Production And Propagation of Sound Video Lecture - Science Class 9

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be covered in this lesson our
introduction production of sound
propagation of sound sound waves and
longitudinal characteristics of sound
waves speed of sound reflection of sound
range of hearing and human ear structure
and function along with applications of
ultrasound so what do you think are we
going to study in this lesson on sound
so whenever is sound there are so many
things that come to our mind I mean I
really don't need to introduce this
lesson you know what do I mean when I
say sound there are so many different
kinds of sounds which we actually hear
right in fact right now I'm speaking
that makes sense because you are able to
hear it because of the sound which is
being produced by me is being heard by
you that's how it makes sense right
sonography we think of any sound we
think of the music which we hear it
might be a musical in the movie it can
be the music produced by a musical
instrument like a flute or a guitar or
the band party which goes generally
during the Indian weddings right the
Baraat which goes during Indians wear at
weddings or the sweet voice of the
couple or if you think of the Bell
whether it's the temple Bell or your
schoolbag so the do the ringing of the
Bell is again a kind of sound which we
hear when somebody is talking like the
way I am talking right now
and you are able to hear so that is
again a sound even try this one example
you stretch a rubber band and leave it
you get a sound slight sound right so if
you think of sound there are so many
different varieties of sounds which we
actually come across in our day-to-day
life right
so in this lesson we are going to talk
about some we're going to see how is
sound actually produced from the others
the sound del is produced how the sound
reaches from one person to another for
example right now I am speaking and you
are able to hear so how is the sound
getting transmitted so we'll talk about
the production of sound we will talk
about the propagation of sound we will
also talk about the various properties
which actually characterizes a sound
right so this lesson is all about sound
okay so with this let's start from the
very basic and the basic question is
what produces sound so how is the sound
actually produced so did you notice
something in the examples which I cited
in the previous slide let us look at
those examples once again let us look at
this guitar how do you produce a sound
with the help of a guitar it is suppose
if I keep a guitar on the table will it
start producing some will I be able to
hear the nice music from the guitar no
unless and until you move the strings of
the guitar you will not hear any sound
right so in a guitar the sound is
produced by the vibration of the strings
so these guitar strings which you have a
guitarist will do what he will actually
move this strings right okay
next if you look at the belly how does
this velcro do sound here what if you
would have gone to a temple you would
have seen that this thing which you have
in the Bell this one so then you move it
what happens it starts vibrating it
starts moving and because of its
movement the sound is reduced similarly
when you an example of somebody speaking
then somebody speak how was the sound
produced in the vocal chords I mean if
you look inside the human structure in
our vocal chords the vocal chords
actually vibrates and because of that by
raishin the soundest reduce if you do
not buy read back no sound hypocrite
rooster said literally the best example
for you to understand you take a rubber
band in your hand it will not make any
sound but when you stretch it and then
give it it makes a sound that is again
because of the vibration so the bottom
line is that sound is produced by
vibration of objects so I hope all of
you are clear with what is vibration
right vibration the term vibration
engine literally it means something
which is shaking so vibration is nothing
but a rapid to and fro motion of an
object so that means an object is
swinging between two positions right
left right left right left so when that
to and fro motion takes place very
rapidly we see that the object is
vibrating so sound is always produced by
vibration of objects so now if you look
at any of the examples which I gave in
the previous slide everywhere you will
see that there is a vibration which is
responsible for the production of sound
right we are good to go
okay so with this let us now talk about
the propagation of sound that means how
does sound travel okay so we understood
that when some object vibrates sound is
produced fair enough
so now the sound is produced in the
guitar but how does that sound
reaching the from the guitar to the
listeners ear so how is that sound
traveling from the guitar to your ear
that is the next question so let us try
to understand how sound propagates so
sound propagates through a medium so
again you have a new term here here
medium so what does this medium knock
through which sound will travel so let
us look at some of the examples before
that it is try to understand what is a
medium an object through which sound is
transmitted that means in order that the
sound is able to reach from one point to
another you mean some object which will
carry the
so that object is nothing but media let
us look at this example let's suppose if
there are two boys
this boy is speaking something so the
sound is travelling to this guy so how
is the sound traveling through the
medium what is the medium here here
there is a broad kind of a structure but
inside the rod what do you have to carry
the sound it is nothing but air right
even if you don't have this tunnel like
structure even if the sky spins and this
guy hears it is like the sound is being
traveled through the air so air is a
medium by default correct so how
actually the propagation of sound takes
place through this medium now this
medium can be a solid or a liquid or a
gas so that means that is so sound can
even travel through a liquid sound can
even travel through gas and it can
travel through some solid object as well
right so tell me anything now the
question is how is the sound getting
propagated through an object so this is
the example when we talked even in that
case air acts as a default medium so air
is not the only medium through which
sound can travel they remain with the
medium for example you can think of and
solid object for example let us suppose
you are in one room and your friend is
in the other room so when you speed from
this room your if you speak nitin loudly
your friend is able to hear that's
