Welcome to the NPTEL lecture on Architectural Acoustics we are in the last week week
number 8; and this lecture is the numbered as 39. This lecture will be on the the
urban noise control planning consideration; and this is a second part of that lecture
. So, in the first part of the lecture, we are
discussed about the the how the source the sound from the noise source can be mitigated.
And in this particular lecture number 39, we will discuss the the urban noise mitigation
measures through some landscaping or some kind of a barrier developing some kind
of barrier what are the different aspect of it. And also we are going to outline
the fundamentals of the noise the barrier design noise barrier design an all that is
our lecture or the learning objective for today's lecture .
So, go ahead with the the noise control. As we know that there are three ways
to control the the urban noise. The first one is to the mitigate from the source point
of view reduce down the noise from the source, and probably that is a one of the best way
to reduce the noise . The second one is the the control of noise in between, in between
in the sense of between the source and the receiver. The source and the receiver this
path is another area, where the the architect or may be urban designer may play
a vital role to reduce down the noise. So, we will discuss on that particular topic
today. So, in that in the noise control at path that is been between the source and
the receiver, there are three ways that at broadly it can be classified into three
ways the mitigation strategies. The first one is there are some kind of a geometric
change we can do we can alter some kind of the dimensional [di/dimensional] dimensional
the properties of the road, and the the setbacks, and the vertical up and down from
the the formation levels or so. So, those all come under the geometrical changes
between the source and the receivers. And then of course, that is basically done
in a newly developed area. And some of the cases I mean when there are some
kind of a new road has been planned, for for a area where there are already road,
and the the other side of the road there are building is already constructed. This
geometrical change may not be that much effective .
So, second one is that the formation of the road. So, formation of the road is that
some time we use some kind of a flyover or maybe some kind of a underpass or maybe
we put the road in a different layer or the different level. So, with respect to,
of course the receiver of the noise. So, those kind of a formation level change in the road
can be also useful for the mitigation of the noise . And the third one is also important
there may be a natural or may be some kind of manmade berries can be can be installed
or can be thought off in between the source and the receiver to make a noise at invasion
are the noise shielding. So, in this regard there are two
effect we have to discuss in this if you if you imagine there is a source of
sound and there is a receiver of the sound, receiver of the sound is maybe a it is
a it is a it is a area . And the source of the sound let us assume that is a road and
that that is a traffic noise. In this in between these two there are two effects is
actually going to play very vital role. The first one is called the ground effect
. The ground effect is due to the various nature of the ground is nature of the grounds
are actually written over here in this slide. The ground characteristics, source to receiver
geometry, and the special character of the the the of the source. Based on that
the particular the the the ground will sometimes increase the level of the sound
or sometimes ground may reduce down the sound or maybe activate that particular sound.
So, we have to look into that particular ground effect very curiously.
If I if you want to design based on the ground properties are the ground
effect. So, the there are two type of ground as for as the the acoustics is concerned.
And this particular two type of the ground is always a architect will actually where
play with in the the area where the there is a the this path, this path between between
the the source and the receiver. And this path between the source and the receiver may
be a footpath maybe a part of the road or maybe some the the grassy area some
pavement areas some setback for the building. So there are different character is possible.
Based on that character, and the based on the acoustical properties of those the
the character of the ground. It can be divided into two part or the two types . The first
one is called the acoustically hard surface , which are maybe a concrete pavement or
maybe a very asphalt cover, even a water, even a like a lack or some water bodies can
also act as a the acoustically hard pavement. And why, it is acoustically hard, because
it is actually reflect the the sound, because it is hard. It is kind of a be
because of the hard is sound is mostly it will be reflected. And as it is
reflected the there is a increment in the the sound pressure level it is noticed
on that So, the second type definitely it will be
acoustically soft, acoustically soft cover, and and that was are the the absorptive
cover. And it mostly absorb the sound by nature, and because of the because of the the texture,
and the kind of the cavities that form in the surface of the particular softcover .
It is considered as a terrain the with a soil terrain or maybe any kind of a grassy field,
and the those are come under that particular or bush kind of a heavy thing bush
in, and around a particular building can act as a very highly absorptive cover an acoustically
soft cover. And by virtue of the absorption almost of
the sound is going to be absorb, and very very very very less proportional
of the sound is going to be reflected. And it will be act as a well very well
damping the sound damper noise damper kind kinds of a thing.
