WEBVTT
Kind: captions
Language: en
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.
Good morning, welcome to the NPTEL course
00:00:18.730 --> 00:00:24.840
on Architectural Acoustics, this is the
last lecture of the second week and this is
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on the Application of Reverberation Time.
So, this lecture will end our the the
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second week and this whole module of the room
acoustics.
00:00:34.660 --> 00:00:41.290
So, in this lecture we will try to do some
the application oriented the version of
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the the reverberation time. If you remember
in the very first two lecture, we know we
00:00:46.660 --> 00:00:52.510
are discussed about the the various fundamentals
of the room acoustics by doctor Gupta and,
00:00:52.510 --> 00:00:59.800
then the following two lectures, I have
discussed with your sound absorption coefficient
00:00:59.800 --> 00:01:03.930
first and, then the reverberation time and
concept of the reverberation time.
00:01:03.930 --> 00:01:11.840
So, today my goal will be or the the the
objective will be will formulate some procedure
00:01:11.840 --> 00:01:17.710
to obtain the reverberation time for any
enclosed space. So, we will go very step by
00:01:17.710 --> 00:01:23.409
step way, how to formulate one after one and
finally, the aim is to get the RT and, also
00:01:23.409 --> 00:01:28.459
we will going to interpret the necessary changes
of the parameter to achieve this required
00:01:28.459 --> 00:01:32.270
the reverberation time.
So, what will be the significant change? So,
00:01:32.270 --> 00:01:37.469
if you know the this formulation this step
by step formulation, then in suppose in the
00:01:37.469 --> 00:01:42.409
second step what can be changed to get the
some effect on the last and, that kind of
00:01:42.409 --> 00:01:48.420
the the interpret this particular the formulation
of the necessary changes in the parameter
00:01:48.420 --> 00:01:53.399
we need to know.
So, here let us again go back to the Sabine's
00:01:53.399 --> 00:02:00.549
equation, which is given as you know that
RT is equal to 0.16 V by S alpha, I have put
00:02:00.549 --> 00:02:05.569
a summation sign because, it is summation
of the S alpha the sum of the various surfaces
00:02:05.569 --> 00:02:09.270
and it is product of the it is the absorption
coefficient.
00:02:09.270 --> 00:02:15.939
Now, let us draw a graph this graph is based
on the volume and the total room absorption,
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but total room absorption is S alpha the this
submission S alpha and, if you see in this
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particular this equation, or may I may
go to a blank page in the blank page.
00:02:32.220 --> 00:02:52.190
So, it is the RT RT is equal to 0.16 V
S alpha which is summation. So, this is the
00:02:52.190 --> 00:03:07.609
total absorption. So, I write capital A and
this is 0.16 V. So, if I say this as the A
00:03:07.609 --> 00:03:23.739
is equal to now 0.16 times RT multiplied by
V. So, this is give you a kind of a equation
00:03:23.739 --> 00:03:31.909
which is a equation of linear equation, where
A can be replaced a represented by x, this
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is some slope of the equation and this is
x y equal to m x kind of thing.
00:03:38.150 --> 00:03:43.780
So, I will get a straight line equation as
I know from this particular way, this is will
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going to be so, I will if I plot this A in
the x and and and sorry A in the y and,
00:03:51.450 --> 00:03:59.450
this V in the x I must get some line like
this, and various lines because of the
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various combinations of the RT. So, I got
those lines I got those lines over here and,
00:04:10.910 --> 00:04:23.170
those lines are actually your this green
line is actually for the RT is equal to 1.5
00:04:23.170 --> 00:04:27.990
and, this blue lines are RT equal to 0.5
the red is for 1.
00:04:27.990 --> 00:04:34.860
So, as the RT value is changes the line and
slope of is also going to change and, as the
00:04:34.860 --> 00:04:41.590
RT value is decreasing the slope is increasing,
that is one of the major observation from
00:04:41.590 --> 00:04:50.440
this particular graph. Next let us go to the
the another empirical formula, where the RT
00:04:50.440 --> 00:04:56.220
can be calculated by virtue of some volume.
So, after this Sabine's equation lot of physicist
00:04:56.220 --> 00:05:04.920
actually go into calculate the RT and, this
RT value is not going to be fixed for same,
00:05:04.920 --> 00:05:08.640
I mean different type of performance will
be required different type of RT.
