Sound:
“Sound is a form of energy which is produced by a vibrating body” Sound Requires Material Medium for its
Propagation.
SOUND WAVES
Like
other waves, the sound is also produced by vibrating bodies. Due to the vibrations of
bodies, the air around them also vibrates and the air vibrations produce the sensation of sound in our ears.
For
example, in a guitar, the sound is produced due to the vibrations of its strings. Our voice results from
the vibrations of our vocal cords. Human heartbeats and vibrations of other
organs like lungs also produce sound waves. Doctors use stethoscopes to hear
this sound.
Longitudinal Nature of Sound
Waves:
Propagation
of sound waves produced by a vibrating tuning fork can be understood by vibrating tuning fork as shown in Figure. Before the vibration of the tuning fork, the density of air molecules on the right side is uniform. When the right prong of the tuning fork moves from mean position O to B, it exerts some pressure on the
adjacent layer of air molecules and produces compression. This compressed-air
layer in turn compresses the layer next to it and so on. A moment later, the
prong begins to move from B towards A. In this way, the sound wave propagates
through the air.
“The
direction of propagation of the sound wave is along the direction of oscillating
air molecules. This shows the longitudinal nature of sound waves. Distance between two consecutive
compressions or rarefactions is the wavelength of the sound wave.”
CHARACTERISTICS
OF SOUND
Sounds
of different objects can be distinguished on the basis of different
characteristics as described below:
1.
Loudness
2.
Pitch
3.
Quality
4.
Intensity
1.
Loudness:
“Loudness
is the characteristic of sound by which loud and faint sounds can be
distinguished.”
When we talk
to our friends, our voice is low, but when we address a public gathering our
voice is loud. The loudness of a sound depends upon a number of factors. Some of
them are discussed below:
(a)
Amplitude of the vibrating body:
The loudness of the sound
varies directly with the amplitude of the vibrating body.
Loud
Sound = Large Amplitude
Quit
Sound = Small Amplitude
(b) Area of the vibrating body:
The loudness
of sound also depends upon the area of the vibrating body.
For
example,
the sound produced
by a large drum is louder than that by a small one because of its large vibrating
area.
(c)
Distance from the vibrating body:
The loudness of sound also depends upon the distance of the vibrating body from the listener.
It is caused by the decrease in amplitude due to an increase in distance.
(d) Physical Condition:
Loudness
also depends upon the physical condition of the ears of the listener. A sound
appears louder to a person with sensitive ears than to a person with defective
ears. However, there is a characteristic of sound that does not depend upon
the sensitivity of the ear of the listener and it is called intensity of sound.
2.
Pitch:
Pitch
is the characteristic of sound by which we can distinguish between a shrill and
a grave sound.
It
depends upon the frequency. A higher pitch means a higher frequency and vice
versa. The frequency of the voice of ladies and children is higher than that of
men. Therefore, the voice of ladies and children is shrill and of high pitch.
3.
Quality: The characteristic of sound by which
we can distinguish between two sounds of the same loudness and pitch is called
quality.
While
standing outside a room, we can distinguish between the notes of a piano and a
flute being played inside the room. This is due to the difference in the
quality of these notes.
4.
Intensity: The sound waves transfer energy from
the sounding body to the listener. The intensity of sound depends on the
amplitude of the sound wave and is defined as:
“Sound energy passing per second through
a unit area held perpendicular to the direction of propagation of sound waves
is called intensity of sound.” The
unit of intensity of sound is watt per square meter (Wm-2).
