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Chapter 11: SOUND
PRODUCTION
OF SOUND
Sound is a form of energy which produces
a sensation of hearing. It is produced by vibrating objects.
Activity 1
•
Strike a tuning fork on a rubber pad to set it
vibrating.
•
When
brought near the ear, a sound can be heard.
•
Touch one of the vibrating prongs with a finger. The vibration will stop.
•
Suspend a table tennis or small plastic ball using a thread. Gently
touch the ball with the vibrating prong. The ball will move.
Activity 2
•
Fill water
in a beaker or a glass up to the brim. Gently touch the water surface with one
prong of the vibrating tuning fork. It creates minimal vibrations in water.
•
Dip the
prongs of the vibrating tuning fork in water. It causes more significant
vibrations and disturbances.
-
Vibration is
a rapid to and fro motion of an object. E.g.,
·
Vibrations
in the vocal cords of human produce voice.
·
In bees,
vibration of wings produces buzzing sound. But flapping of bird’s wings doesn’t
produce noticeable sound because flapping is slow.
·
A
stretched rubber band when plucked vibrates and produces sound.
PROPAGATION
OF SOUND
-
Sound
moves through a medium (solid, liquid, or gas) from the source to the listener.
-
When an
object vibrates, the surrounding particles of the medium also vibrate. So the particle
is displaced from its equilibrium position and exert a force on the adjacent
particle, causing it to displace. Then the first particle returns to its
original position. This repeats until the sound reaches the ear. i.e., the
disturbance, not the particles, travels through the medium.
-
A wave is
a disturbance that moves through a medium by setting neighbouring particles
into motion. Sound is a mechanical wave characterized by the motion of
particles in the medium.
-
Air is the
most common medium for sound travel.
-
As a
vibrating object moves forward, it compresses the air, creating a high-pressure
region called compression (C), which moves away from the object. When
the object moves backward, it creates a low-pressure region called rarefaction
(R). The rapid back-and-forth movement forms a series of compressions and
rarefactions that propagate as sound waves.
-
Pressure
is related to the number of particles of a medium in a given volume. More
density of the particles gives more pressure and vice versa. Thus,
propagation of sound can be seen as propagation of density or pressure
variations in the medium.
SOUND WAVES ARE LONGITUDINAL WAVES
Activity
•
Stretch a slinky
by holding its two ends.
•
Give it a
sharp push. A compression wave travels along the slinky.
•
Push and
pull the slinky alternately to see a series of compressions and rarefactions
traveling along it.
•
Mark a dot
on the slinky. The dot will move back and forth parallel to the direction of
the propagation of the disturbance.
-
The
regions where the coils become closer are called compressions (C) and
the regions where the coils are further apart are called rarefactions (R).
-
Sound
propagates as a series of compressions and rarefactions, similar to a
disturbance in a slinky. These are longitudinal waves, where particles
move parallel to the direction of wave propagation.
-
Particles do
not travel but oscillate back and forth about their position of rest. This is how
a sound wave propagates. So sound waves are longitudinal waves.
-
In a transverse
wave, particles oscillate up and down about their mean position,
perpendicular to the direction of wave propagation. E.g., a pebble dropped in a
pond creates transverse waves.
-
Light is
also a transverse wave, but its oscillations are not of medium particles or
pressure or density. So it is not a mechanical wave.
CHARACTERISTICS OF A SOUND WAVE
-
A sound
wave is described by its frequency, amplitude and speed.
-
Graphically,
a sound wave represents changes in density and pressure as it moves through the
medium. The density and pressure at a given time vary with distance,
oscillating above and below the average values.
-
Compressions are the regions where particles are crowded together and represented
by the upper part of the curve. Here, density and pressure are high. The peak
represents the region of maximum compression.
-
Rarefactions are the regions of low pressure where particles are spread apart
and are represented by the valley (lower part of the curve). A peak is called
the crest and a valley is called the trough of a wave.
-
The
distance between two consecutive compressions or rarefactions is called the wavelength
(λ). Its SI unit is metre (m).
-
As sound
propagates through a medium, the density of the medium oscillates between
maximum and minimum values. One complete oscillation is the change from maximum
to minimum density and back to maximum.
-
The number
of oscillations per unit time is the frequency (ν) of the sound wave.
Counting the compressions or rarefactions that cross a point per unit time
gives the frequency (ν). Its SI unit is hertz (Hz).
-
The time
taken for one complete oscillation (i.e., time taken by two consecutive compressions
or rarefactions to cross a fixed point) is called the time period (T) of
the sound wave. Its SI unit is second (s).