00:05:08.640 --> 00:05:14.050
So, various physicist actually work in this
area now and, some of them has some come up
00:05:14.050 --> 00:05:19.520
with a some formula the the some kind of
the empirical formula. So, this is one of
00:05:19.520 --> 00:05:26.240
the empirical formula, where RT can be projected
by this. And here the V is the volume of the
00:05:26.240 --> 00:05:33.251
room and K is a constant and, this K is based
on different type of performance. So, suppose
00:05:33.251 --> 00:05:40.960
if it is a speech just like this particular
the the lecture room is for speech. So,
00:05:40.960 --> 00:05:46.090
this case four, suppose it is a orchestra
where there are some instruments some singing
00:05:46.090 --> 00:05:50.240
and all it is 5.
Suppose if it is a choir kind of a thing where
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is a group song in a kind of a thing then
it is 6. So, again by virtue of this V and
00:05:57.530 --> 00:06:04.160
RT values, the RT is kept in the y axis and
the V is volume in the x axis. Again you can
00:06:04.160 --> 00:06:11.610
plot for different volume what should be the
proposed RT value. So, we can see the green
00:06:11.610 --> 00:06:20.020
one which is for choir, which is higher and
the speech which is blue which is lower.
00:06:20.020 --> 00:06:27.980
So, for the speech you required some low
value of RT for a music you require little
00:06:27.980 --> 00:06:33.930
more. So, let us go to a kind of a band where,
we can see the what are the different standard
00:06:33.930 --> 00:06:40.200
reverberation time for different space,
or different performance. So, there are 5
00:06:40.200 --> 00:06:47.900
such classification room characters
classification very soft soft where the RT
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is very very low.
See the RT when it is 0.2 to 0.25 second it
00:06:53.340 --> 00:06:59.080
is actually required for the recording studio
the TV studio for the radio, or any kind of
00:06:59.080 --> 00:07:06.240
broadcasting cases. Suppose any television
broadcasting where there is a speech, any
00:07:06.240 --> 00:07:13.470
commentary box for any sports arena, you required
very low RT value 0.2 to 0.25 second. And
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the mean absorption is almost 0.4. So, you
have to put lot of absorber and that 0.4 as
00:07:19.160 --> 00:07:25.220
to be achieved.
The soft is 0.4 to 0.5 the RT value will between
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0.4 to 0.5 it is for the theater, where there
is again there is a speech there is lecture
00:07:29.900 --> 00:07:36.670
hall, just like this is the speech restaurants
and all. The most cases are normal cases which
00:07:36.670 --> 00:07:44.900
is 0.9 to almost 1.1, where it is cinema
hall, the office building, the library, the
00:07:44.900 --> 00:07:50.460
multipurpose auditorium residence etcetera
are in normal category, where the the mean
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absorption coefficient is almost about 0.15.
The hard where the the the reverberation time
00:07:57.020 --> 00:08:04.610
is little more, who is 0.1 0.8 to 2 is charge
and the the hospitals. And very large is the
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factory and the charges 2.5 and 4.5, you see
the charge and the religious place you always
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required the very high RT value. So, perhaps
we have already learned the the classical
00:08:19.780 --> 00:08:25.090
architecture and the history of architecture,
if you go to the history of architecture lecture
00:08:25.090 --> 00:08:29.610
and the books, you see the all the church
in the European church and very high in volume
00:08:29.610 --> 00:08:36.979
and also it is having very hard surfaces.
The church required very high RT not only
00:08:36.979 --> 00:08:43.070
church, any any short of religious song
and religious deliberations, it is actually
00:08:43.070 --> 00:08:49.520
demand high RT value, it may be a bell of
church, it may be some singing in the temple,
00:08:49.520 --> 00:08:56.200
it may be some kind of a beating of drums
in the the for the monastery Buddhist monastery,
00:08:56.200 --> 00:09:02.720
or maybe it is the azan in the Islamic
the the the mosque and all. So, you required
00:09:02.720 --> 00:09:10.620
all there is prolong version of the sound,
which demand high amount of the reverberation
00:09:10.620 --> 00:09:14.490
time.
In other sense when it is a speech which is
00:09:14.490 --> 00:09:20.760
recording studio, or maybe theatre or lecture
hall in this particular domain, you require
00:09:20.760 --> 00:09:27.640
a very very discrete and concrete and very
split of sound, that should not be any prolong
00:09:27.640 --> 00:09:33.430
kind of a case, then there will be kind of
the the hearing problem and, the the intigilability
00:09:33.430 --> 00:09:37.950
problem and the kind of the confusion will
create in time of hearing.
00:09:37.950 --> 00:09:45.050
So, let us next stage let us try to calculate
the how the reverberation time can be calculate
00:09:45.050 --> 00:09:50.420
what are the steps what are the formulations.
