SOUND 1
CHAPTER
CONTENTS HOW DOES SOUND PROPAGATE
Introduction
When a person speaks, the molecules in the air
How is sound produced near his mouth are disturbed. Due to this, these
How does sound propagate molecules start vibrating to-and-fro about their
mean positions. These vibrating molecules then
Oscillations disturb the nearby molecules. This process
Audible and Inaudible sound continues until the molecules in the air next to the
listener's ear start vibrating. These vibrating
Applications of Ultrasonic sound molecules then cause vibrations in the diaphragm
Sound needs a medium to propagate
of the listener's ear and the sound is heard.
Speed of sound OSCILLATIONS
Reflection of sounds
Have you carefully watched a child on a swing ? The
Characteristics of sound swing repeats its updown and forward-backward
Noise-A health hazard
motion in a regular fashion. The swing moves to-
and-fro on the same path with its mean position in
The human ear the middle. The motion like that of a swing is
called oscillatory motion.
Some examples of oscillatory motion are :
(a) motion of the pendulum of a wall clock,
INTRODUCTION
(b) vibrating string of a musical instrument.
Sound is a form of energy that produces the (c) motion of the heart muscles in a healthy
sensation of hearing in our ears. The speed of person.
light is 3 × 108 m/s and the speed of sound in the When a body undergoes an oscillatory motion, it
air under normal conditions is 340 m/s. So, the passes through a particular position at regular
light travels almost instantaneously, whereas intervals of time. Therefore, oscillatory motion is
sound takes some time. a periodic motion.
Oscillation
HOW IS SOUND PRODUCED
The movement of a body from one extreme
Sound is produced by vibrations. Thus, vibrating position to the other and back is called an
bodies produce sound. oscillation. In the figure shown the movement of
In some cases, the vibrations are easily visible to the bob from B to C and back to B is one
our naked eyes but in some cases they can only be complete oscillation.
felt and not seen.
, The sound in the audible range (20 to 20000
Hz) is called sonic sound. An infant (about 1
C B year old) can hear sounds up to 35000 Hz.
A This limit gradually comes down to 20000 Hz
Also, the motion of the bob from A to B, B to C for an adult.
and then from C to A is one complete oscillation. The sound of frequencies greater than 20,000
Amplitude of Oscillation (A) Hz is called ultrasonic sound.
The maximum displacement of a body from its The sound of frequencies lower than 20 Hz is
mean position is called the amplitude of called subsonic or infrasonic sound.
oscillation. Thus, in the figure shown, the subsonic sound Less than 20 Hz
displacement AB or AC is called amplitude of the sonic sound 20 Hz to 20000 Hz
oscillating bob. Amplitude is denoted by A.
ultrasonic sound Greater than 20000 Hz
For a body oscillating in the air, the amplitude of
oscillation gradually decreases due to the air-
resistance. APPLICATIONS OF ULTRASONIC
SOUND
Time period (T)
The time taken to complete one oscillation is called Certain animals such as dog, leopard, monkey and
its time period. Time period is denoted by T. In the deer can hear ultrasonic sounds. Certain birds like
figure, the time taken by the bob to travel from B bat can produce sounds of very high frequencies.
to C and back to B is called is its time period. A bat is able to locate any obstacle or its prey in
As long as the amplitude of oscillation is small, its path due to reflection of the ultrasonic wave
the pendulum takes equal intervals of time to from the object. Dolphins use ultrasonic sound to
complete each oscillation. locate their prey.
Frequency of Oscillation Technological / Industrial Applications of
The number of oscillations made by an oscillating Ultrasonic Sound
body in one second is called the frequency of Ultrasonic waves have short wavelength.
oscillation.
These short wavelength sound waves can be
Frequency () is related to the time period (T) by reflected back from the smaller objects. Thus,
the relationship, ultrasound can detect or 'see' smaller objects
Frequency, = 1/T (< 1 cm size). The ultrasonic waves do not get
The unit of frequency of oscillation is hertz (Hz). scattered.
1 Hz = 1 cycle per second Some important technological and industrial uses of
ultrasonic waves are described below :
AUDIBLE AND INAUDIBLE SOUND Ultrasonic waves are used to drive away rats,
cockroaches etc.
The human ear can hear the sounds having Ultrasonic waves are used for detecting any
frequencies between 20 Hz to 20000 Hz. This is deformity in the unborne baby.
called the audible range. Thus, the audible range Ultrasonic waves are also used for
of a normal human ear is 20 to 20000 Hz. determining the depth of sea.
