8 Pius Augustine Sound 1

1
Appeal: Please Contribute to Prime Minister’s or Chief
Minister’s fund in the fight against COVID-19
Dr. Pius Augustine, Dept of Physics, Sacred Heart College, Thevara
Sound -1
we will
overcome

Dr. Pius Augustine, S H College, Kochi

Apdi pode pode ……………..

Why do thermocol balls on speaker
dance?
When music is being
played, vibration of cone
paper of speaker makes
it dance.
No music – no dance.

How will you account for sound as
energy form?
Sound is produced due to vibration,
transmitted through surrounding
medium.
Mech energy is required for vibrn.
Sound can vibrate material
bodies.(ear drum)
ie. sound can do work and is a form
of energy Dr. Pius Augustine, S H College, Kochi

1.What causes sound?
2. What is sound? How
is it produced?

Why absolute silence in outer space?
Mechanical or elastic wave
( solids, liquids and gas)
Can not travel through
vacuum.
Eg. No sound in moon or
outer space. Dr. Pius Augustine, S H College, Kochi

Requisites of the medium
i. Elastic : particles have
tendency to come back to
their mean position
ii. Inertia : capacity to store
energy.
iii.Frictionless : no loss of
energy in propagationDr. Pius Augustine, S H College, Kochi

Describe in brief, with the
aid of a sketch diagram,
an experiment to
demonstrate that a
material medium is
necessary to transmit
sound.

Bell jar experiment
Electric bell inside air tight glass bell jar.
Jar is connected to vacuum pump.
On completing circuit, sound is heard and
hitting of hammer on gong is visible.
Air is removed gradually – intensity of
sound heard decreases.
Fully evacuates – no sound, hammer hits.
Air is permitted again – sound starts.

Describe an experiment to
show that sound requires a
medium for propagation and
light does not.
Mobile phone in
bell jar.

Waves
Disturbance advancing through a
medium.
Energy transfer without actual
translation of particles of the med.
Elastic and inelastic waves.
Elastic waves require medium.
Inelastic or em waves donot
require medium.

What is the
difference between
ultrasonic and
ultraviolet waves?

Elastic waves -2 types
Longitudinal – alternate compressions
and rarefactions.
Transverse – alternate crest and
trough.
Wavelength λ: Distance travelled by the
wave during the time particles of the
medium makes one complete
vibration.
Frequency f : no. of waves produces
per second. Dr. Pius Augustine, S H College, Kochi

Electromagnetic waves

Name two kinds of
waves in form of
which sound travels
in a medium?

Sound is longitudinal
generally, except on free
surface of a liquid, where
particles are free to move
perpendicular to advancing
waves as there is no
obstruction, which make it
transverse.Dr. Pius Augustine, S H College, Kochi

What is longitudinal
wave? In which
medium – solid,
liquid or gas, can it
be produced?

Energy transfer through a
medium due to periodic
vibrations of particles of
medium parallel to
advancing wave.
Produce alternate
compressions and
rarefactions.Dr. Pius Augustine, S H College, Kochi

Longitudinal waves
Compressions: region of higher
pressure, particles of the medium
closer than normal
Rarefactions: lower pressure, more
separated than normal.
One compression and one
rarefaction together makes one
wave.
Possible in solids, liquids and gases

Describe a simple
experiment which
demonstrates that the
sound produced by a
tuning fork is due to
vibration?Dr. Pius Augustine, S H College, Kochi

Expt . Produces due to vibration
Excite tuning fork. Faint sound will
be heard.
Ends of tuning fork appear blurred
due to vibration.
Freely hanging tennis ball jumps to
and fro, when vibrating fork is so
close.
When no sound , to and fro jump
also stopped. Dr. Pius Augustine, S H College, Kochi

Explain with an
example how
sound travels in a
medium.

Sound is propagating through a
medium due to periodic vibrations
of particles of the medium which
produces alternate compressions
and rarefactions.
When the prongs of tuning fork
vibrate it compel air molecules to
vibrate which will be transmitted
through the medium.

What is transverse
wave? In which
medium – solid, liquid
or gas, can it be
produced?

Transverse waves
Alternate crests and troughs.
Crest : raised positions
Trough: lowered position
One crest and trough together makes
one wave.
Possible only in solids and surface of
liquids and not possible inside liquids
and gases.
Eg. Ripples on water surface.

Describe an experiment to show
that in wave motion, the energy
is transferred , but the particles of
medium do not move.
Paper boat on
water surface.

• Place a paper boat on free surface
of water.
• Put a stone little distance away to
make ripple.
• When ripple reaches paper boat, it
moves up and down.
• It does not move along with ripple.
• Ie. Particles of the medium do not
move with wave Dr. Pius Augustine, S H College, Kochi

Draw a displacement –
time graph of a wave and
mark on it, the amplitude
of wave by the letter a and
wavelength of wave by
the letter λ

Show on a
displacement – time
graph of a wave, the
amplitude and time
period of the wave?