So, I have drawn a plan of a hall of 30 meter
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by 15 meter and, the height of the hall is
6 meter which is given in the section and
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this hall is having 4 doors of 2 meter by
2.5 meter and 3 meter by 1.5 meter 6 windows
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which is plotted in this plan in blue colour.
So, first what will do in the very first step,
00:10:12.170 --> 00:10:21.480
we first find out the what is the interior
area. So, we calculate the door area is 20
00:10:21.480 --> 00:10:32.520
and the window area as 27, the ceiling
is 30 by 15 is 450 floor is also 450 and wall
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area, I calculate the perimeter and I multiply
the perimeter by height 6 and I deduct from
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that 20 plus 17, because that is the window
and door in is in the part of the wall.
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So, 493 is the the total wall area total
wall area, and the volume of the hall is
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15 into 30 into 6 which is equal to
2700 2700 meter cube. So, first you calculate
00:11:09.930 --> 00:11:16.750
the all the the surface area, then go to
the next and try for the next data that
00:11:16.750 --> 00:11:23.060
is given for this particular. So, what the
next set of data. So, next set of data is
00:11:23.060 --> 00:11:28.870
given as the door and window as glass and
aluminum, which is having alpha value of 0.01,
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because this is very very reflective the ceiling
and wall wall is also very reflective as 0.02,
00:11:36.029 --> 00:11:43.280
it is plaster and the floor is wooden floor
is 0.12. So, if you see all the three materials
00:11:43.280 --> 00:11:47.870
what I have given as a part of the problem
are most reflective material.
00:11:47.870 --> 00:11:53.920
The alpha value is very small very close to
0 for the first one 0.12 for the the floor.
00:11:53.920 --> 00:12:00.839
So, I am expecting I am expecting a very
high RT value for this for this room, then
00:12:00.839 --> 00:12:07.670
I start calculating the next stage, where
I multiply this alpha value with the the
00:12:07.670 --> 00:12:12.620
the corresponding area and, I am finding the
total absorption of that corresponding area
00:12:12.620 --> 00:12:16.620
by meter square 7.
So, I have multiplied everything, ceiling
00:12:16.620 --> 00:12:23.860
is 0.02 and the floor is 0.12, again wall
is 0.02 because both are plaster. So, and
00:12:23.860 --> 00:12:34.100
I add all this 5 and, then I got a total absorption
is 73.33 meter square 7. So, my second
00:12:34.100 --> 00:12:39.339
the the stage, or the formulation is over.
So, now, I know what is the sum S sum S
00:12:39.339 --> 00:12:46.870
sum of S alpha I also know the V so, I can
can calculate the RT now. But let us go
00:12:46.870 --> 00:12:52.700
to further some some character of the
room.
00:12:52.700 --> 00:12:58.320
Suppose this hall is having a capacity of
300, 300 people can sit and listen something
00:12:58.320 --> 00:13:03.860
over there and, there are two options, suppose
there is a cane chair or may be a cushioned
00:13:03.860 --> 00:13:09.710
chair like this like this cushioned
chair. And there is a audience also so,
00:13:09.710 --> 00:13:16.850
I have given the per seat, what is the absorption
cushioned chair is 0.5 cane chair is 0.01
00:13:16.850 --> 00:13:23.250
and audience is 0.4. And by virtue of the
clothing and all these things, but the hair
00:13:23.250 --> 00:13:29.250
and all you have the kind of the absorption
for human being also.
00:13:29.250 --> 00:13:34.310
Suppose this is summer cloth. So, it having
a absorption coefficient less, suppose it
00:13:34.310 --> 00:13:40.770
is in winter if I having a jacket, or maybe
some kind of a woolen, my for the audience
00:13:40.770 --> 00:13:45.940
absorption per audience absorption is also
will be very very high. So, what happened
00:13:45.940 --> 00:13:50.670
is that now I go for the third calculation
for the absorption for the chair and all.
00:13:50.670 --> 00:13:56.330
So, only cane chair if I use, then it is this
much because per chair is 0.01 and, there
00:13:56.330 --> 00:14:01.820
are 300 chair only cushioned chair, or no
audiences is if it is there, then is 150 and
00:14:01.820 --> 00:14:07.520
if the audience is full I mean there is a
full audience, then the 300 audience per audiences
00:14:07.520 --> 00:14:13.710
is 0.4 this is 120.
So, either it is a full house or maybe a it
00:14:13.710 --> 00:14:18.790
is empty or maybe it is half full so, some
conditions will be there and, based on that
00:14:18.790 --> 00:14:25.640
I can take the permutation combination and,then
I can again find out some amount of absorption.
