WEBVTT
Kind: captions
Language: en
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Good morning students welcome to the NPTEL
lecture on architectural Acoustics.
00:00:25.669 --> 00:00:30.689
This is the third lecture where we will
discuss the frequency and the octave.
00:00:30.689 --> 00:00:38.080
If you remember in the lecture number 2, we
have discussed about the the general general
00:00:38.080 --> 00:00:44.110
propagation of the sound in a media like air
or in any other kind of a media.
00:00:44.110 --> 00:00:49.900
And we have actually thought of some kind
of the parameters and we have actually
00:00:49.900 --> 00:00:54.081
given some kind of a relations to one parameter
to the other.
00:00:54.081 --> 00:00:59.420
We also found out the, what is what should
be the appropriate propagation equations are
00:00:59.420 --> 00:01:00.420
so.
00:01:00.420 --> 00:01:07.189
So, there we have discussed one typical parameter
of the sound has the frequency and this frequency
00:01:07.189 --> 00:01:13.800
plays a major role in building acoustics
or any kind of the acoustical phenomena.
00:01:13.800 --> 00:01:17.860
So, in this lecture there are 2 learning objective.
00:01:17.860 --> 00:01:25.000
So, if there are various combinations of the
frequencies, how to differentiate at the distinguish
00:01:25.000 --> 00:01:29.060
between them that is the first objective of
this lecture.
00:01:29.060 --> 00:01:35.070
And second objective is the interpret the
super imposition of the wave motion.
00:01:35.070 --> 00:01:40.690
That is also given to be happened because
of the if sound cannot be cannot
00:01:40.690 --> 00:01:45.540
be a single frequency sound, it it is actually
in the mixed frequency sound.
00:01:45.540 --> 00:01:50.560
So, how the sound is super imposed with the
other and that kind of a scenario we have
00:01:50.560 --> 00:01:57.060
to see .
So, the frequency as we discussed as we already
00:01:57.060 --> 00:02:01.360
discussed in the lecture number 2 is the it
is a number of oscillations of the or the
00:02:01.360 --> 00:02:06.940
cycles that is going to be displaced
in the unit time in a wave motion and its
00:02:06.940 --> 00:02:13.920
the unit is hertz or cps, and out of the hundreds
are out of the the thousands of the frequency
00:02:13.920 --> 00:02:21.220
is available in the nature, the human being
can only response to 20 to 2 20000 hertz
00:02:21.220 --> 00:02:22.220
frequency.
00:02:22.220 --> 00:02:27.329
Now, this spectrum is also very huge, huge
spectrum of the frequence frequency we can
00:02:27.329 --> 00:02:33.260
actually mapped in our brain .
Now, let us let us discuss about this frequency
00:02:33.260 --> 00:02:39.529
how this frequency has create this kind of
a sensation and that is called as a tone a
00:02:39.529 --> 00:02:45.390
sensation sound sensation because of some
kind of a frequency is called tone.
00:02:45.390 --> 00:02:49.489
There are 2 type of tone, the one is called
the pure tone.
00:02:49.489 --> 00:02:55.579
The pure tone is the single frequency sound,
only a particular frequency nothing else a
00:02:55.579 --> 00:03:01.439
sticking of some kind of a vibrating object
like is tuning fork or maybe a some kind of
00:03:01.439 --> 00:03:07.249
a guitar string is a pure tone its involve
only one frequency.
00:03:07.249 --> 00:03:13.709
There is second type of tone is called the
complex tone, where there are 2 or more than
00:03:13.709 --> 00:03:16.230
2 number of frequency mixed together.
00:03:16.230 --> 00:03:21.549
So, that is a speech that speech I am giving
to you or may be some kind of a music, any
00:03:21.549 --> 00:03:29.559
kind of noise in roadside and practically
in nature 99 percent of the sound is a complex
00:03:29.559 --> 00:03:36.969
tone or the mix frequency sound . So, when
this mixed frequency goes into that . So,
00:03:36.969 --> 00:03:39.370
we will have some kind of superimposition.
00:03:39.370 --> 00:03:51.090
The superimposition will go to in a way that
a sound wave is having suppose a simple motion
00:03:51.090 --> 00:03:58.849
like this, which can be talked as suppose
this is the first sound wave that is why I
00:03:58.849 --> 00:04:18.609
am writing y 1 is suppose sum amplitude 1
sin omega 1 t or this can be written as A
00:04:18.609 --> 00:04:27.010
1 sin 2 pi f t.
