuning fork in (2) was detected under water,
what frequency would be heard?
4. If the wavelength in (3) of water is 4.5m, at what speed does the
wave travel through the water?
Chapter 8: Sound
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