-
Frequency
and time period are related as:
-
Consider a
violin and a flute are played simultaneously in an orchestra. Both sounds
travel through the same medium (air) at the same speed and reach the ear at the
same time. However, the sounds differ due to their distinct characteristics,
such as pitch.
-
How the
brain interprets the frequency of a sound is called
its pitch. Faster vibrations result in higher frequency and
pitch. Thus, a high pitch sound corresponds to more compressions and
rarefactions passing a fixed point per unit time.
-
Objects of
different sizes and conditions vibrate at different
frequencies to produce sounds of different pitch.
-
The
magnitude of the maximum disturbance in the medium on either side of the mean
value is called the amplitude (A) of the wave. For sound, its unit is
that of density or pressure.
-
The loudness or softness of a sound is determined by its amplitude.
-
Amplitude
depends on the force causing vibration of an object. A light strike on a table
produces a soft sound with lower amplitude (less energy). A hard hit results in
a louder sound with greater amplitude. As the sound wave spreads out from its
source, its amplitude and loudness decrease.
-
Louder
sounds travel farther due to their higher energy.
-
The quality or timbre of sound is the characteristic that distinguishes sounds with
the same pitch and loudness. A more pleasant sound is said to have a rich quality.
-
A sound of single frequency is called a tone.
-
A sound produced by a mixture of several frequencies is called a note and is
pleasant to listen to.
-
Noise is unpleasant to the ear. Music is pleasant to hear and is of rich
quality.
-
The speed
of sound is defined as the distance which a point on a wave, such as a compression
or a rarefaction, travels per unit time.
-
Speed, v
= distance / time.
-
Speed of
sound is almost the same for all frequencies in a given medium under the same
physical conditions.
Example: A sound wave has a frequency of
2 kHz and wave length 35 cm. How long will it take to travel 1.5 km?
Solution:
Frequency, ν = 2
kHz = 2000 Hz
Wavelength, λ =
35 cm = 0.35 m
Speed of the wave,
v = λ ν
= 0.35 m 2000 Hz = 700
m/s
Time
taken by wave to travel a distance, d of 1.5 km is
-
The amount
of sound energy passing through unit area per second is called the intensity
of sound.
-
Loudness
and intensity are not the same; loudness measures
the ear's response to sound.
-
Even two
sounds have equal intensity, one sound may be heard as louder than the other because
our ear detects it better.
SPEED OF SOUND IN DIFFERENT MEDIA
-
Sound
propagates through a medium at a finite speed.
-
The delay
between hearing thunder and seeing lightning shows that sound travels much
slower than light.
-
Speed of
sound in a medium is affected by temperature. It decreases from solid to gas. As
temperature increases, the speed of sound also increases. E.g., speed of sound
in air is 331 m s–1 at 0 °C and 344 m s–1 at 22 °C.
|
Speed of
sound in different media at 25°C |
||
|
State |
Substance |
Speed in m/s |
|
Solids
|
Aluminium Nickel Steel Iron Brass Glass (Flint) |
6420 6040 5960 5950 4700 3980 |
|
Liquids |
Water (Sea) Water (distilled) Ethanol Methanol |
1531 1498 1207 1103 |
|
Gases |
Hydrogen Helium Air Oxygen Sulphur dioxide |
1284 965 346 316 213 |
REFLECTION OF SOUND
-
Sound
bounces off solids or liquids like a rubber ball bounces off a wall.
-
Like
light, sound reflects at the surface of a solid or liquid according to the same
laws of reflection. The angles of incidence and reflection are equal to the
normal at the point of incidence, all within the same plane. Reflection
requires a large obstacle, polished or rough.
Activity
•
Arrange two
identical pipes on a table near a wall.
•
Keep a
clock near the open end of one pipe and try to hear the sound of the clock
through the other pipe.
•
Adjust the
pipes to best hear the clock's sound.
•
Measure the
angles of incidence and reflection. The angles are equal.
•
Lift the
pipe on the right vertically to a small height. The sound will become harder to
hear.
ECHO
-
It is the
repetition of a shout or clap heard later after reflecting off an object like a
building or mountain.
-
The
sensation of sound persists in brain for 0.1 s.
-
To hear a
distinct echo, the time interval between the original sound and reflected one
must be at least 0.1 s.
-
If the
speed of sound is 344 m/s at 22 °C, the total distance traveled by the sound
(to the reflecting surface and back to listener) must be at least 34.4 m (344 x
0.1 s). Thus, to hear distinct echoes, the minimum distance from the source to
reflecting surface must be 17.2 m (half of total distance).