00:04:27.010 --> 00:04:36.690
This is another frequency
suppose something like this, which can be
00:04:36.690 --> 00:04:59.629
written in like this A 2 sin 2 pi f 2 t, where
your this this f 2 and f 1 f 1 are the 2 different
00:04:59.629 --> 00:05:09.110
frequencies .
Now, at a particular time t, this both the
00:05:09.110 --> 00:05:18.000
frequency if you add when the both the frequency
is actually merged together, we will get a
00:05:18.000 --> 00:05:29.539
different type of frequency and this 2 superimposition
in a particular time t gives you some different
00:05:29.539 --> 00:05:45.400
type of the the addition of this y 1 plus
y 2 will be equal to your A 1 sin 2 pi f 1
00:05:45.400 --> 00:05:53.610
t plus A 2 sin f 2 t.
00:05:53.610 --> 00:05:59.330
And because of the variation of this A 1 and
A 2 because of the variation of your f 1 and
00:05:59.330 --> 00:06:06.180
f 2 you will get different type of frequency
and different type of the amplitude in nature
00:06:06.180 --> 00:06:08.990
.
So, it is only 2 there maybe 3 there may be
00:06:08.990 --> 00:06:14.370
4 and if it is much more the things will be
very much complicated .
00:06:14.370 --> 00:06:17.090
So, let us take a complex tone.
00:06:17.090 --> 00:06:23.509
In the complex tone is a mixed frequencies
. So, I have taken a series of complex
00:06:23.509 --> 00:06:29.840
series of frequency, which gives me a complex
tone its starts from 50 then 75 like that
00:06:29.840 --> 00:06:37.229
. So, in this I have identified a first frequency
the minimum one the lowest one out of this
00:06:37.229 --> 00:06:44.610
particular bracket and this minimum one in
a particular complex tone, that minimum frequency
00:06:44.610 --> 00:06:49.480
or the lowest frequency is called the fundamental
tone.
00:06:49.480 --> 00:06:55.469
And the rest frequency which is the higher
than this particular fundamental tone or called
00:06:55.469 --> 00:07:01.240
over tone or the integral frequencies or the
partials there are lot of names.
00:07:01.240 --> 00:07:06.039
So, what tones probably the best fitted kind
of a thing .
00:07:06.039 --> 00:07:14.520
Now, in that over tone if I see there is a
good relation with the fundamental frequency
00:07:14.520 --> 00:07:21.199
there are some frequency may exist in this
overtone which may have a very good relation
00:07:21.199 --> 00:07:24.000
with the fundamental tone.
00:07:24.000 --> 00:07:32.850
Suppose this red color ones the 50 is a fundamental
tone, you see this 100, there is 250 also
00:07:32.850 --> 00:07:39.300
there is a 450 also and there is a 750 also
their the a numerical multipliers of the fundamental
00:07:39.300 --> 00:07:48.840
tones . So, 20 2 multiplied by 50 is your
100 something like the 9 multiplied of 50
00:07:48.840 --> 00:07:51.979
is your the 450.
00:07:51.979 --> 00:08:00.939
So, those red color frequencies in that complex
tone is called the harmonics . So, harmonics
00:08:00.939 --> 00:08:08.539
is a part of the partials or the part of the
over tone . So, I may say that all harmonics
00:08:08.539 --> 00:08:16.659
are overtones, but all over tones are not
harmonic .
00:08:16.659 --> 00:08:24.680
Then let us take this particular band of the
the the spectrum of the complex tone, where
00:08:24.680 --> 00:08:35.050
this 25; 25 is your the fundamental tone
and it has lot of I mean I mean in in fact,
00:08:35.050 --> 00:08:37.550
it is a series of it is a harmonic.
00:08:37.550 --> 00:08:43.400
So, it is all are harmonic, in every fair
frequency is the the multiplier the interior
00:08:43.400 --> 00:08:50.370
multiplier of the the first frequency the
lowest frequency or the the fundamental frequency,
00:08:50.370 --> 00:08:54.780
but again there is a good relation exist between
them.
00:08:54.780 --> 00:09:04.870
If you take 25, if you take 100, if you take
200, 400 and 800 they are definitely harmonics,
00:09:04.870 --> 00:09:11.560
they are definitely multiplier the the integer
multiplier, but they are double of other.