00:14:25.640 --> 00:14:30.839
So, what happened in case of there is a empty
hall. So, if there is a hall is empty there
00:14:30.839 --> 00:14:39.240
is no audience and, I have only cane chair
which is very very very reflective in nature
00:14:39.240 --> 00:14:47.360
and there is no absorption at all. So, the
total S alpha value will be 73.33 plus 3 why
00:14:47.360 --> 00:14:54.020
73.33 that that comes from the absorption
of the surfaces and, why this is 3 if you
00:14:54.020 --> 00:14:59.279
go to the previous slide, if it is cane chair
no audience it is 3.
00:14:59.279 --> 00:15:08.180
So, total absorption is 76.33 and the RT value
by virtue of your volume and 76.33 and 0.16
00:15:08.180 --> 00:15:17.340
is 5.7, I predicted that I told that if there
are less amount of absorption and there are
00:15:17.340 --> 00:15:22.620
so, much of reflective surface, I am going
to get a the high reverberation time and I
00:15:22.620 --> 00:15:29.899
am getting that and this hall is empty
hall no audience. Now, let us see if I replace
00:15:29.899 --> 00:15:39.010
the chair, cane chair by cushioned chair.
So, 73.33 will remain same, but 150 150 comes
00:15:39.010 --> 00:15:43.560
from comes from this cane chair and no audience
case.
00:15:43.560 --> 00:15:53.890
So, this two gives me 223.33 as the S alpha
and finally, this will be my RT value 1.9
00:15:53.890 --> 00:16:02.959
drop down 5.7 to 1.9 it is improve. Now, let
us see if it is full house this full of people.
00:16:02.959 --> 00:16:10.399
So, all chairs are full of peoples. So, the
audience is now going to absorb, then this
00:16:10.399 --> 00:16:19.430
is 120 is for the full audience and 73.33
is give me 193 and the corresponding RT value
00:16:19.430 --> 00:16:23.480
is 2.2.
So, you see if there is a empty hall, or maybe
00:16:23.480 --> 00:16:29.740
a full house, or if it is a cane chair, or
some other type of chairs, it is also impart
00:16:29.740 --> 00:16:36.649
a physically the RT value of a room. Now,
let us take a a design based case, I am
00:16:36.649 --> 00:16:42.160
going to design this particular room for a
base case, where I have going to use the cane
00:16:42.160 --> 00:16:47.810
chair and, the half the hall is occupied so;
that means, the half the hall will be occupied
00:16:47.810 --> 00:16:53.899
and rest half is cane chair and, this will
be my best design condition and for that I
00:16:53.899 --> 00:16:58.730
want to set up a particular the design
the RT value.
00:16:58.730 --> 00:17:04.370
So, for that I recalculate the what will be
the empty chair conditions 150 empty chair
00:17:04.370 --> 00:17:12.529
with 1.5 absorption and, the 150 people will
0.4 equal to 60 is the meter square Sabine.
00:17:12.529 --> 00:17:22.039
So, total is 61.5 and, for that condition
this 73.33 remain same surface I have not
00:17:22.039 --> 00:17:30.419
changed. So, finally, 134.83 and it is gives
me 3.2 reverberation time and please remember
00:17:30.419 --> 00:17:37.330
this 3.2 is my base I want to improve it improve
it means, reverberation time improvement means,
00:17:37.330 --> 00:17:42.730
you have to reduce the reverberation time
3.2 is too much supposes this hall is for
00:17:42.730 --> 00:17:50.279
speech we require almost point 6 or so,
if it is for music or so, 1 second or 1.2
00:17:50.279 --> 00:17:55.750
second like that.
So, 3.2 is something like a charge. So, this
00:17:55.750 --> 00:18:03.049
cannot be actually take in so, I have to
improve the the other surfaces. So, I have
00:18:03.049 --> 00:18:11.019
first case as I will do some covering, I will
cover the door and window by heavy curtains
00:18:11.019 --> 00:18:18.610
having alpha value 0.65. And the wall be treated
with the absorption some absorptive tiles
00:18:18.610 --> 00:18:23.929
having alpha point. So, I have taken this
two decision, I will treat the wall and I
00:18:23.929 --> 00:18:29.139
will treat the the door and window by virtue
of some the curtains.
00:18:29.139 --> 00:18:35.940
But the the ceiling and floor I will not
going to treat, that will be as usual as earlier.