because sound is able to travel through
the solid medium that is to the one so
the wall is not acting as a barrier your
friend is also able to hear you he that
means that sonic substance is acting as
a medium so there are many different
mediums that can be so a liquid of gas
but air is by default a medium which
helps in propagation of sound but the
important point here is that sound needs
a medium to propagate
if there is no medium sound will not be
able to properly so we will talk about
that part also little later
so right now let us try to see
how actually the propagation of sound
takes place how actually the sound is
transmitted from one point to another so
here we have taken an example of a
speaker let us say this is the source of
some so this object is producing sound
and this is the ear of the listener so
this is the person who is listening so
how will some traveling from this point
to this point
Takas a question so what happens in this
case is sound is produced here because
of some vibration right there will be
some library sheet a convinced consent
so as a result then some vibrations in
this object as a result particles of
this medium which are in direct contact
with the object gets displaced from
their equilibrium position let us
suppose let me draw a rough diagram let
us say this is my object records the
source right so this object vibrates so
vibrates means it will move like this
it'll come back like this again it will
move like this it'll come back like this
so this kind of to and fro motion is
known as vibrations so this object
vibrates so when this object comes this
side it is suppose there is a particle
one so this particle is nothing but
these are the particles of the medium so
we assume that if the medium is air so
air will have so many particles in it
right suppose there is one particle in
the air which is in the neighborhood of
this object so this particle will also
get pushed in this direction right so
now so the particle one gets displaced
from its equilibrium position so now
this particle will exert some force on
another particle later suppose there is
some particle to here so this particle
one will exert some force F on this
particle right now after exerting this
force F on particle to this particle one
will come back to list right so by
moving the side it came back right when
one went to this side it pushed two so
to again vent the second so what we'll
do is exert some force F on particle
three so it will exert some force on
particle three and again it will come
back so as I designed this particle
three will start moving in this way so
what do you see
tell me what is happening so every
particle of the medium is actually
vibrating they are actually moving on
its own position so that means every
time this this object is also vibrating
so it will keep will coming this side
they're going back this side again
coming this side going by the same so
every time this object is coming in the
right direction it is pushing one to
right one will push two to right - will
push three - right so as a result every
object is vibrating or it is oscillating
in its own position but as a result this
vibration on this disturbance this is a
kind of disturbance right because of the
vibration of the source there is a
disturbance which will keep on moving
from particle one into particle to
particle 3 because this disturbance is
getting transmitted and this disturbance
is nothing but sound so if you look at
this diagram very carefully you will see
that just observe any one particle
you'll be able to when you look at this
picture what do you see you feel that
you can see very clearly that the
disturbance is moving along this
direction right so this disturbance
which you see I mean you do not see the
particles individually you just look at
this picture as of hold what do you see
you see you it looks as if everything is
moving towards the right side so that is
the disturbance which is getting
transmitted this disturbance which I
told you this object pushed one one
pushed two - pushed three three pushed
four so the disturbance is actually
moving towards the right direction that
is known as propagation of sound but
when you look at each particle of the
medium just observe any one particle for
example it let me take this particle
that is one particle here if you look at
only this particle do not see anything
else what do you see how is the particle
moving the particle is not going
anywhere it is just oscillating about
its mean position once it is going right
who wants it is going left
once it is going right once it is going
left similarly you look at any other
particle let me consider this particle
now if you look at this particle this
particle is again oscillating about the
red dot right so every particle of the
medium
none of the particles are actually
moving from one point to another so the
particles of the moon' medium and
helping the sound sound is nothing but
the disturbance it is actually helping
to transmit the disturbance through the
neighboring particles but the particles
of the medium do not move as such ok so
what is the conclusion the first
conclusion is that particles of the
medium do not move forward themselves
even though it appears that a
disturbance moves forward but the
particles of the medium
do not move forward right second is
disturbance establish forward through
the particles of the medium so here the
disturbance is started forward so these
are two different things one thing is
the particles of the medium what are
they doing they are just awesome eating
they are just vibrating about their mean
positions but as as a whole if you look
at it the entire disturbance is getting
transmuted and this disturbance is
nothing but sound right so what do we
see
so this disturbance is the sound and
that sound is again a wave so this shows
that it is nothing but a wave sound when
I talk of sound sound is nothing but a
sound wave actually so it sound is a
form of wave so what does me if I am
sure all of you know what is we bean is
nothing but it is a disturbance in a
medium see if you do not have the source
of sound
there will be no disturbance in the
medium the particles will not oscillate
at all so if the particles do not
oscillate there will be no sound but
wave is something wave is a disturbance
which is created in a medium by some
external source so here the external
source is the source of sound that is
the speaker right so it was a source
which created a disturbance in the
entire medium so this disturbance is
known as a wave and here the kind of
wave which we have gives some sound wave
so now you understood how some