The next effect other than that particular the ground effect is called as atmospheric
effect. The atmospheric effect is also sometimes gives you a increment in the sound pressure
level, on sometimes it is maybe decrease the sound pressure level . So, the prevailing
condition of the wind prevailing condition of the atmosphere the condition of the air,
and the presence of the moisture in the air, presence of the some suspended particle in
the air is going to give you this kind of the the absorptive criteria of the
the at as atmospheric effect. So, it has actually has three classification, three the
sub classification . The one called the as atmospheric absorption; the another
called is the atmospheric reflection, and the third one is turbulence.
Now, atmospheric absorption is the it is you know it is because of the nature, and
because of the air, and the the moisture present in the air. And this actually the function
of those this particular amount of absorption if you talk about it is actually depend
upon the frequency of the sound, it is also depend upon the temperature of the sound.
It is I am sorry the frequency of the sound, and the temperature of the wind or
maybe the air and also the humidity in the wind or humidity in the air itself. And
also it is depend upon the atmospheric pressure between the source and the receiver .
Out of these four the frequency is one of the prime important criteria . The temperature
and humidity is also one of the prime criteria. As if if you remember, you know all I think
in the second lecture we have discussed about that the how temperature plays a vital role
in the velocity of the sound, so that is going to give you a more propagation, more
velocity means it is quick propagation or the quick the shifting from one source to
the receiver . And humidity is also going to give you a way absorption criteria, atmospheric
pressure is also also also plays a role, but atmospheric pressure is it is
not that much predominant as why have talked about the other three. So, those are the atmospheric
absorption criteria. There are atmospheric reflection criteria
is also . It is caused by the temperature and the wind gradient. Then then the how the
wind actually flowing from, which point to the other that means it is is it from the
source to the receiver or from the receiver to the source or in between the receiver of
the source, because it may it may go parallel in that way, so because of that gradient of
the wind on also from the temperature of the wind . This particular phenomena of the atmospheric
reflection is going to change or it is going to vary .
So, atmospheric refraction or this also is going to give you kind of a the sound
um the the bending of the sound. And it also give some kind of the temperature the gradient
an all. And those kind of things also will be depend upon the upwind condition,
and the the the the condition for the away from the wind, and that kind of a scenario.
And it will going to decrease or increase in case of with this scenarios and
the if can in case of the upwind scenario, it will decrease the level of the sound, And
if it is a down condition or the downwind condition is prevailing, then the [sou/sound]
sound is going to be sound level will be going to be increase in the in the point
of the receiving source receiving point .
The last one is the [Vocalized noise] that air turbulence. Air turbulence is is sometimes
occur in the atmosphere. And this is a very unpredictable scenario. And as it is a very
unpredictable scenario. The changes in the sound pressure level because of the the air
turbulence is going to be fluctuated in the the in it is fluctuated I mean the its
not may it may not give a kind of a very direct relation the due to turbulence are the type
of the turbulence it will increase or decrease know it is very the the unpredicted
in nature . So, this refraction of the atmospheric sound,
and the the absorption of the atmospheric sound place a real vital, and the important
role in the mechanism between the the reduction of the noise from the source to the receiver
. So, this two effect the wind effect, and
the effect of the um the atmosphere is the the prime prime the parameter
for the mitigation. So, now let us discuss about those three things, which I have
just talked in the very first slide that the geometric changes and formation level changes
and all. So, what happened I have drawn a very small sketch of a house the this
blue colour a house habitat, and there is a road, and there is a car are that indicate
some kind of the road noise . So, from the car in the chassis level or the
level of the wheel from that particular level the sound is going to propagate I mean that
can be take it as a source of the sound . And source of the sound of course, there is a
breaking, due to the breaking there is a sound there is a horn that sound maybe
it up, but mostly that the friction between the tire and the road, the engine sound and
the breaking sound comes from that particular level. So, it is a point source. So,
I have drawn the the spherical propagation of the sound from that particular source.
So, the building is directly exposed to the road noise. And this one is a why it is
directly exposed. So, I have told in the last lecture also it
is a vision and sound propagation is very very similar. So, there is a direct line
of vision between the noise source and the receiver. So, there is a direct propagation
of the sound. So, somehow we have to nullify this direct vision line or direct
the straight line between the source and the receiver. So, it need to you need to block
this particular line of the vision so reduce the noise. And this blocking can be a done
by various way the formation change, the barrier then the the geometric the changes, and
there may be a lot of other things can be thought of .