00:18:35.940 --> 00:18:42.090
So, if I do that I have taken the same
the calculation table from the previous. So,
00:18:42.090 --> 00:18:50.100
door is now a new way we have to calculated,
the 20 will remain as 20 the area area has
00:18:50.100 --> 00:18:56.580
not going to changed, but instead of this
wooden door or the glass door and the glass
00:18:56.580 --> 00:19:02.879
window, now there is a curtain. So, 0.65 comes
from here and this is got cancelled.
00:19:02.879 --> 00:19:12.850
So, the 2 2 meter square Sabine which was
earlier is now, 13 it is improved, 2.7 for
00:19:12.850 --> 00:19:20.389
the window is no improve to 17.55 by virtue
of induction of this two curtains ceiling
00:19:20.389 --> 00:19:26.200
and floor, I told I have not I will not going
to treat. So, this 2 remain same 9 and 54,
00:19:26.200 --> 00:19:31.239
but wall is now going to change, what was
earlier the earlier was it is the plaster
00:19:31.239 --> 00:19:40.070
0.2. Now, I have treated with that the wall
with a sound absorptive tiles 0.45. So, this
00:19:40.070 --> 00:19:50.549
4 9 3 the total area of the wall was multiplied
with 0.45 and 9.86 is improve to or increase
00:19:50.549 --> 00:19:57.210
to 221.85.
So, totals Sabine total amount of Sabine are
00:19:57.210 --> 00:20:03.039
the meter square Sabine at the total amount
of absorption of the room was 73.33. Now,
00:20:03.039 --> 00:20:12.330
it is 315.5 so definitely I am expecting a
low version of the or the low RT so, let us
00:20:12.330 --> 00:20:22.090
see how much is RT. So, cane chairs and half
audience give me 61.5 and, this 315.4 from
00:20:22.090 --> 00:20:33.649
the surfaces total is 376.9 you see earlier
it was 3.2 and this is 1.15, much more controlled
00:20:33.649 --> 00:20:39.710
was achieved.
But if it is in for speech if it is for music,
00:20:39.710 --> 00:20:46.679
for some kind of musical performance 1.15
or if this hall is for some movie or the the
00:20:46.679 --> 00:20:53.690
cinema it is ok, but if it is for speech for
any kind of lecture room, it is not still
00:20:53.690 --> 00:20:58.970
now is not suitable it is too high we have
to come down to point 67.
00:20:58.970 --> 00:21:06.309
So, I have a case 2 also so, in case 2 what
I did I remain this two as it is curtains
00:21:06.309 --> 00:21:15.070
in the door and window, sound absorb absorptive
styles in the this wall 0.45, what I did
00:21:15.070 --> 00:21:24.320
here is that, I created a suspended ceiling
with some tiles of alpha value 0.3 and, I
00:21:24.320 --> 00:21:30.830
hang this particular ceiling at a depth of
1 meter. So, why what I did I did a new surface,
00:21:30.830 --> 00:21:37.059
I put a the suspension ceiling at a depth
of a 1 meter from the the the structural
00:21:37.059 --> 00:21:45.690
ceilings of it, and that gives me 2 benefit
1 is I also treated that particular surface
00:21:45.690 --> 00:21:52.570
with the acoustical tiles of 0.3. So, my the
surface which was clustered was hidden now
00:21:52.570 --> 00:21:58.830
and, the absorption coefficient is also increased
and, the second one is by virtue of lower
00:21:58.830 --> 00:22:04.220
down the the ceiling I decrease the volume
of the room also.
00:22:04.220 --> 00:22:10.940
Now, you see in the the volume of the room
has to be calculated as height 5 meter not
00:22:10.940 --> 00:22:18.539
6 meter because, 1 meter is actually taken
by the suspension ceiling. So, let us recalculate
00:22:18.539 --> 00:22:27.429
the volume 15 into 30 into 5 which was 2700
earlier now it is 2250 and the reflect new
00:22:27.429 --> 00:22:35.440
surface of the wall, also will decrease because,
the one meter band in the roof, which was
00:22:35.440 --> 00:22:40.629
now hidden by the suspended ceiling has to
taken out from my calculation.
00:22:40.629 --> 00:22:48.679
So, now the perimeter is multiplied with 5
instead of 6 6 was earlier. Now, it is 5 because
00:22:48.679 --> 00:22:55.989
this portion is exposed to the sound and 20
plus 27 is the door and window was detected.
00:22:55.989 --> 00:23:05.600
So, now, new the area of the wall is 403.
So, based on this new to volume of the wall,
00:23:05.600 --> 00:23:11.700
volume of the room and the the surface of
the wall let proceed for the calculation.