00:09:11.560 --> 00:09:20.940
I mean there is this one is also going to
be sorry .
00:09:20.940 --> 00:09:31.710
Doubled of other and this is called octave
. So, now let us go to the octave band central
00:09:31.710 --> 00:09:38.250
frequency, the problem with says that we
have lot of frequency to be handle from 20
00:09:38.250 --> 00:09:39.250
to 20000.
00:09:39.250 --> 00:09:48.880
So, out of that, this this 10 frequencies
are identified, which are a compute set of
00:09:48.880 --> 00:09:58.400
octave which covers the total set of audible
frequency its starts from 31.5, then double
00:09:58.400 --> 00:10:06.450
of that 30 double of 31.5 is 63, then almost
double of that is 125 and like that its goes
00:10:06.450 --> 00:10:11.230
up to 16000 .
So, this 10 frequencies are called octave
00:10:11.230 --> 00:10:18.570
band, and this octave band has taken as
the criteria for any kind of a testing
00:10:18.570 --> 00:10:24.810
or any kind of the statistical output to
generate for any kind of a noise or any kind
00:10:24.810 --> 00:10:31.760
of a sound atmosphere, to test or maybe making
some kind of a benchmarking or so.
00:10:31.760 --> 00:10:37.970
So, here if I take the logarithmic of that
particular octave frequency and multiplied
00:10:37.970 --> 00:10:38.970
that by 10.
00:10:38.970 --> 00:10:46.810
So, it is 10 of log of that f, f is that frequency
. So, something like 10 log 31.5 is almost
00:10:46.810 --> 00:10:47.870
going to be 15.
00:10:47.870 --> 00:10:55.680
Similarly 10 log 63 the next one is 18 . So,
this particular 10 log the frequency that
00:10:55.680 --> 00:10:57.360
called band number.
00:10:57.360 --> 00:11:05.610
So, these are the band number for each those
10 octave band central frequency and very
00:11:05.610 --> 00:11:09.350
interestingly, this band number is jumping
by 3.
00:11:09.350 --> 00:11:17.570
First it starts with 15, then it is 18, 21
24 like that where the frequency actually
00:11:17.570 --> 00:11:24.540
going to be doubled with another it is a gp
series, but this is a arithmetic progression
00:11:24.540 --> 00:11:25.540
series.
00:11:25.540 --> 00:11:34.230
Now this 31.5 or 63 whatever may be the frequency,
let us say that frequency is f and it has
00:11:34.230 --> 00:11:43.330
2 arm one is lower arm and one is the upper
arm . So, lower arm is f minus 0.3 times f
00:11:43.330 --> 00:11:50.390
are almost 0.7 f is the lower bandwidth limit,
and the upper bandwidth limit or the upper
00:11:50.390 --> 00:11:55.050
arm is something like f plus 0.4 times f which
is 1.4 f.
00:11:55.050 --> 00:12:05.320
So, if I go to the 31.5 frequency, by virtue
of this calculation I find 22 is the lower
00:12:05.320 --> 00:12:14.720
band lower limit of that band of 31.5
and 44 is the upper limit of the band of 31.5.
00:12:14.720 --> 00:12:19.260
Now let us see the second one second one 63.
00:12:19.260 --> 00:12:27.140
So, similarly if I put 63 and this f is now
twice of f, because it is in the in the in
00:12:27.140 --> 00:12:28.140
the octave.
00:12:28.140 --> 00:12:33.180
So, I will find this is point one fourth of
f this 2 are matches with each other and this
00:12:33.180 --> 00:12:43.890
is 0.28 2.8 f . So, now, if I multiply
with 0.7 into this 63, which comes from this
00:12:43.890 --> 00:12:54.970
particular here it is 44 and this is 88 .
So, the 44 and the 88 is the limit of lower
00:12:54.970 --> 00:13:05.490
and upper limit of 63, 44 and 22 are the limit
for 31.5 . So, like that each will have its
00:13:05.490 --> 00:13:13.270
lower and upper limit and once the frequency,
which is the earlier frequency the upper limit
00:13:13.270 --> 00:13:17.560
will be the equal to the lower limit of the
other next higher frequency.
