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SOUND IS MADE OF VIBRATIONS
We hear sounds from various sources such as moving vehicles, television music
system, machines and so on. Sound is basically a form of energy like heat and light. It
produces a sensation of hearing in our ears. When we talk about sound, so many
questions come to our mind. How is sound produced? How do we hear sound? How
does sound travel from one place to another? What is music? What is noise pollution?
Vibration
When you pluck a string of a guitar, you notice that the string moves back and forth
rapidly making a sound. When you place your palm on the speaker of a music system,
what do you feel? The back and forth movement which is rapidly done about its mean
position is called vibration.
All sounds are caused by vibrations. The prongs of a tuning fork vibrate. If the ends of
the prong are dipped in water, you can see the effect of vibration.
Sound is produced in a vibrating body.
Why do vibrating bodies produce sound?
Do all vibrating bodies produce sound?
It is a vibration in our ear caused by the vibration of the air in our ear. For an object to
make a sound it must therefore cause the air to vibrate.
How do you show sound waves?
Have a slinky spring. Move one end of the spring forward and backwards. The coils
move closer together then further apart again as the wave transmits energy. When the
coils of the spring are closer together, it is called compression. When the coils are
further apart, you call it as rarefaction.
HOW DOES A HUMAN EAR WORK
The ear is the organ of hearing. The ear is usually described as having three parts—
the outer ear, middle ear and the inner ear. The outer ear consists of the pinna and ear
canal. The middle ear consists of the ossicles (malleus, incus, stapes) and ear drum.
The inner ear consists of the cochlea, the auditory (hearing) nerve and the brain.
The compressions in the air reach the outer ear called pinna which directs the sound to
travel through the ear canal to reach the ear drum or tympanic membrane. Due to these
compressions, the membrane starts vibrating. This thin membrane vibrates and relaxes
depending on the amplitude of compression in air. The vibration is then passed to
three bones in the ear connected to the ear drum: hammer anvil and stirrup and is
amplified about 20 times than the actual vibration.
After the amplification by ear bones, this vibration is passed through the oval ear
called cochlea. A liquid is present in the cochlea. When the vibrations are sent to this
liquid, it creates waves and gets the electrical signal out of the cochlea. Now from
cochlea it is sent to brain through the auditory nerve. Brain reads the electrical
impulses and detects it as sound. We have an Eustachian tube at the bottom which
connects the ear with the nasopharynx, a part of nose.
Find out what happens when you have a problem on hearing? What is hearing aid?
LOUDNESS, AMPLITUDE AND INTENSITY
It is almost impossible to go anywhere without being surrounded by noise. It is a
common experience that when you shout you produce a louder sound. Similarly when
you whisper, you produce a feeble sound.
Loudness is a characteristic of sound by which a loud sound can be distinguished from
a faint sound. It is measured accurately by amplitude of vibration of the sound source.
It has also been found that loudness of a sound is directly proportional to the square of
the amplitude of the vibration producing the sound. For example if the amplitude
becomes twice, the loudness increases four times.
Smaller amplitude and larger amplitude
When you strike a drum softly, its skin vibrates with smaller amplitude and a soft sound is
produced.
When you strike the drum harder, its skin vibrates with greater amplitude and a louder skin is
produced.
There are other factors also which affect the loudness of sound like surface area of a vibrating
body, density of the medium etc.
We define sound by a special unit called decibel(dB). On the decibel scale, an increase of 10
dB means that the sound is twice as loud. This means that a lawn mover is twice as loud as a
home stereo. If your ear is 15cm away from a trumpet, then it is four times louder than a
pneumatic drill.
What do you understand by loudness and amplitude?
Loudness is the way in which the human ear perceives sound wave amplitude, and the larger
amplitude is, the louder the perceived sound. Amplitude refers to the magnitude of the sound
wave, the pressure of its successive compressions and rarefactions.
FREQUENCY AND PITCH
The number of times a guitar string or loudspeaker vibrates every second is the frequency of
the vibration. Frequency is measured in hertz.