00:23:11.700 --> 00:23:19.880
Door and window remain same 0.65 multiplication
ceiling is now different because, now this
00:23:19.880 --> 00:23:29.129
0.02, which was clustered which gives me 9
meter square Sabine is now 0.3. So, 450 into
00:23:29.129 --> 00:23:36.899
0.3 that is 135 Sabine, I have not treated
the floor. So, it is remain as at as it is
00:23:36.899 --> 00:23:44.629
54, but there is a again change in the wall,
there is no change in the acoustical coefficient
00:23:44.629 --> 00:23:51.450
of the tiles which is 0.45, but there is a
decrease in the surface area of the tiles
00:23:51.450 --> 00:24:00.539
which was 493, now it is 403 almost 90
meter square of the reduction in the area.
00:24:00.539 --> 00:24:13.249
So, I I got almost 181.85, which was earlier
221. So, now, my new total absorption of
00:24:13.249 --> 00:24:23.289
the room is 400 almost 401 409 meter square
Sabine, if you add this all now let us calculate
00:24:23.289 --> 00:24:30.359
and, if you calculate with the same half full
audience and the cane chair this is 61.5 plus
00:24:30.359 --> 00:24:38.070
400.9 462 and, if you divide that and use
that one for your calculations and please
00:24:38.070 --> 00:24:45.010
remember the volume is also changed, it is
not 2700, it is 2250 now and now your reverberation
00:24:45.010 --> 00:24:52.309
time is 0.8.
So, which was 3.2 in the base case untreated
00:24:52.309 --> 00:25:01.039
case, which was now one point was 1.15
in the case one now it is 0.8. So, let us
00:25:01.039 --> 00:25:09.909
draw a graph for that the case one base case
was 3.2 very high, all surface untreated what
00:25:09.909 --> 00:25:16.580
happened next the case two, wall is treated
and curtains and all these things are given
00:25:16.580 --> 00:25:23.470
in the base case in the doors and, the windows
and wall is treated 1.15 and finally, in the
00:25:23.470 --> 00:25:30.879
case 2, it is 1 meter drop of suspension ceiling
and wall and this things are as previous treated
00:25:30.879 --> 00:25:37.600
suspension ceiling, it is 0.8 probably we
can adopt that for a lecture room or so.
00:25:37.600 --> 00:25:43.710
So, there are how to determine this application
of the RT from that point of view how to determine
00:25:43.710 --> 00:25:48.860
the sound absorption coefficient of the panel.
Suppose, I want to test a panel and, I want
00:25:48.860 --> 00:25:54.529
to find out what is the alpha value of the
panel is so, how to test it so, by with
00:25:54.529 --> 00:25:59.690
this equations and all. So, suppose I have
a room and this room is empty and this empty
00:25:59.690 --> 00:26:05.390
rooms gives me RT 1, this is the first condition.
So, I can find out the a alpha by virtue of
00:26:05.390 --> 00:26:08.690
just change this particular rearrange this
particular equation.
00:26:08.690 --> 00:26:14.759
Now, what I did I put a additional acoustical
tiles, additional panel in the that, that
00:26:14.759 --> 00:26:19.889
particular panel I am going to test. So, the
RT value will going to change because, there
00:26:19.889 --> 00:26:25.629
is a new edition of material the A is the
new the area of the material and, alpha
00:26:25.629 --> 00:26:30.629
new I am interested in how to find out what
is the alpha of this particular panel. So,
00:26:30.629 --> 00:26:36.440
again I rearrange this equation in this.
Now, I replace this S alpha here and I can
00:26:36.440 --> 00:26:43.869
easily find out the equation the from this
equation, I am what I am I did is that this
00:26:43.869 --> 00:26:50.649
is S alpha S alpha I have replaced here. And
then I can rearrange the equation and the
00:26:50.649 --> 00:26:57.999
alpha new of this particular panel can be
found out, if I know the RT 1 and RT 2, if
00:26:57.999 --> 00:27:02.739
I know the volume of the room and, if I know
the area of the this new panel, I do not need
00:27:02.739 --> 00:27:09.760
any any else, or anything else I do not need
the the alpha of the the the room walls
00:27:09.760 --> 00:27:16.500
and, all the the surface areas and all
I can have some other methods.
00:27:16.500 --> 00:27:24.070
Suppose I know that one material which is
I have drawn here is a blue and, this particular
00:27:24.070 --> 00:27:30.279
value is alpha one of this particular material
is known to me. Suppose, I know this material
00:27:30.279 --> 00:27:37.470
what is the value is known to me and, I found
out the RT by this situation, this is a total
00:27:37.470 --> 00:27:45.370
S that is the total surface area minus A into
alpha plus A into alpha 1, because I assume
00:27:45.370 --> 00:27:51.269
that the rest of the area is having the alpha.