So, what are the geometric changes, the first geometric changes can be occur like direct
line of vision can be still here, but we can increase the distance between the noise source
and the receiver. So, we push the building little back. And we as for as possible we
push the little back. And this is the probably in the one of the
the first thing that comes in our mind that if we segregate or if you make I mean, if
you provide a higher distance higher set back from the source to the receiver road to the
building, the the noise will be [redu/reduce] reduce definitely it is so, but it is not
because of the cutting of or the the this direct vision line, but it is as we know that
the the sound level this is inversely proportional to the distance. And it is reduced by 3 dB
by doubling of the distance. So, normally we can use this particular the
fundamentals of the sound physics, and you can go for a larger setback, but um students
this particular the criteria or this particular solutions is sometimes not feasible,
because we may not get get that much amount of the setback in the front setback or sometimes
you may get. So, it is a chance, it is a depend on your the site the ama the site,
and the dimension of the side and the what will be the total FER what will be the
total ground coverage, what is the demand from the client. So, there are lot of other
things you may not get that much amount of the distance.
And number two it is only reduced by 3dB. Let us, assume that the noise is almost
65 dB in the road . So, if you want to reduce it by around 10 dB, we have to go why a huge
distance from the source to the the receiver, source to the receiver. So, sometimes it is
not feasible. And it will only act as an academic kind of an academic kind of a
solutions kind of a thing. So, next one is may be a better or feasible solutions is that
change the level between the noise source and the receiver. We can make the building
little up or maybe we can sung the the the road little low. Sung thing up are the
this depressing the road little low sometimes also may not feasible, because
it is not in your hand. It is the already road is being laid or
so, but if you are go go I mean go going to have that road layout for a new area,
if you are actually working in a terrain area or maybe hilly site, then you can thought
of that kind of entry or those kind of a think with a different differential level . And
due to this change of the level of the road, this direct line will now going to be little
bit pain. So, the the lower part of the the source in
which is very near to the source those is kind of a blocked, of course it is the
exposed to the upper portion; and if the building will be under some kind of shadow some kind
of a noise shadow or so. And the this particular the efficiency of this reduction how much
will going to reduce the sound is definitely depend upon the depend upon the two things.
One is how much is the differential level between the source and the receiver, and also
what is the the the distance between the source and the receiver. So, this is the second one.
There can be a third thought also or the third solution also. We can go for some kind of
a physical barrier, which will talk little later. And this physical barrier may be a
tree, this physical maybe an earthbound or maybe a kind of a the the manmade wall
concrete or break wall . If you provide a physical barrier, then you are going to cut
the whole vision line. So, it will be directly nullify your nul noise, and the receiver
will be under full the shadow condition of the from the noise point of view. And this
also depend upon the receiving I mean the receiver to the the source distance, and the
what is the the type of on the height, and height and type of the barrier.
So, these are common that this particular the the geometric changes or so. So, next
let us discuss about the the road formation. So, road formation is um again I I must tell
you this road formation is another way of another sub category of the geometric changes,
because again I am going to change the levels, change the heights and whatever. And then
try to put my buildings and the habitats under a sound shadow zone. Ah
But again another disadvantage of this particular the the methods of reduction is that. Ah
It is not possible for an area where there is already a road is laid it say already there
are building is under construction or building is constructed on you think of doing some
changes no may not be possible, but yes it say some newly developed area you are creating
some kind of a the the are in a particular site is self you are doing some kind of a
level changes in the movement for the vehicular entries or may be some kind of a service entry
for the lories, truck and all then it is in your in in your scope of your work.
And you can also go for this kind of a road formation change.
So, whatever I have drawn here a small sketch. Two sketches are giving you a kind of a road
formation . The first case the upper one there is no such change in the level. So, suddenly
I thought these two buildings are has to be shield or that invited from the noise.
So, I can create some kind of earthbound or some kind of a the the earth berm, and that
is going to give you kind of a shielding. And sometimes we can sunken down the road
level is sunken down with respect to the with respect to the the the the the sides.
Where there are the the building plots and definitely it can create a shadow it can
create a shadow to the the building building to that in the habitat area.
There are [ano/another] another three the sketches. So, in this first top one the the
top one the noise shadow has been created, but kind of a the berm and the depressed section
kind of a thing. So, some of the areas in the, where there is a terrain development
or so or maybe there are some area is where there can be a kind of the the design
required, some kind of a sunken kind of the the habited to be developed. There
we can also integrate that with the noise reduction criteria.
These trees are also going to be very very important. I already told about this pai particular
area, where there is a grassy field or the grassy cover, which actually going to at in
way it, because of the the ground coverage. Here also it is a retaining wall sections
in the see this figure. A retaining wall sections is can be thought of and then the this
level of the rode level of the habitat area can be lifted above and create a kind
of sound shadow. And the raised sections is also possible, and by virtue of the distance
and raised sections can also sound shadow can be created.