Musicians refer to the pitch of the note. A high pitched note has a high frequency. A low
pitched note has a low frequency.
Pitch is the characteristic of sound by which you can distinguish a shrill sound from a hoarse
sound. Pitch is a subjective measurement. However, the pitch of a sound depends upon the
frequency of vibration.
The pitch of the sound produced by an object vibrating with a low frequency is low and the
sound is called a flat sound.
The pitch of the sound produced by an object vibrating with a high frequency is high and the
sound is called a shrill sound.
The higher the frequency of the sound, the higher will be the pitch and vice versa. For
example, the voice of children and ladies is shrill as compared to the voice of men. This is
because children and women have short vocal chords which therefore, vibrate with a high
frequency to produce high pitched shrill voice. Men have large vocal chords which vibrate
with a low frequency to produce low pitched flat sound.
When the skin of the table or the strings of the guitar is tightened, they give out a sound of
higher pitch than when they were loose.
It is important for you to understand what is
frequency and pitch, loudness and amplitude
QUALITY
It is the characteristic of a sound that enables us to distinguish between two sounds of
the same pitch and loudness produced by two different sources. It depends upon the
waveform of the sound. For example, you can recognise the family members by their
voice without seeing them because each of them has a voice of different quality.
Quality of a sound is called its tone.
A same note sounds different on different music instruments. If the same note is
played on a violin and a flute, it will sound different. You can tell which instrument is
being played without looking because each one sounds different. So the quality of
sound depends upon the musical instrument that is vibrating.
Sounds may be characterized by pitch, loudness and quality. Sound “quality or
“timbre” describes those characteristics of sound which allow the ear to distinguish
sounds which have the same pitch and loudness.
Audible range: young people can usually hear sounds between 20 Hz and 20000Hz.
This is known as audible range. By the time you reach 30, you may hear only upto
16000Hz.
SPECIES
Bat
cat
cow
horse
rat
sheep
whale
AUDIBLE RANGE IN Hz
2000 – 110000
45 – 64000
23 – 35000
55 – 33500
200 – 76000
100 – 30000
1000 – 123,000
ECHO
Shout in front of a high mountain or a deep well or in a big empty hall. You hear your sound
back after a few seconds. This is known as echo.
An echo is a repetition of the original sound heard after the sound is reflected from a distant,
dense and rigid object such as high building, cliff etc.
The human ear can hear two sounds separately only when they reach the ear after an interval
of 1/10th of a second. How do you calculate what is the minimum distance for an echo?
We know
Speed of sound in air
=
340m/s
Minimum time required for formation of echo
=
1/10 of a second
Therefore distance travelled by sound
distance/time)
=
speed x time ( because speed =
=
340 x 1/10
=
34 m
Remember the sound is travelling rom you to the building and travels back.
So the minimum distance for an echo
=
34/2 = 17m
Find about Golconda fort near Hyderabad. The most astonishing is a dome near the entrance
to the fort. If one claps at a particular point under the dome, echoes are heard in the durbar
hall, the highest point in the fort, about a kilometre away. This was devised as a warning
system.
NOISE POLLUTION
Noise from any source that causes disturbance or discomfort of any kind in the environment
is called noise pollution. Decibels are the unit of loudness. A sound of about 90 dB and
above, heard over a long time causes noise pollution. The sources of noise pollution can be
o
o
o
o
o
Excessive use of horns
Moving aeroplane
Loudspeaker
Crackers
Loud music system
Harmful effects of noise pollution



Causes nervous tension, irritation, earache, head ache and high blood pressure
Lack of concentration in work or studies
Prolonged exposure to high level noise can affect the eardrum and may lead to
temporary or even permanent loss of hearing
Measures to reduce noise pollution
 Automobiles should be fitted with silencers and soft horns
 Airports and noise making factories should be shifted away from the residential area
of the city
 Should not play loud music in cars and other vehicles when driving because it affects
the concentration of the driver
Recent studies by the World Health Organization suggest that the stress caused by noise
may have caused as many as 3300 deaths a year in England due to heart disease.