So, the rest of the area multiplied in the
00:27:51.269 --> 00:27:56.019
alpha and, this blue area which is the a is
alpha 1.
00:27:56.019 --> 00:28:04.320
So, this keeps me the total absorption and
this RT, next I replace this by a another
00:28:04.320 --> 00:28:10.590
material, another same area by another
panel the red in color and, this alpha
00:28:10.590 --> 00:28:15.869
two of this new material is unknown to me.
So, I can rewrite this equation by RT 2 I
00:28:15.869 --> 00:28:23.080
will get some different RT and this is alpha
2. Now, just rearrange this equation because,
00:28:23.080 --> 00:28:33.849
the S alpha S alpha minus A is is common for
this this one S alpha minus A and, then you
00:28:33.849 --> 00:28:40.649
can rearrange and finally, you can calculate
the alpha you can calculate the the alpha
00:28:40.649 --> 00:28:45.769
2 from this equation by this rearranging this
things. .
00:28:45.769 --> 00:28:53.629
Now, there are way to handle this particular
scenario, am I understanding is that
00:28:53.629 --> 00:29:02.639
I being a designer being a acoustical designer,
I have to provide some RT, or some design
00:29:02.639 --> 00:29:09.529
RT for a particular room and, when I got a
particular room or a hall, it is all reflective
00:29:09.529 --> 00:29:14.020
surface plastered surface and all reflective
surface. So, I have to render it with some
00:29:14.020 --> 00:29:19.360
different material and finally, I have to
propose the material and finally, I have to
00:29:19.360 --> 00:29:24.009
give some kind of the RT I mean achieve
some RT value.
00:29:24.009 --> 00:29:30.049
So, if you remember this graph I have drawn
in the very first slide today. And these are
00:29:30.049 --> 00:29:35.639
the absorptions room absorption total room
absorption which is S alpha and in x axis
00:29:35.639 --> 00:29:41.789
is the volume. So, this green line indicate
or or may be there is red line indicate the
00:29:41.789 --> 00:29:48.600
RT value is 1 and, this blue line indicate
the RT value is 0.5.
00:29:48.600 --> 00:29:56.169
Suppose I I I am I am targeting a 0.5, I am
targeting to achieve 0.5 and my present value
00:29:56.169 --> 00:30:04.499
of RT of this room suppose 1. So, I have to
reduce the RT value how to reduce from this,
00:30:04.499 --> 00:30:13.210
particular graph I can go from red to green
blue by virtue of a horizontal shift and,
00:30:13.210 --> 00:30:19.120
if I go with this this is my option 1, I have
to reduce the volume see I have to reduce
00:30:19.120 --> 00:30:25.169
the volume from 4000 to 2000.
Another interesting point you see if I want
00:30:25.169 --> 00:30:32.519
to go from green to red same same see the
the the difference is 0.5 second, I have
00:30:32.519 --> 00:30:39.749
to go smaller, I have to go smaller, but if
I want to go from one second to 0.5 second
00:30:39.749 --> 00:30:47.639
same the difference is again same I have to
go higher. So, where you go towards the lower
00:30:47.639 --> 00:30:54.129
part of the reverberation time you have to
give more effort more rendering more absorption.
00:30:54.129 --> 00:31:00.509
But in this option one is reduce reduction
of the volume from this to this, is sometimes
00:31:00.509 --> 00:31:07.289
not possible why it is not possible, because
the volume of the room and the dimension of
00:31:07.289 --> 00:31:13.830
the room is basically comes from the the
occupancy the the the rows, number of rows,
00:31:13.830 --> 00:31:19.090
the the size and the leg space and the chair
and, the table and lot of other furniture
00:31:19.090 --> 00:31:25.859
details and the prime factor is some kind
of a circulations and, thus there are some
00:31:25.859 --> 00:31:30.419
the occupancy requirement and all.
So, sometimes and also the other dimension
00:31:30.419 --> 00:31:36.529
the aspect ratios of the hall also. So, sometimes
or probably many times it is not possible
00:31:36.529 --> 00:31:42.399
to reduce the volume and achieve that. So,
the option one is not practical.
00:31:42.399 --> 00:31:49.749
So, how to touch this blue line, from red
line, the another way if you go vertical.
00:31:49.749 --> 00:31:57.550
So, if you go vertical also from this 4000
in rather decreasing the volume, you can
00:31:57.550 --> 00:32:04.850
also touch, but what is this you have to increase
the room absorbent. So, from here it is almost
00:32:04.850 --> 00:32:10.629
620 or something like that or may be 650 2
almost 12000 plus.