The next and last one is the noise barrier . A noise barrier as I told you that it
is a kind of a barrier, which a community the boundary wall kind of a thing.
It may be manmade or it may be some kind of the vegetation the group of vegetations
and the trees and all, so that can be also thou thought of this noise barrier . It is
a wall, particularly it is a wall it may be a soft wall or maybe a very hard wall by virtue
of its nature of its the density, and the the the the weight has been constructed and
over material has been taken for that construction and all.
So, this wall suppose this is a source this red the colour dot is the source of
the noise . So, there are lot of phenomena I will going to occur. The first is the reflection.
So, this is a reflection can be occurred. If it is very hard, if it is soft, may not
be reflected much. Then there is an absorption the things will be absorb, the sound will
be absorb . Some sound may transmitted also in the other part of the wall. And the one
of the interesting criteria is that the the sound which will move at the top of this
wall can be diffracted . And this diffraction is depend upon the atmospheric conditions,
and some other properties also. So, here let us talk about this diffraction,
because that is the important criteria to design a the noise barrier or a wall also.
So, I have put a [ro/road] road. And there are two sides of the road one side is
probably the noise source and other side of the road is my building or whatever are
the noise sensitive zone. So, there is a sound source or the noise source, which is marked
as S and there is a receiver is R receiver. What is a receiver receiver point may be a
window, receiver point may be a kind of a balcony, receiver point may be a kind of a
open area, where people are seating for cup of tea or may be in relaxing for some kind
of a part of the day. So, those are the areas or those are the point
it can be consider as a receiver of the particular the the sensitive zone. So,
the sound will be going to diffract sound will be going to diffract. And this diffraction
is basically due to the the densi it is a governing by the density of the air layer,
the temperature, the humidity, and the gradient of the winds also, so but this this diffraction
will also not very the the equal amount for the the the frequencies.
For the higher frequency this diffraction angle will be very less. So, it will be re
diffract very less. Just a slight diffraction will occur, it will fly through . Almost it
will going to fly through . But if the frequency of the sound is low low frequency sound, which
is probably the sound comes from the road noise mostly the noise from the the engine,
the the friction from the tyre, and all. And there are another noise that actually comes
from the the the any any kind of a operations are the those kind of a things, automobile
operations those are basically a the low frequency sound. And those low frequency sound
is going to be diffracted much and that is a a higher degree of the diffraction . And
this higher degree of the diffraction will bend the sound from the top to lower.
So, is this particular sound this low frequency sound, which actually comes from the the automobile
source will be diffracted much, and the receiver may come under that particular sound . So,
this sound shadow is created by the noise barrier will not be same for the high frequency
sound or the low frequency sound. Low frequency sound has a much more wider appearance
much more wider the the domain, but the high frequency sound S. There are some sound high
frequency sound also comes from the road side the automobile, the hunk, the horn, and the
breaking sound or the high peak sound. Those will be fly away from the top.
So, from that I can understand that this is the first one, which is flying away from the
top of the this noise barrier, which is high frequency. And this is the low frequency is
tending back tending towards the ground. It is divide the whole area in the side of the
noise source into 3 zone. The first zone this called bright zone, which is above that particular
flying line or the line of the side. Why it is bright zone, because all the frequencies
are present here. All the high frequency and the low frequencies are present here. So,
suppose if you design a very tall building like this, so the upper part of the tall building
will be under this bright zone. And those balconies, and all in the maybe in the 5th
floor, 4th floor, 6th floor level will be exposed to all the sound. There is no vision
cut, there is this is that particular bright zone.
Next is there is a transition zone, where the partial frequencies are present. So, it
is a partial shadow kind of a scenario, because the high frequency will tend like this it
is not going to diffract. Some will go like this. And as in when the frequency increases,
it is moves back. So, here in the 3rd 4th or maybe the second floor level, there
are some partial frequencies will be available available over there, but some frequencies
definitely will be will be takeoff will be will be will be will be eliminated from
the zone . And then probably this is the last frequency
or the um the smallest frequency or the the lower lowest frequency that travels, and it
is deflected much higher amount of diffraction, so I can get a third zone, which is the lower
part of that this triangular triangulation, which is called the shadow zone, and almost
low frequency in present over here. And it is a truly reduced area, and the that can
be the the ground floor area or maybe any kind of the outdoor area, which can be
under that particular shadow zone also. So, next is you must thing that how can
this design can be formulated. So, again if I go back to this particular fundamentals
of the vision line and the sound line goes as in the same direction, so what we can do
is that we can calculate the the differential path or the path difference. This path a plus
b is your diffracted path, because of the diffraction. And suppose if there is no barrier,
so this S to R this path c this is your direct path. Of course, this direct path the
sound cannot me, because if there is a barrier. So, what is the path difference the diffracted
path minus the direct path that is a plus b minus c, and this a plus b minus c this
path difference are delta plus very vital role. Our our our the our role will be
as being a architect or maybe a urban designer. We have to increase this path difference by
any means we have to increase this path difference. As even when increase the path difference
you will going to get more amount of area as a sound shadow area . You will be able
to hide more amount of frequency. And we will get the most of the best benefit in the
receiving end. So, this is the path difference is one of the criteria.