Differentiate between the transverse and longitudinal waves
Transverse
i. Vibration -
perpendicular
ii. Crests and troughs
iii. Crest – displacement
is positive max.
iv. Only in solids and
surface of liquids.
v. No variation in density
and pressure in the
medium due to
propagation .
Longitudinal
i. Vibrn – parallel
ii. Compression and
rarefaction .
iii. Comprsn - +ve
max.
iv. Solids, liquids and
gases.
v. Density and
pressure max at
compression and
min at rarefactions.

Amplitude of wave
Maximum
displacement from
mean position
Unit – meter.

Time period
Time for complete to
and fro motion of
particle.
Time for one vibration .

Wavelength
• Distance travelled by the
wave during the time
particles of the medium
make one complete
vibration

Frequency f
No of vibrations made by a
particle in one second.
It is the no of waves passing
through a point in one
second.
Frequency f = 1/ T
SI unit is Hz or s-1

Wave velocity
Distance travelled by
the wave in one
second.
Unit – m/s
v =λ / T = fλ

How are the wave
velocity V, frequency
n and wavelength λ
of a wave related?
Derive relationship.

Sound
Form of energy
produces
sensation of
hearing.

Expt - Produces due to vibration
Stretched rubber band
between mouth and
hand.
Pluck at middle.
Sound is heard as long as
it is in vibration.

Expt. Produces due to vibration .
Stretched wire between 2
nail (guitar)
Place paper rider – flies off
when sound is heard
Stays back if placed, when
no sound is heard.

Why do thermocol balls on speaker
dance?
When music is being
played, vibration of cone
paper of speaker makes
it dance.

How will you account for sound as
energy form ?
Sound is produced due to vibration,
transmitted through surrounding
medium.
Mech energy is required for vibrn.
Sound can vibrate material
bodies.(ear drum)
ie. sound can do work and is a form
of energy Dr. Pius Augustine, S H College, Kochi

Why absolute silence in outer space ?
Mechanical or elastic wave
Can not travell through
vacuum.
Eg. No sound in moon or
outer space.

Supersonic flyby: A U.S. Air Force F-22
Raptor breaks the speed of sound

Factors affecting speed of sound
i. Elasticity E of the
medium
ii.Density ρ of the
medium
V = √E/ρDr. Pius Augustine, S H College, Kochi

Newton’s Formula for vel of sound
He assumed process is isothermal.(
temp is constant as sound
propagates in a gas )
Isothermal elasticity for gas = pressure.
V = √P/ρ
For air, P = 1.01 * 105 Pa, ρ = 1.293 kg/m3.
Calculated value of velocity V = 279.5 m/s
Experimental value of V = 330 m/s

Laplace correction
Propagation of sound in air is so fast that,
process should be adiabatic.( no
exchange of heat b/w system and
surrounding)
adiabatic elasticity for gas = γP
γ – ratio of specific heats = 1.4
V = √γP/ρ
For air, P = 1.01 * 105 Pa, ρ = 1.293 kg/m3
Calculated value of velocity V = 330m/s
Experimental value of V = 330 m/s

Speed of sound in different media
Speed of sound is more in
solids, less in liquids and
least in gases, due to
corresponding variations
in elasticity.

Arrange the speed
of sound in gases,
solids and liquids
in ascending
order?

Compare approximately
the speed of sound in air,
water and steel
1 : 4 : 15

The sound of an explosion on the
surface of a lake is heard by a boat
man 100m away and by a diver 100m
below the point of explosion.
i. who would hear the sound first
ii. Give reason
iii. If sound takes time t to reach the
boatman, how much time
approximately does it take to reach the
diver ?

Ans: i. diver
ii. Sound travels faster in
water than in air.
iii. If velocity in water is
1400m/s , aproximately 4
times that in air, then to cover
same distance in water , it will
take only 1/4th time. ie. 0.25 t.

Speed of sound in various media
Gases i. Air - 330 m/s
ii. CO2 - 260 m/s
iii. H2 - 1270 m/s
Liquids i. Alcohol - 1210m/s
ii. Turpentine - 1325 m/s
iii. Water - 1450 m/s
Solids i. copper - 3560 m/s
ii. Steel - 5100 m/s
iii. Glass - 5500 m/s
iv. Granite – 6000 m/s

Factors affecting the speed of
sound in a gas
i. Density
ii. Temperature
iii. Humidity
iv. Wind

How does speed of sound in air
vary with density ?
Vα 1/√d
Inversely proprtional to the
square root of density of the
gas.
D 0xigen = 16 D hydrogen
V
h = 4 VO

How does the speed of sound in
air vary with temperature?
V α √T
Increases with increase in temperature.
As temperature increases, density
decreases due to increase in volume.
Vt = Vo + 0.61t
61 cm/s is the temperature coefficient
of velocity of sound

vary with humidity? Explain.
Increases with increase in
humidity.
Density of water vapour is 5/8th
times density of dry air.
ie. Increase in moisture reduces
the density of air and hence
decreases effective density.Dr. Pius Augustine, S H College, Kochi

vary with velocity of wind?
Wind blows in the direction of
sound , speed of sound
increases and vice versa.
Effective speed = v + W or v- W
V- velocity of sound and W- wind
velocity.