00:13:17.560 --> 00:13:26.100
And by virtue of that I can rewrite this particular
band number and the bandwidth like 22 and
00:13:26.100 --> 00:13:34.230
44 is for 31.5, 44 and 88 is for 63; 88 and
175 is for 125 like that .
00:13:34.230 --> 00:13:41.950
So, all the spectrum and the the octave
band central frequency that is OBCF the band
00:13:41.950 --> 00:13:49.350
number and the band width are ready for you
. So, then if you see from the band width
00:13:49.350 --> 00:13:58.450
or the frequency from the 22 frequency 22400
which is in our audible range is covered by
00:13:58.450 --> 00:14:04.370
all those 10 OBCF.
00:14:04.370 --> 00:14:06.600
Now what is the necessity of the octave band?
00:14:06.600 --> 00:14:10.980
The necessity of the octave band is very simple
and straight forward.
00:14:10.980 --> 00:14:19.250
The first one the octave band gives a logical
set of some frequency out of many.
00:14:19.250 --> 00:14:25.670
Because I have problem I have problem, because
I have I have frequencies from 20 to 20000
00:14:25.670 --> 00:14:30.570
which is audible; audible by a human being
.
00:14:30.570 --> 00:14:38.010
So, from that if I want to select something,
select some may be 10 15, I need a certain
00:14:38.010 --> 00:14:43.140
amount of logical understanding and its gives
me that.
00:14:43.140 --> 00:14:49.630
Number 2 the logarithmic logarithm of the
octave frequencies is separated by the equal
00:14:49.630 --> 00:14:56.450
distance, because I have seen that the logarithmic
of one is separated by 3 to the next higher
00:14:56.450 --> 00:14:57.960
on the 3 to the next higher.
00:14:57.960 --> 00:15:04.970
So, while I will plot a graph, plot a graph
in the in in some to test some material
00:15:04.970 --> 00:15:14.010
or some kind of the the environmental acoustics,
I have only 10 such equal distribution line.
00:15:14.010 --> 00:15:22.430
If I have taken the all the 20 to 20000, then
the x axis of the particular frequency will
00:15:22.430 --> 00:15:24.740
be too high to handle.
00:15:24.740 --> 00:15:30.660
The third important thing is that the octave
band central frequency provide a common platform
00:15:30.660 --> 00:15:36.990
for any kind of material testing and assess
any kind of a acoustical data.
00:15:36.990 --> 00:15:46.391
So, worldwide globally it has been taken
as this octave band that 31.5 to 16000 are
00:15:46.391 --> 00:15:53.470
this 10 frequencies are set for any kind of
acoustical testing and any kind of material
00:15:53.470 --> 00:15:58.930
data handling and all the kind of the noise
and acoustical data purpose .
00:15:58.930 --> 00:16:09.190
Now, there is a one third octave band, people
are not very happy with this 10 frequency.
00:16:09.190 --> 00:16:14.320
Now they go much more better because there
are lot of parameters in the noise or maybe
00:16:14.320 --> 00:16:22.020
the sound transmission, through which can
be understood by this 10 frequencies, but
00:16:22.020 --> 00:16:28.060
we need may be some more set of frequency
to understand that particular phenomena better
00:16:28.060 --> 00:16:34.870
. So, that is why let us consider some
other set of frequency and let us make that
00:16:34.870 --> 00:16:43.060
particular set larger instead of 10 may be
more than that 20 25 and how to generate that
00:16:43.060 --> 00:16:47.400
one that is the very important and the the
logical way let us generate.
00:16:47.400 --> 00:16:56.590
So, as we know this 31.5 is octave band and
also the 63, whose band number is 15 and band
00:16:56.590 --> 00:16:57.760
number is 18.
00:16:57.760 --> 00:17:06.510
So, in between this 15 and 18 we have 2 more
number 17, and 16and 17 and like that.
00:17:06.510 --> 00:17:09.270
So, let us write all those band number.
00:17:09.270 --> 00:17:16.640
So, fill the gap or between the 15 and 18
by virtue of 16 and 17 and then do the
00:17:16.640 --> 00:17:24.980
antilogarithmic of that and divided by 10
and antilog of that and then if you get you
00:17:24.980 --> 00:17:33.890
will get the another set of frequency .
So, we will feel this particular the band
00:17:33.890 --> 00:17:42.420
number by 2 more digit and equally spaced,
and antilogarithmic of that will give you
00:17:42.420 --> 00:17:49.470
another set of frequency and that frequency
set is your octave band I am sorry the one
00:17:49.470 --> 00:17:50.840
third octave band.