-
This
distance varies with the temperature of air.
-
Echoes can
occur multiple times due to successive reflections. E.g., the rolling of
thunder is due to repeated reflections of sound from clouds and the land.
REVERBERATION
-
In a big
hall, sound persists through repeated reflections from the walls until it is inaudible.
This persistence of sound due to repeated
reflections is called reverberation.
-
Excessive
reverberation in auditoriums is undesirable. To reduce it, sound-absorbent
materials such as compressed fiberboard, rough plaster, or draperies are used
on the roof and walls. Seat materials are also chosen for their sound-absorbing
properties.
Example: A person clapped near a cliff
and heard the echo after 2 s. What is the distance of the cliff from the person
if the speed of the sound, v is taken as 346 m s–1?
Solution:
Speed of sound, v
= 346 m s–1
Time taken for hearing
the echo, t = 2 s
Distance travelled by
the sound
= v × t =
346 m s–1 × 2 s = 692 m
In 2 s sound has to
travel twice the distance between the cliff and the person. Hence, the distance
between the cliff and the person = 692 m/2 = 346
m.
USES OF MULTIPLE REFLECTION OF SOUND
1.
Megaphones
(loudhailers), horns, musical instruments like trumpets and shehnais, are
designed to send sound in a particular direction. They use a tube and a conical
opening to reflect and guide most sound waves towards the audience.
2.
Stethoscope is used to listen to internal body sounds, mainly in the heart or lungs.
The sound of heartbeat reaches the doctor’s ears
by multiple reflection of sound.
3. Ceilings in concert halls, conference halls, and cinema halls are often curved to ensure sound reflects and reaches all corners. A curved soundboard may also be placed behind the stage to evenly distribute sound across the hall.
RANGE
OF HEARING
-
The
audible range of sound for human is 20 Hz to 20000 Hz (one Hz =
one cycle/s).
-
Children
under the age of five and some animals (e.g., dogs) can hear up to 25 kHz (1
kHz = 1000 Hz).
-
As people
age, their ears become less sensitive to higher frequencies. Frequencies below
20 Hz are called infrasonic sound or infrasound. If we could hear
infrasound, we would hear the vibrations of a pendulum
just as we hear the vibrations of the wings of a bee.
-
Rhinoceroses communicate using infrasound of frequency as low as 5 Hz. Whales
and elephants produce sound in the infrasound range.
-
Some animals
get disturbed before earthquakes.
Earthquakes produce low-frequency infrasound before the main shock
waves begin which alert the animals.
-
Frequencies
higher than 20 kHz are called ultrasonic sound or ultrasound. They
are high frequency waves.
-
Ultrasound
is produced by animals like dolphins, bats, rats, and porpoises. Certain moth
families can detect the high-frequency squeaks of bats, allowing them to sense
nearby bats and escape capture.
Hearing Aid: It is an electronic, battery-operated
device. It receives sound through a microphone which converts the sound waves
to electrical signals. These signals are amplified by an amplifier and sent to
a speaker, which converts them back into sound for clearer hearing.
APPLICATIONS
OF ULTRASOUND
Ultrasounds can travel along well-defined paths even in the presence
of obstacles. Their uses are given below:
•
Ultrasound cleans hard-to-reach parts, such as spiral tubes and electronic components. Objects are placed in a cleaning solution, and ultrasonic waves detach particles of dust, grease, and dirt,
thoroughly cleaning them.
•
It detects
cracks and flaws in metal blocks used in construction and scientific equipments.
Ultrasonic waves pass through the metal, and detectors identify any reflected
waves from defects.
Ordinary sound of longer wavelengths cannot be used for such purpose
as it will bend around the corners of
the defective location and enter the detector.
•
Ultrasonic
waves are made to reflect from various parts of the heart and form the image of
the heart. This technique is called echocardiography.
•
Ultrasound
scanner is an instrument which uses ultrasonic
waves to create images of internal organs, such as the liver, gall bladder,
uterus, and kidneys.
It helps to detect abnormalities, like stones or tumours.
The ultrasonic waves travel through the body tissues and get
reflected from a region where there is a change of tissue density. These waves
are converted into electrical signals to generate images of the organ. These
images are displayed on a monitor or printed on film. This technique is called ultrasonography.
Ultrasonography is also used for examination of the foetus during
pregnancy to detect congenital defects and growth abnormalities.
•
Ultrasound
can be used to break small kidney stones into fine grains which are then
flushed out with urine.
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