propagates some wave propagates through
the particles of the medium but the
particles of the medium do not move
themselves they actually oscillate about
their mean positions and as a result the
disturbance or the sound is carried
forward to the particles of the medium
let us now talk about compressions and
very factions of a sound wave now let us
see what our compressions and
rarefactions let's take the same example
what is happening here
the sound is getting transmitted from
the source to the listener right so what
are compressions compressions and
rarefactions are the two terms which are
used to characterize sound wave so what
are compressions compression means
something which is compressed the term
itself means something which is
compressed something which is very
congested right so here if you see there
are certain regions now when the object
moves left think of what we discussed in
a previous slide how is this movement
happening when this object vibrates what
happens let us suppose when it move
forwards it pushes the neighboring
particles right so when it pushes the
neighboring particles it is actually
compressing the region in the front that
means let us suppose let us understand
it in this way it is suppose if this is
my object if this is my source so when
it vibrates and it moves in this
direction what is happening let us
suppose if these are the particles of
the medium so this will move the side so
it will actually compress the particles
here so the particles will come close
back right again when this object comes
back when it comes backwards what
happens this particles and wide away
from each other they are not compressed
now so the when they are compressed that
means when this object moves forward in
that case the neighboring region becomes
a region of high pressure or we can see
that the density of the particles is
high if you look at it here you can see
that when this particle moves in this
direction what happens a high
compression region app exists here that
means a high pressure region so the
region where the density of particles is
more the region where the congested
condition is more it is more congested
right so if you look at this figure as a
whole you will see that there is one
region
the particles are more congested there
is one region where particles are wide
away from each other so the regions of
high pressure or high density is known
as compression and the region for loop
of low pressure and low density is known
as very airy faction so these are the
two very important terms compression and
rarefaction
so combinations happen when the object
vibrates in the forward direction
because when it vibrates in the forward
direction it exerts a force in the
neighboring particles as a result a
neighboring particle comes closer so the
density of particles increases that at
that time it has more pressure so it is
high pressure high density that is
called compression similarly when the
object moves backward so then there is
less pressure on the neighboring
particle as a result the neighboring
particles can move away from each other
so a region of low pressure is created
and that is known as the Rayleigh
faction so in this case we can say that
this is compression I mean if if you do
not take an animated figure of this so
you can say that this is this is
compression and this is really faction
again this is compression again this is
really fashion so you have alternative
compeition and very airy factions so in
really faction you see the particles are
quite away from each other but in
compression they are very close to each
other so come from the term compression
you can remember compression with
something which is compressed so when
you compress something all the particles
will come very close by so the density
of the particles will be more so the
pressure will also be more right so that
is how you can remember what is
compression and what is very refraction
now as I mentioned before also that we
we spoke about that sound means a medium
to travel or to propagate now can some
travel without a medium that is a
Pacific device hammer medium if there is
no air no water no solid object nothing
so do you think that sound can travel
without the medium can sound travel
through a vacuum vacuum means an empty
region where there is nothing not even
here so that is well as vacuum the sound
means
medium to propagate it cannot propagate
through vacuum so that means medium
presence of a medium is a must for
Soundwave so how was this room to prove
this there was an experiment which was
performed which is popularly known as
the bench our experiment so what was
this bench our experiment in this what
was done was that an electric bed so you
can see here it is an electric band if
you press it sound should come right so
this electric Bell was placed inside an
airtight glass jar so this is the chair
and inside the child we had this
electric bench that is why this one has
been join experiment okay so this was
airtight so you cannot enter into this
yes right and then this was connected to
that this jar was connected to a vacuum
pump so that means it is a pump which if
you switch it on so vacuum will start
entering inside vacuum will start
entering inside means air will keep
going out so gradually after some time
it will be all vacuum
there will be no here at all correct now
what was saying that as vacuum was
pumped in air was pumped Pumped
gradually that is obvious what was
observed is that the sound of this Bell
became dimmer that means as more and
more vacuum pump then the sound of the
Bell started becoming dimmer and at a
certain point of time when there was no
air inside and it was all back soon the
sound of the Bell was not at all heard
so what did that prove that proved that
sound cannot travel through vacuum so it
actually means the medium and that
despite that makes sense also because we
saw that how sound propagates it
actually propagates because of the
vibration of the particles of the medium
so if you do not have a medium that
means you do not have the particles of
the medium if you do not have the
particles of the medium then how and how
the song
propagate there is nothing to vibrate
right because normally how it is getting
propagated is getting propagated because
of the vibration of the particles of the
medium so the particles are vibrating
that is how they are making the
neighboring particles also vibrate so if
you do not have a medium you do not have
the particles at all so the sound will
not be able to propagate so that is why
we say that sound means a medium it
medium is a must for sound to properly
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