00:17:50.840 --> 00:17:58.280
This one third octave band is filled by 2
more frequencies in between the octave frequency.
00:17:58.280 --> 00:18:05.800
So, see this 31.5 and 63, which are the octave
which is also called one by one octave band
00:18:05.800 --> 00:18:19.360
is field by another 2 frequency 40 and P in
between and make it as a one third octave
00:18:19.360 --> 00:18:23.940
band .
So, each of those 10 we will now produced
00:18:23.940 --> 00:18:30.930
like this ; so, this is red colors on the
one by one octave and this black colors of
00:18:30.930 --> 00:18:37.780
are the one third octave . So, as we know
the one, one by one octave is twice of other
00:18:37.780 --> 00:18:44.290
31.5 into 2 is your 63, but what is the relation
between them?
00:18:44.290 --> 00:18:52.620
If you just divide 31.5 the 25 40 with 31.5
will get almost like 1.66 one points I am
00:18:52.620 --> 00:19:03.550
sorry 1.26 1.25 like that and if you see generally,
this one third octave band is separated by
00:19:03.550 --> 00:19:11.110
the other with the cube root of 2, this
particular factor which is very near to 1.2599
00:19:11.110 --> 00:19:16.500
.
So, here in this table, we have this purple
00:19:16.500 --> 00:19:25.370
color the column which is the octave and
there there are the series of band number,
00:19:25.370 --> 00:19:32.110
the antilog of that band number and again
this with this last column gives you the
00:19:32.110 --> 00:19:37.800
one third octave .
And this is the till that we can go till the
00:19:37.800 --> 00:19:42.270
200000 as the one the the last one is the
one third octave .
00:19:42.270 --> 00:19:51.800
So, there are 30 such one third octave
band frequency starts from 25 to 20000 and
00:19:51.800 --> 00:20:01.180
out of that we have 10 number of this red
color one, which are the octave band which
00:20:01.180 --> 00:20:08.920
is the one by one octave band . So, 10 octave
band and 30 one third octave band ; So, most
00:20:08.920 --> 00:20:15.720
of the cases for testing or any kind of a
general purpose we take 10 octave band, but
00:20:15.720 --> 00:20:21.980
sometimes we need to do the critical analysis,
we need to do some kind of the the confirmatory
00:20:21.980 --> 00:20:27.110
kind of analysis for any kind of a material
testing or any kind of a acoustical surroundings
00:20:27.110 --> 00:20:33.330
to check whether it is perfect or not,
then we go to the this one third octave band
00:20:33.330 --> 00:20:36.110
and the 30 such frequencies.
00:20:36.110 --> 00:20:41.710
And this 30 frequencies or the 10 frequencies
are very much wide again.
00:20:41.710 --> 00:20:43.470
Again it is very much wide.
00:20:43.470 --> 00:20:51.300
It starts from 25 to 20000 and here it is
that is from 31.5 to 16000 . So, here the
00:20:51.300 --> 00:20:58.990
behavior of the sound also not perfectly synchronized
with the all frequencies .
00:20:58.990 --> 00:21:06.500
So, here for that we are categorized the frequencies
as a low medium or the high frequency.
00:21:06.500 --> 00:21:14.290
So, as you see this is 20 to the 20 16000,
where all the octave are separated by the
00:21:14.290 --> 00:21:21.520
equal distance, which is maybe the 3 by the
logarithmic in by virtual its logarithm,
00:21:21.520 --> 00:21:32.030
and if I draw to perpendicular line from 250
hertz and the 2000 hertz, this total spectrum
00:21:32.030 --> 00:21:35.740
is divided subdivided into 3 parts.
00:21:35.740 --> 00:21:43.170
At in this 3 parts the first part which is
called the lower frequency, which is 20 to
00:21:43.170 --> 00:21:50.500
250, the middle frequencies are 250 to to
20000 and the higher frequencies 20000 to
00:21:50.500 --> 00:21:57.790
that onward to till the 20000 22,000 to 20,000
.
00:21:57.790 --> 00:22:03.740
And the human can listen 20000 to 2000 frequencies
as you know, in the spectrum is mainly have
00:22:03.740 --> 00:22:08.481
to this 3 categories which I have just now
tell you like the low medium and the high
00:22:08.481 --> 00:22:13.080
frequency is also has the different name bass.