Sound Intensity Level:
The
loudness (L) of a sound is directly proportional to the logarithm of intensity
i.e.,
L
α log I
L
= K log I ……… (1)
Where K = constant of proportionality & L = loudness of a sound
Let Lo be the loudness of the
faintest audible sound of intensity Io and L be the loudness of an unknown sound of intensity I, then Eq. 1 can be written as:
Lo = K log Io ……… (2)
Subtracting
Eq. (2) from Eq. (1), we get:
L
– Lo = K (log
I - log
Io)
L – Lo = K log
This
difference, (L- Lo), between the loudness L of an unknown sound and
the loudness L is called the intensity level of the unknown sound. Therefore,
the intensity level of an unknown sound is given by:
Intensity
level = K log ……… (3)
The
value of K depends not only on the units of I and
Io but also on the unit of intensity
level. If the intensity I of any unknown sound is 10 times
greater than the intensity of Io the faintest audible sound i.e., I=10Io, and the intensity level of such a
sound is taken as the unit, called bel, the value of K becomes 1. Therefore, using
K =1, Eq. (3) becomes:
Intensity level = log (bel) ………
(4)
bel
is a very large unit of the intensity level of a sound. Generally, a smaller unit
called decibel is used. Decibel is abbreviated as (dB). 1 bel = 10 dB. Eq. (4)
becomes:
Intensity level = 10 log (dB)……… (5)
Eq. 5 shows the intensity level of sound on the “decibel
scale”.
REFLECTION (ECHO) OF SOUND:
“When
sound is incident on the surface of a medium it bounces back into the first
medium. This phenomenon is called echo or reflection of sound”
When
we shout in a large or vacant space, the sound is reflected back. Suppose you
are standing in front of a high wall and clap your hand; you can hear the same
sound of the clap back again. This is an echo. Echoes may be heard more than
once due to successive or multiple reflections.
SPEED OF SOUND:
Sound
waves can be transmitted by any medium containing particles that can vibrate.
They cannot pass through the vacuum. However, the nature of the medium will affect
the speed of the sound waves. In general, the speed of sound in a liquid is five
times that in gases; the speed of sound in a solid is about fifteen
times that in gases. The speed of sound in air is affected by changes
in some physical conditions such as temperature, pressure, humidity, etc. The
speed of sound in air is 343 ms-1 at one atmosphere of pressure and
room temperature (21°C). The speed varies with temperature and humidity. The
speed of sound in solids and liquids is faster than in air. The following relation
can be used to find the speed of sound:
v = f λ
here,
v
= speed of sound wave.
f
= frequency of the sound wave.
λ
= wavelength of the sound wave.
Measuring Speed of Sound by
Echo Method:
Apparatus: Measuring tape, stopwatch, a flat wall
that can produce a good echo. Procedure:
1.
Use the tape to measure a distance of 50 meters from the wall.
2.
Now clap your hands in front of the wall at a distance of 50 meters and check
if you can clearly hear an echo from the wall. Make sure the echo is not coming
from any other wall in the area. The time taken by the sound to travel 100
meters is the time difference between the clap and the echo.
3.
Now restart the clapping and start the stopwatch at the first clap. Count the
number of claps, and stop the clapping and the stopwatch when you hear the echo
of the 10th clap (say).
4.
Now find the average time for 10 claps. After calculating the time interval t
between claps and using the formula S = vt, we can calculate the speed of the
sound.
NOISE POLLUTION:
sounds that produce unpleasant effects
on our ears are called NOISE POLLUTION such as the sound of machinery, the slamming
of a door, and the sounds of traffic in big cities. Sound which has a jarring
and unpleasant effect on our ears is called noise. Noise corresponds to
irregular and sudden vibrations produced by some sounds.
Noise pollution has become a major
issue of concern in big cities. Noise is an undesirable sound that is harmful
to the health of humans and other species. Transportation equipment and heavy
machinery are the main sources of noise pollution. For Example, the noise of the machinery in industrial areas, loud vehicle horns, hooters, and alarms. Noise
has negative effects on human health as it can cause conditions such as hearing
loss, sleep disturbances, aggression, hypertension, and high-stress levels. Noise
can also cause accidents by interfering with communication and warning signals.
A safe level of noise depends on two
factors: the level (volume) of the noise; and the period of exposure to the
noise. The level of noise recommended in most countries is usually 85-90 dB
over an eight-hour workday. Noise pollution can be reduced to an acceptable level
by replacing the noisy machinery with environment-friendly machinery and
equipment, putting sound-reducing barriers, or using hearing protection
devices.