CHAPTER
CONTENTS HOW DOES SOUND PROPAGATE
Introduction
When a person speaks, the molecules in the air
How is sound produced near his mouth are disturbed. Due to this, these
How does sound propagate molecules start vibrating to-and-fro about their
mean positions. These vibrating molecules then
Oscillations disturb the nearby molecules. This process
Audible and Inaudible sound continues until the molecules in the air next to the
listener's ear start vibrating. These vibrating
Applications of Ultrasonic sound molecules then cause vibrations in the diaphragm
Sound needs a medium to propagate
of the listener's ear and the sound is heard.
Speed of sound OSCILLATIONS
Reflection of sounds
Have you carefully watched a child on a swing ? The
Characteristics of sound swing repeats its updown and forward-backward
Noise-A health hazard
motion in a regular fashion. The swing moves to-
and-fro on the same path with its mean position in
The human ear the middle. The motion like that of a swing is
called oscillatory motion.
Some examples of oscillatory motion are :
(a) motion of the pendulum of a wall clock,
INTRODUCTION
(b) vibrating string of a musical instrument.
Sound is a form of energy that produces the (c) motion of the heart muscles in a healthy
sensation of hearing in our ears. The speed of person.
light is 3 × 108 m/s and the speed of sound in the When a body undergoes an oscillatory motion, it
air under normal conditions is 340 m/s. So, the passes through a particular position at regular
light travels almost instantaneously, whereas intervals of time. Therefore, oscillatory motion is
sound takes some time. a periodic motion.
Oscillation
HOW IS SOUND PRODUCED
The movement of a body from one extreme
Sound is produced by vibrations. Thus, vibrating position to the other and back is called an
bodies produce sound. oscillation. In the figure shown the movement of
In some cases, the vibrations are easily visible to the bob from B to C and back to B is one
our naked eyes but in some cases they can only be complete oscillation.
felt and not seen.
, The sound in the audible range (20 to 20000
Hz) is called sonic sound. An infant (about 1
C B year old) can hear sounds up to 35000 Hz.
A This limit gradually comes down to 20000 Hz
Also, the motion of the bob from A to B, B to C for an adult.
and then from C to A is one complete oscillation. The sound of frequencies greater than 20,000
Amplitude of Oscillation (A) Hz is called ultrasonic sound.
The maximum displacement of a body from its The sound of frequencies lower than 20 Hz is
mean position is called the amplitude of called subsonic or infrasonic sound.
oscillation. Thus, in the figure shown, the subsonic sound Less than 20 Hz
displacement AB or AC is called amplitude of the sonic sound 20 Hz to 20000 Hz
oscillating bob. Amplitude is denoted by A.
ultrasonic sound Greater than 20000 Hz
For a body oscillating in the air, the amplitude of
oscillation gradually decreases due to the air-
resistance. APPLICATIONS OF ULTRASONIC
SOUND
Time period (T)
The time taken to complete one oscillation is called Certain animals such as dog, leopard, monkey and
its time period. Time period is denoted by T. In the deer can hear ultrasonic sounds. Certain birds like
figure, the time taken by the bob to travel from B bat can produce sounds of very high frequencies.
to C and back to B is called is its time period. A bat is able to locate any obstacle or its prey in
As long as the amplitude of oscillation is small, its path due to reflection of the ultrasonic wave
the pendulum takes equal intervals of time to from the object. Dolphins use ultrasonic sound to
complete each oscillation. locate their prey.
Frequency of Oscillation Technological / Industrial Applications of
The number of oscillations made by an oscillating Ultrasonic Sound
body in one second is called the frequency of Ultrasonic waves have short wavelength.
oscillation.
These short wavelength sound waves can be
Frequency () is related to the time period (T) by reflected back from the smaller objects. Thus,
the relationship, ultrasound can detect or 'see' smaller objects
Frequency, = 1/T (< 1 cm size). The ultrasonic waves do not get
The unit of frequency of oscillation is hertz (Hz). scattered.
1 Hz = 1 cycle per second Some important technological and industrial uses of
ultrasonic waves are described below :
AUDIBLE AND INAUDIBLE SOUND Ultrasonic waves are used to drive away rats,
cockroaches etc.
The human ear can hear the sounds having Ultrasonic waves are used for detecting any
frequencies between 20 Hz to 20000 Hz. This is deformity in the unborne baby.
called the audible range. Thus, the audible range Ultrasonic waves are also used for
of a normal human ear is 20 to 20000 Hz. determining the depth of sea.