Now, there are two reduce are increase this path difference. Of course, we will try to
increase the path difference always to mitigate the noise, there are sound factors.
Let us discuss those 5, 6 factors. Source is that red colour. S and the receiver is
in the the right side that is R receiver yellow colour. So, first factor is the distance
between the source to the barrier S. If you change the distance, if you just move the
source away from the receiver, if you go very close to the the receiver, this particular
barrier is very close to the receiver or so, there will be change of the in the path.
What is the next, next is definitely to the distance between the receiver and the barrier,
source and receiver and the barrier. These two the mutual distance are very much
important to get a particular path difference or maximize the path difference. The next
one is the height of the source from the road level . The next one is the height of the
receiver from the the the level, of course from the road level. These are four from
the point of view of the source and the receiver. And the last one is the height of the barrier
that is also true, because if the height of the barrier smaller, suppose all distance
are same. Now, the height of the this is very small, so the direct path, and this the other
diffracted path will be same, so maybe the less or so. So, you would not get any kind
of the benefit. So, and these are all geometric property that plays a role in this particular
noise barrier. There are so other factors. Also I have already told you that frequency
of the sound matters, because it is diffract the sound, and it is the high frequency sounds
is fly away or may not go that . And the porosity of the barrier, whether it is a solid barrier,
whether is a green cover, whether is a only a fens, so those porosity is actually going
to be important. And as and we know that this if it is a solid, this definitely
give you a lot of sound reflected back. Poros will again allow some kind of this, and also
the punchers in the barrier, which is may be a solid wall, but there are some punchers
that also give you some kind of a noise leak. So, there are some control on the barrios.
Also so here there maybe expose sound in the first figure, what I have drawn just exposed
to the building, noise expose to the building. And the second figure you see there are some
barriers, which is earth berm or maybe a kind of a the soft barrier like the trees, and
all which can create some kind of a sound shadow zone. In the this is outdoor space
can be very enjoyable from, and that will cut of the noise from the sound source also
. There are some kind of a guidelines also may
be there are two type of the road the this light, road light the street light,
and the tree maybe placed. One cases, the tree may be placed in the side, an another
cases trees maybe in the central path that is the road median. And the in the side there
are definitely the the in the side there are in the the the street lights in the sides,
so which one is good, which one you thought that is good, definitely, the first one, because
that will give you some kind of a barrier of sound, because of the vegetations and landscaping
screen. And another one I must say that if you
want to provide some trees the rows of trees in a like a boundary wall between the the
road and the the the the building, you must plan different type of tree one plan same
type of tree, because the amount of the sound observe by the the foliage of the tree is
depend upon the texture of the foliage. And also it is depend upon the how much cavities
it is form by the leaves. So, if you go for the same type of tree, the texture of the
trees will be same, and the the cavities created by the the leaves also will be the same or
of same nature. So, a particular band off sequence will be
observed ok . So, and if you change the type of the tree maybe in two layers or maybe
zigzag in that way, and then the different foliage and the different cavities formation
will be there, and because of that your um the a higher band of frequency can be observed,
because you know the the texture, and the absorption of the frequency has a vital role
or the cavities has a vital role. So, we are in the end of this lecture. And
the as usual I have some homework for you. I will give you a small exercise like are
take an example of a housing society was in a you may stay in the housing society
or maybe your friend is staying in that is kind of society society where visited lot
many times. And you know about that society take a particular
real life situation of a housing society. Try to draw the plan of that particular society
housing society and develop a schematic noise mitigation strategies for the housing, so
what can we can do for reduce the noise from the road to the particular housing society
that is one . And second one you prepared a list of list for design an effective
noise barrier, what are the the things you will be thought when somebody will
ask you to do a effective noise barrier . So, these two are the homework.
And that is all for this lecture number 39th, which was on the urban noise control the the
the planning consideration. And this is a second part, and the last lecture will be
on the architectural consideration. Thank you.