IMPORTANCE
OF ACOUSTICS:
“The technique or method used to absorb undesirable sounds by soft and porous surfaces is called acoustic protection.”
Reflection
of sound is more prominent if the surface is rigid and smooth, and less if the
surface is soft and irregular. Soft porous materials, such as draperies and
rugs absorb a large amount of sound energy and thus quiet echoes and softening
noises. Thus, by using such material in noisy places we can reduce the level of
noise pollution. However, if the surface of classrooms or public halls is too
absorbent, the sound level may be low for the audience. Sometimes, when sound
reflects from the walls, ceiling, and floor of a room, the reflecting surfaces are
too reflective and the sound becomes garbled. This is due to multiple
reflections called reverberations. In the design of lecture halls, auditoriums,
or theater halls, a balance must be achieved between reverberation and
absorption. It is often advantageous to place reflective surfaces behind the
stage to direct sound to the audience. Generally, the ceilings of lecture
halls, conference halls, and theatre halls are curved so that sound after
reflection may reach all the corners of the hall. Sometimes curved soundboards
are placed behind the stage so that sound after reflection is distributed evenly
across the hall.
AUDIBLE FREQUENCY RANGE:
“The
range of the frequencies which a human ear can hear is called the audible
frequency range.”
A
normal human ear can hear a sound only if its frequency lies between 20Hz and
20,000 Hz. In other words, a human ear neither hears a sound of frequency less
than 20 Hz nor a sound of frequency more than 20,000 Hz. Different people have a different range of audibility. It also decreases with age. Young children can
hear sounds of 20, 000 Hz but old people cannot hear sounds even above 15, 000
Hz.
ULTRASOUND:
“Sounds
of frequency higher than 20, 000 Hz which are inaudible to the normal human ear are
called ultrasound or ultrasonics.”
Uses of Ultrasound
1) Ultrasonic waves carry more energy and
higher frequency than audible sound waves. Therefore, according to the wave
equation v = f λ, the wavelength of ultrasonic waves is very small and is very
useful for detecting very small objects.
2) Ultrasonics are utilized in medical and
technical fields.
3) In the medical field, ultrasonic waves are
used to diagnose and treat different ailments. For diagnosis of different
diseases, ultrasonic waves are made to enter the human body through transmitters.
These waves are reflected differently by different organs, tissues or tumors, etc. The reflected waves are then amplified to form an image of the internal
organs of the body on the screen. Such an image helps in detecting the defects
in these organs.
4) Powerful ultrasound is now being used
to remove blood clots formed in the arteries.
5) Ultrasound can also be used to get pictures of the thyroid gland for diagnosis purposes.
6) Ultrasound is used to locate underwater
depths or is used for locating objects lying deep on the ocean floor,
etc. The technique is called SONAR, (sound navigation and ranging). The sound
waves are sent from a transmitter, and a receiver collects the reflected sound.
The time-lapse is calculated, knowing the speed of sound in water, the distance
of the object from the ocean surface can be estimated.
7) SONAR ranging is also used to see the shape and the
size of the object. Cracks appear in the interior of moving parts of high-speed
heavy machines such as turbines, engines of ships, and airplanes due to
excessive use. These cracks are not visible from the outside but they can be very
dangerous. Such cracks can
be detected by ultrasonics. A powerful beam of ultrasound is made to pass
through these defective parts. While passing, these waves are reflected by the
surface of these cracks and flaws. The comparison of the ultrasonic waves
reflected from cracks and from the surfaces of these parts can give a clue to
the existence of the cracks.
8) Germs and bacteria in liquids can also be destroyed by using high-intensity ultrasonic waves.
Quastions
11.12
Describe the effect of change in amplitude on loudness and the effect of change
in frequency on the pitch of the sound.
Ans. The
loudness of the sound varies directly with the amplitude of the vibrating body.
The sound produced by a sitar will be loud if we pluck its wires more
violently. Similarly, when we beat a drum forcefully, the amplitude of its
membrane increases and we hear a loud sound.