00:22:13.080 --> 00:22:19.820
The bass is the low frequency range of sound,
which is cover almost 20 to 20 250 hertz,
00:22:19.820 --> 00:22:27.090
which is very sound like a tabala or sound
like some kind of sound which is having some
00:22:27.090 --> 00:22:32.690
not a that much of intense speech kind of
the sensation .
00:22:32.690 --> 00:22:39.680
The middle frequencies are come within that
250 to 2 2000 hertz, which is very normal
00:22:39.680 --> 00:22:45.720
and which is very much widely used for any
kind of the music or any kind of speech.
00:22:45.720 --> 00:22:51.920
The treble sound which is called the high
frequency sound, which is more than 20000
00:22:51.920 --> 00:22:57.610
hertz also some part of the music and some
part of the human voice also comes into that
00:22:57.610 --> 00:23:05.600
particular particular the zone, the
treble zone where there are some kind of a
00:23:05.600 --> 00:23:11.320
machine some some kind of the the the equipment
gives some kind of a treble sound, which is
00:23:11.320 --> 00:23:16.660
very high frequency zone .
Next let us discuss the frequency range of
00:23:16.660 --> 00:23:18.290
the audible sound and all.
00:23:18.290 --> 00:23:24.650
In inaudible sound also in case of that also
we have have this the octave in separated
00:23:24.650 --> 00:23:32.300
by equal distance and this audible sound is
generally of something like this nature.
00:23:32.300 --> 00:23:38.550
The hearing range of the young person like
you people, are almost the full spectrum,
00:23:38.550 --> 00:23:41.420
but if you are olds somebody is old.
00:23:41.420 --> 00:23:47.140
So, then the spectrum of the hearing will
gradually spring down and its maybe limited
00:23:47.140 --> 00:23:51.540
to four the 4000 frequencies or so.
00:23:51.540 --> 00:23:53.860
Now let us discuss about the male and the
female voice.
00:23:53.860 --> 00:24:02.179
Male voice stars from little bit of bass towards
the for 4000 also female voice is separated
00:24:02.179 --> 00:24:03.179
a bit.
00:24:03.179 --> 00:24:09.620
There the the the female voice is bit of for
the treble kind of a thing, where it is 250
00:24:09.620 --> 00:24:14.040
to the 8000 is the range of the female
voice or so.
00:24:14.040 --> 00:24:19.940
Music next is the music, the musical instrument
again take a very wide range it is the bass
00:24:19.940 --> 00:24:27.150
music which is in this zone till 250 is
are so, there are mid frequency is also and
00:24:27.150 --> 00:24:33.010
there are very high frequency is also . So,
by virtue of that we can have differentiate
00:24:33.010 --> 00:24:39.270
frequency level, we can have different type
of the the quality of the music based on that
00:24:39.270 --> 00:24:47.600
particular thing and let us go to let us
go to the a excel chart and we will
00:24:47.600 --> 00:24:52.050
see how a superimposition can be done in a
excel chart .
00:24:52.050 --> 00:24:59.040
So, let us open this one . So, I have I have
3 frequencies can be mapped over here .
00:24:59.040 --> 00:25:08.090
So, in the first frequency if I say, suppose
this is some frequency like 25 and its gives
00:25:08.090 --> 00:25:12.260
me the the frequency 25 in pink color.
00:25:12.260 --> 00:25:18.780
And the second one suppose I give some frequency
let us suppose 69 . So, its give some frequency
00:25:18.780 --> 00:25:22.170
like 69 this is in green color second one.
00:25:22.170 --> 00:25:29.030
And the third one and this 2 the first
frequency 69 and the second frequency 25 you
00:25:29.030 --> 00:25:34.940
superimpose suppose and you will find out
this the this one, this one is the your the
00:25:34.940 --> 00:25:41.620
superimposed signs frequency of the song
are the propagation of the sound.
00:25:41.620 --> 00:25:50.929
Let us take the third frequency as some some
little higher frequency, suppose 1200 and
00:25:50.929 --> 00:25:57.470
then you will get the blue lines, which
is 1300 this one and the if you add this this
00:25:57.470 --> 00:26:01.870
and this .
This 3 wave front you will get a very complex
00:26:01.870 --> 00:26:02.870
kind of a thing.