In Germany and other developed countries, as many as four to five million people, that is,
12 to 15 percent of all employed people, are exposed to noise levels of 85 decibels or
more (World Health Organization 2001).
How loud would you expect the noise to be in your school library?
Suggest why trees are planted next to a motorway that runs close to a housing locality.
SOUND LEVEL IN EXAMPLE OF SOUND
dB
10
Soft whisper
EFFECT
40
Talking quietly (in a village street)
quiet
60
Normal speech, telephone ringing
Disturbs conversation
80
Traffic at a junction
intrusive
110, 120
Electric drill, thunder, rock concert
Can damage hearing after 15
minutes per day
140
Gun shot, jet engine 30m away
Loudest recommended level
with ear protection
150
Trumpet 15 cm away
Possible
damage
Can just be heard
permanent
ear
SOUND TRAVELLING IN SOLIDS, LIQUIDS AND GASES
Sound waves need to travel through a medium such as a solid, liquid, or gas. The
sound waves move through each of these mediums by vibrating the molecules in the
matter.
The molecules in solids are packed very tightly.
Liquids are not packed as tightly as solids.
And gases are very loosely packed.
The spacing of the molecules enables sound to travel much faster through a solid than
a gas. Sound travels about four times faster and farther in water than it does in air.
This is why whales can communicate over huge distances in the oceans. Sound waves
travel about thirteen times faster in wood than air. They also travel faster on hotter
days as the molecules bump into each other more often than when it is cold.
At freezing (0º Celcius), sound travels through air at 331 meters per second (about
740 mph). But, at 20ºC, room temperature, sound travels at 343 meters per second
(767 mph).
In fresh water, sound waves travel at 1,482 meters per second (about 3,315 mph).
That's well over 4 times faster than in air!
The exact speed of sound in steel is 5,960 meters per second (13,332 mph)
ACTIVITY
Ask students if sound would travel better in solids, liquids, or gases. Divide students
into three groups and quickly model the vibration traveling through the different states
of matter. In the gas group, students stand far apart and it is difficult to pass, wiggle,
or push along. In the liquid group, students stand close together but not super tight and
it is easier. In the solids group, students are packed tightly and the vibration easily
goes through all the molecule-kids.
Have students test how sound travels through solids. Students will work in pairs at
their seats. One student will tap lightly on his/her desk and the second student will
record what they hear through the air. The same person will tap lightly again while the
second person lays his or her ear on the desk. The students should compare the sounds
and record what they hear through the solid. Have the students try the experiment one
last time, tapping louder, and, again, recording the results.
SOUND REQUIRES A MEDIUM
Sound requires a medium to travel. It cannot travel in vacuum. In air, sound travels in
all directions. By the time it reaches the ear, most of the sound waves dissipate.
Sound is a mechanical wave that needs a material medium like air, water, steel, etc., for
its propagation. We can describe a sound wave by its frequency, wavelength and velocity.
The sound wave is a longitudinal wave, ie., the particles of the medium vibrate in a
direction parallel to the direction of the propagation of the wave..
The bell jar experiment is a common experiment used to demonstrate that sound needs a
medium to travel.
What is a bell jar?
A bell jar is a laboratory equipment used for creating a vacuum. It is so named as its
shape is similar to that of a bell. A bell jar is placed on a base which is vented to a hose
fitting that can be connected via a hose to a vacuum pump. By pumping the air out of the
bell jar, the air pressure inside the jar can be varied.
The experiment is done by placing an electrical bell in the bell jar. As the air is pumped
out of the sealed bell jar, the sound from the bell jar fades. At a particular vacuum, no
more sound is heard from the bell, but we can see that the hammer continues hitting the
gong and sound is produced. However, the sound is not audible to our ears because of the
vacuum inside the jar. This demonstrates that the sound wave cannot travel through
vacuum. That is, a sound wave needs a material medium for its propagation.