Pitch
is the characteristic of sound by which we can distinguish between a shrill and
a grave sound. It depends upon the frequency. A higher pitch means a higher
frequency and vice versa.
11.13
If the pitch of the sound is increased, what are the changes in the following?
a.
the frequency b- the wavelength c. the wave
velocity d. the amplitude of the wave.
Ans.
(a) If the pitch of the sound is increased frequency will also
increase.
(b) With
the increase of pitch wavelength of the waves decreases according to the
relation. λ = v/f.
(c) If the
pitch increases then the velocity of the wave also increases according to the
relation. V = fλ.
(d) Pitch
is independent of the amplitude of the sound wave so the amplitude of the wave does not
change with the change of pitch.
11.14
If we clap or speak in front of a building while standing at a particular
distance, we rehear our sound after some time. Can you explain how this happens?
Ans. If we
clap or speak in front of a building while standing at a particular distance,
we rehear our sound after some time. This is due to the reflection of sound waves from the surface of the building.
11.15
What is the audible frequency range for the human ear? Does this range vary with
the age of people? Explain.
Ans. The
range of the frequencies which a human ear can hear is called the audible frequency
range. It lies between 20Hz and 20,000Hz. Different people have a different range of audibility. It also decreases with age. Young children can hear 20,000Hz but
old people cannot hear sounds even above 15,000Hz.
11.16
Explain that noise is a nuisance.
Ans.
Noise Pollutions: Noise
pollution has become a major issue of concern in big cities. Noise is an
undesirable sound that is harmful to the health of humans and other species.
Sources
of noise pollution: Transportation equipment and heavy machinery
are the main sources of noise pollution. For Examples, the noise of the machinery in
industrial areas, loud vehicle horns, hooters, and alarms.
Negative
effects of sound: Noise has negative effects on human health
as it can cause conditions such as hearing loss, sleep disturbances,
aggression, and high-stress levels. Noise can also cause accidents by interfering
with communication and warning signals.
Safe
Level of Noise: A safe level of noise depends on two
factors:
- the level (volume) of the noise; 2. the
period of exposure to the noise.
The
level of noise recommended in most countries is usually 85-90 dB over an
eight-hour workday.
Ways
to reduce noise: Pollution can be replaced to an acceptable
level by replacing the noisy machinery with environment-friendly machinery and
equipment, putting sound-reducing barriers, or using hearing protection
devices.
11.17
Describe the importance of acoustic protection.
Ans.
The technique or method used to absorb undesirable sounds by soft and porous
surfaces is called acoustic protection.
Explanation: Soft,
porous materials, such as draperies and rugs absorb a large amount of sound
energy and thus quiet echoes and soften noises. Thus, by using such material in
noisy places we can reduce the level of noise pollution. However,
- If the surfaces of classrooms or public halls are too
absorbent, the sound level may be low for the audience.
- Sometimes, when sound reflects from the wall, ceiling,
and floor of a room, the reflecting surfaces are too reflective and the
sound becomes grabbled. This is due to multiple reflections called
reverberations.
- In the design of lecture halls, auditoriums, or theater
halls, a balance must be achieved between reverberations and absorption.
It is often advantageous to place reflective surfaces behind the stage to
direct sound to the audience.
- Generally, the ceilings of lecture halls, conference
halls, and theatre halls are curved so that sound after reflection may
reach all the corners of the hall. Sometimes curved soundboards are placed
behind the stage so that sound after reflection is distributed evenly
across the hall.
11.18
What are the uses of ultrasound in medicine?
Ans.
1- In the medical field, ultrasonic waves are used to
diagnose and treat different ailments. For diagnosis of different diseases,
ultrasonic waves are made to enter the human body through transmitters. These waves
are reflected differently by different organs, tissues, or tumors. The reflected
waves are then amplified to form an image of the internal organs of the body on
the screen. Such an image helps in detecting the defects in these organs.
2- Powerful
ultrasound is now being used to remove blood clots formed in the arteries.
3- Ultrasound can also be used to get pictures of the thyroid gland for diagnosis purposes.