00:26:02.870 --> 00:26:07.740
So, this is your complex frequency or the
complex tone and this is your one tone pure
00:26:07.740 --> 00:26:10.790
tone this is another pure tone this is another
pure tone.
00:26:10.790 --> 00:26:16.990
So, if those pure tones are in a in a some
order, suppose this is 25.
00:26:16.990 --> 00:26:24.361
So, suppose I made it like 50 and if this
is I made it like 75 . So, you will get some
00:26:24.361 --> 00:26:29.220
kind of a hm the rhythmic action rhythmic
kind of a thing in the harmonic.
00:26:29.220 --> 00:26:31.670
So, they are in harmonics, they are in harmonic.
00:26:31.670 --> 00:26:34.320
So, they get some kind of a rhythmic harmonic
.
00:26:34.320 --> 00:26:43.350
So, as an when you are the frequencies
in the complex tone are more in harmonics,
00:26:43.350 --> 00:26:49.050
you will find the the music or this particular
the sound is very interesting very interesting
00:26:49.050 --> 00:26:51.710
in character there are some ups and downs.
00:26:51.710 --> 00:26:55.130
Let us see if I have something in octave.
00:26:55.130 --> 00:26:58.760
So, suppose this is 2 200.
00:26:58.760 --> 00:27:11.410
So, the next in case of octave, it is twice
of that and this is . 800 and you see it is
00:27:11.410 --> 00:27:17.670
also gives you some kind of a beat some high
and low kind of the the propagation on the
00:27:17.670 --> 00:27:22.929
those kind of a thing .
So, we understand that how a tone and the
00:27:22.929 --> 00:27:32.809
3 such hm the 3 such tones pure tones can
be made into a complex tone in the in this
00:27:32.809 --> 00:27:38.710
by addition of this frequencies and in
the equation the frequency equation.
00:27:38.710 --> 00:27:50.809
So, again let us go to the power points light
and . And come to that the last slide .
00:27:50.809 --> 00:27:57.070
And as again it says kind of a homework
kind of a thing . So, the thing is that the
00:27:57.070 --> 00:28:05.190
can you hm can you the distinguish the
harmonics and octave from a given kind of
00:28:05.190 --> 00:28:12.679
a complex tone, let us suppose if I give you
a kind of a set frequencies like 23 36
00:28:12.679 --> 00:28:17.980
and all can you find out the octave
and the harmonics out of the complex tone
00:28:17.980 --> 00:28:19.480
it is very simple.
00:28:19.480 --> 00:28:27.330
Now next one probably you can just to
calculate, you can just calculate in a
00:28:27.330 --> 00:28:31.390
piece of paper .
Ah Suppose there are 2 different sound wave
00:28:31.390 --> 00:28:32.520
I have given to view.
00:28:32.520 --> 00:28:40.330
The sound wave one is having a amplitude of
50 mm and the frequency of 75 point I am
00:28:40.330 --> 00:28:50.780
sorry 79.577 hertz and the second wave
having a lower amplitude, but higher frequency
00:28:50.780 --> 00:28:59.140
318.39 hertz is the frequency .
So, they they are to typical fundamental
00:28:59.140 --> 00:29:06.860
or the typical 2 tone 2 2 pure tone, now can
you superimpose them and can you find out
00:29:06.860 --> 00:29:13.990
what could be the the particle displacement
the value of the y that is y 1 plus y 2.
00:29:13.990 --> 00:29:19.370
Y 1 comes from the sound wave and y 2 comes
from the sound 2 and can you find out the
00:29:19.370 --> 00:29:24.710
what should be the that y 1 plus y 2 that
is the particle displacement after a time
00:29:24.710 --> 00:29:31.630
interval of 0.25 try it .
Ah If you have any problem, you can contact
00:29:31.630 --> 00:29:40.750
for that, but try it try it by your own
and in the next lecture we will go to the
00:29:40.750 --> 00:29:43.450
we will go to the the the.
00:29:43.450 --> 00:29:50.550
Sound pressure level and the sound intensity
level, which is going to create a kind
00:29:50.550 --> 00:29:58.581
of the the initial benchmark scenario for
understanding the reverberation time and
00:29:58.581 --> 00:30:05.700
all other sound physics level, which is very
very important in the building acoustics
00:30:05.700 --> 00:30:06.700
so.
00:30:06.700 --> 00:30:07.850
Thank you very much .