00:32:10.629 --> 00:32:16.979
So, how to increase the room absorbent and,
what is room absorbent S alphas, you cannot
00:32:16.979 --> 00:32:23.419
change S mostly, or what you can do is that
you can increase the value of alpha, what
00:32:23.419 --> 00:32:30.269
how can you increase the value of alpha, we
can some kind of a acoustical tiles of course,
00:32:30.269 --> 00:32:35.119
if you do that it is required money because,
you have to render it with some different
00:32:35.119 --> 00:32:42.029
acoustical material, it required cost and
so, you have to be careful also I mean
00:32:42.029 --> 00:32:45.470
how much is actually going to achieved or
so.
00:32:45.470 --> 00:32:53.019
So, there is this is the option two the
increase of the room absorbent also give you
00:32:53.019 --> 00:32:59.679
reduce your particular RT there is a third
option also. You can touch this particular
00:32:59.679 --> 00:33:07.139
blue line from the red line by going angular,
what is that mean going angular means, actually
00:33:07.139 --> 00:33:15.909
I am increasing the room absorption and parallelly
I am decreasing the room volume. So, if you
00:33:15.909 --> 00:33:22.390
decrease the room volume and increase the
adsorption, which was my last case by creating
00:33:22.390 --> 00:33:29.769
a false ceiling I decrease the volume and,
I render the surface with the room absorbent
00:33:29.769 --> 00:33:35.909
this two also going to help me.
So, again if I go back there are 3 options
00:33:35.909 --> 00:33:43.090
to achieve a particular design design RT,
either you reduce the volume which always
00:33:43.090 --> 00:33:49.809
not going to possible, you increase the room
absorption, which is a bit costly of course,,
00:33:49.809 --> 00:33:58.460
but mostly we do like that, or sometimes we
can increase the volume we reduce the volume
00:33:58.460 --> 00:34:05.179
and increase the room absorbent, which may
be come into a most practical scenario. So,
00:34:05.179 --> 00:34:09.810
you are almost are may be at the end of our
lecture number 10.
00:34:09.810 --> 00:34:16.470
So, as usual we have some kind of homework
for you. So, I have given a just a single
00:34:16.470 --> 00:34:22.490
problem the calculate the reverberation
time of a room, which is empty condition there
00:34:22.490 --> 00:34:29.490
is no furniture no the audience for the
following data, the dimensions I have given.
00:34:29.490 --> 00:34:36.389
And the all surfaces having a the sound absorption
coefficient is 0.1. So, again you can
00:34:36.389 --> 00:34:41.819
expect a very high reverberation time for
as a answer.
00:34:41.819 --> 00:34:49.950
So, I have further the same problem
is further extended like, what should be
00:34:49.950 --> 00:34:57.490
the sound absorption coefficient of that need
to be provided only in the ceiling to get
00:34:57.490 --> 00:35:02.700
the reverberation reverberation time one seconds.
So, definitely in the first problem I will
00:35:02.700 --> 00:35:09.920
get reverberation time more than 1 second
and so, I want to make it 1 second. So,
00:35:09.920 --> 00:35:15.640
then only I am going to treat the ceiling
surface only the ceiling surface. So, what
00:35:15.640 --> 00:35:21.630
will be the alpha value of that that particular
tiles, I need to place in the the ceiling
00:35:21.630 --> 00:35:30.740
that I that that is that is my question. So,
at the end I hope you have learned about the
00:35:30.740 --> 00:35:37.930
room acoustics.
And understand the basic principles of
00:35:37.930 --> 00:35:43.200
the room acoustics in the first two lecture,
third lectures, you understand probably the
00:35:43.200 --> 00:35:49.619
the way we handle the reflections and the
absorptions and, we define the reflection
00:35:49.619 --> 00:35:56.200
coefficient the sound reflection coefficient
of any panel, or kind of material.
00:35:56.200 --> 00:36:01.369
And finally, in the last two lecture we
discuss the concept of the reverberation,
00:36:01.369 --> 00:36:07.910
reverberation time and, methodology or the
formulation how we can actually calculate
00:36:07.910 --> 00:36:16.420
the the reverberation time and we design
for reverberation for a particular room.
00:36:16.420 --> 00:36:25.020
So, the next week we will start the acoustical
material various type of acoustical
00:36:25.020 --> 00:36:31.170
material and Dr. Sumana Gupta will deliver
her lecture on that.
00:36:31.170 --> 00:36:32.160
Thank you very much.