How does speed of sound in
air is affected if its frequency
or wavelength or amplitude
of sound is changed?
No effect.

What is the effect of variation of
atmospheric pressure on speed of sound?
No effect. V = √γP/ρ
P/ρ - remains unchanged with the change
in pressure.(as P increases, V decreases )
Eg. If P is doubled, V reduced to half
according to Boyle’s law.
Density gets doubled as mass remains
const.
Show that if pressure increases 4 times,
velocity is unaffected. Dr. Pius Augustine, S H College, Kochi

If you place your ear close to an iron railing
which is tapped some distance away you
hear the sound twice. Explain why?
Because sound travels with different
velocity in different medium.
Sound heard first in this case comes
through solid rails, due to higher
velocity in solid.
Second sound comes through air.

Comparison of speed of sound
with speed of light
V = 3 x108m/s
V decreases
in denser
medium
V = 330 m/s
V increases in
denser
medium

Consequence of large difference
in the speed of sound and light
i. Thunder and lightning
ii.Athletic starter gun
iii.Spectators watching
cricket

Describe simple
experiment to determine
the speed of sound in air.
What approximation is
made in the method
described by you?

In which medium
speed of sound is
more: humid air or dry
air? Give a reason to
your answer?

Describe a simple experiment to
determine the speed of sound in
air. What approximation is made
in the method described by you?
Approximation : velocity of
light is much greater than
velocity of sound .

Expt. Speed of sound
A and B at high altitudes having
separation d (1km or more) in air.
Observer A fires gun and B starts clock
on seeing the flash of light.
Stop the clock on hearing sound,
Time is noted as t1.
Now B fires gun and as before A noted
time t2.
V = d/t, t – average time

The heart of a man beats 75 times
a minute. What is its frequency and
time period ?
Frequency = no. of
beating in one second =
75/ 60 = 1.25 s-1
Time period = 1/f = 0.8s.

The time period of a simple
pendulum is 2s. Find its
frequency?
Frequency =
1/T = ½ =
0.5Hz. Dr. Pius Augustine, S H College, Kochi

The separation between two
consecutive crests in a transverse
wave is 100 m .if the wave velocity
is 20m/s, find the frequency of
wave.
Given λ= 100m
V = 20m/s
We have eqn v = fλ
f = v/λ = 20/100 = 0.2 HzDr. Pius Augustine, S H College, Kochi

A longitudinal wave travels at a
speed of 0.3m/s and the frequency
of wave is 20Hz. Find the
separation between two
consecutive compression
Question is to find wavelength.
Given V and f
λ = v/f = 0.3 / 20 = 1.5 x 10-2m

A source of wave
produces 40 troughs
and 40 crests in 0.4s.
What is the frequency
of wave?

An observer A fires a gun
and another observer B at a
distance 1650 m away from
A hears its sound. If the
speed of sound is 330m/s,
find the time when B will hear
the sound after firing by A

The time interval between a
lightning flash and first sound
of thunder was found to be
5s. If the speed of sound in
air is 330 m/s, find the
distance of the flash from the
observer.

A boy fires a gun and
another boy at a distance
hears the sound of the fire
2.5s after seeing the flash. If
the speed of sound in air is
340m/s, find the distance
between the boys.

An observer sitting the line of the
tanks, watches the flashes of two
tanks firing at each other at the
same time. He hears the sound
of the two shots 2s and 3.5s
after seeing the flashes. If the
distance between the two tanks
is 510m, find the speed of
sound.Dr. Pius Augustine, S H College, Kochi

How long sound will take to
travel in an iron rail and air,
both 3.3km in length? Take
speed of sound in air to be
330m/s and in iron to be
5280m/s. Dr. Pius Augustine, S H College, Kochi

Assuming the speed of
sound in air equal to 340m/s
and in water equal to
1360m/s, find the time taken
to travel a distance 1700m by
sound in i) air and ii) water.

What do you mean by audible
range of frequency?
What is the audible range of
frequency for humans?
The human ears are most
sensitive to which range of
frequencies?

Which has the
higher frequency -
ultrasonic sound or
infrasonic sound?

Explain how do bats
locate obstacles and
prey in their way

State two
applications of
ultrasound?

What is ultrasound?
State two properties of
ultrasound that make
it useful to us?

Range of hearing

Ultrasound sensors for parking space measurement

Eye surgery

New Antarctic Seabed Sonar Images Reveal
Clues to Sea-Level Rise

Velocity of sound

Will continue in sound 2 ……

105
Appeal: Please Contribute to Prime Minister’s or Chief
Minister’s fund in the fight against COVID-19
Dr. Pius Augustine, Dept of Physics, Sacred Heart College, Thevara
we will
overcome
Thank You
http://piusaugustine.shcollege.ac.in
https://www.facebook.com/piustine

8 Pius Augustine Sound 1

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