So, good morning this is the the lecture number 37, in our the Architectural Acoustics
NPTEL course. Welcome to the lec, welcome to this lecture we will discuss about the
Environmental Acoustics also in this lecture . As, the lecture topic says this is environment
ac environmental acoustics 2 . So, if you remember in the previous lecture,
you have just the figure out and the the outline the particular the the the acoustical phenomena's,
which actually import are the sources which actually import the the noise and the noise
pollution in the environment. So, in this particular lecture also we will
go into bit of detail and also we will try to do some kind of the the indexes for the
noise exposure and all. So, that will be demonstrated in this lecture as a lecture objective or
the learning objective. And, also we will categorize the ambient noise the what are
the different types and how it will be actually functional in those the type of ambient
noise. So, if I go with this ambient noise this
is defined as this means that this kind of a background noise, which sustain for sometimes
or maybe a longer duration the time is also very important of course, and it is presence
in a specific area and it is kind of a geographical area or maybe sometimes, it is also specified
in a in a in a in and around surrounding a building built up environment,
or it may be a community, or it may be sometimes industry, or it may be a kind of a suppose
if you take a the shopping area, or may be a bazar or kind of a market area those
are another area. So, what is the ambient noise level of
that area? So, that is the the particular geographical conditions, which may or may
not have that kind of the the built environment, but it is may should have some kind of background
noise. Presence and duration is also important maybe a for a smaller duration of the time
and for it is sometimes for a whole day or whole night also.
So, those are the the typical the definitions are the which you can start with amb[ient]-
ambient noise, but ambient noise if you just go or if you just go or if you just
think or if you just remember. This ambient noise is are time is specific and also
the nature specific; that means, it has 3 types as such; one is called a continuous
nice ambient noise and one is intermediate intermittent noise which is not continuous.
But, but tend to be a continuity having kind of a continuity. And, the third one is
not at all continuous, which is impulse if kind of a noise which is a noise it is comes
sometimes in some uncertain way or sometimes, it may not come may be in a day sometimes.
So, it is with that particular definitions and with that particular ambient noise
ambient noise a classification. Let us discuss this 3 type of the noise ambient
noise. First one is continuous as you know continuous meaning is without interruption.
So, this noise comes almost in a mode same mode of frequencies or same group of frequencies.
And, it is actually an interrupted from the timeline point of view an in interrupted.
And, also almost similar amount of the sound pressure level with exist. So, almost
everything is similar the the time specifics it is not time specific, it is a continuous,
it is a not specific frequency specific, it is a kind of a band of frequency which
go for a longer deviation of the time. So, you required some kind of a manual equipment
to for some supposing if you just go for a testing for those particular or the recording
those particular noise and try to measure that particular noise. So, if you just spend
some minutes of time and find out what is the the the the total level of the sound.
So, it is look like this it is very steady kind of a the SPL level and if you
just frequency if you analyze also it will be some kind of a steady.
So, the examples are some kind of a blower, or some kind of a pump, you know the
same kind of the mixed frequency as comes in a specific longer duration of the time
. Intermittent Noise; Intermittent noise is
that which is tend to be continuous it is a kind of a continuity, but there is some
the gaps or some drops. So, it is operates for a some certain time which in that there
is a plateau kind of a thing and then it is drops and then again it is goes like that.
So, to measure this particular sound you required a longer duration little bit longer
duration the time and for the those specific 2 3 hours' time, it is of course, depend
upon the the source, my specific also and you take the particular duration for
the measurement for those duration longer duration and then analyze that one, it is
look like this. It is a some steady portion sometime this minimum particular time, it
exit and there is a drop and again this goes. So, you need at least some longer duration
measurement and then you have to find out. Now, what are the if you take a very small
duration measurement suppose your measurement duration is here, you cannot actually sense
the actual measure or if you can actually do not go to do not we will not get the
actual measurement also. So, it actually see the longer duration measurement. Periodic
running of machines, specific traffic rush hour is another suppose it is morning some
8 to 10 30 or 11, and this is a evening time and in the afternoon time it is little
bit low because of there is no traffic in the road. Some pass train movement or
those kind of a thing, and of course, the landing and the takeoff are some kind of a
aircraft, which is also um that kind of a specific .
Last one is impulsive noise. Impulsive noise is a kind of a noise which
operate for a few second or may be a very few second and it is very much uncertain when
actually it will be going to occur unexpected . So, it is monitoring also very tough,
because suppose for a full day there is no such noise, but there may be a day where there
are 3 or 4 such noise going to be a may be happened for that particular area .
So, you manual operations manual recording is impossible So, you required a kind
of a data logger with equip data logger digital recording machines and data logger which can
take for longer period of data. And, after you receive those data and you analyze
then what is the peak values also it sometimes look like this. So, there is a very few second
there is a this huge or the high amount of noise and, for a longer duration there
is no and the occurrence of base is also un expected .
So, suppose there is blasting of mine in the mine, there are blasting some punching machine
operation gunshot those kind of a thing or I may say suppose a the the um the very
high speed bike that moves in a particular road gives a very tremendous amount of sound,
but it does not move sports bike are those kind of a thing is may not move very
frequently, it can be it may be a example of impulsive noise also.
So, as we understand from the the earlier 3 slides, it is not only the the amount of
noise that is a SPL of the noise, it is not only the the frequency of the noise. And also
the the character of the noise and the source and all those kind of a thing, it is one of
the important the the character of the urban noise or the environmental noise is that is
exposed time the it is very much time dependent. So, suppose a very high level noise of
suppose 90 dB also, it is act for a very few second it may not give you a that much amount
of impact in your brain, but suppose a little bit of lower decibel level in 65 decibel level
sound, stay for a continuous period continuity . So, that particular noise may be for a
half an hour also that particular we are exposed to a particular that particular environment
65 dB noise also may give you a very much problematic from the point of view of
the the health also. So, the noise exposure how much time you are
exposed to noise is also important. So, what we have seen in the earlier lecture is that,
I am going to find out the average and the total amount of noise and the evaluated noise
also. Which, now in this particular exposure point
of view has to be taken in consideration of the in the time so, what frequency? And, which
amount of the decibel level stays for how much amount of time? So, time factor has to
be also embedded or the in builted with this particular the the the index system and find
out the what is the total noise exposed. So, I have told you if this is the character
characteristics of the the SPL and the noise some is stay for a longer duration some is
not stay for a longer duration, but the total SPL level is very much peak very much peak
is very very high. So, maybe this is problematic this may not be not this may not be So,
now how I can actually now tell of the SPL and the time, this is a fusion between the
SPL and the time is required for this kind of the noise exposure. .
So, what we have can do is that we can actually find out the continuous sound level, which
is equivalent continuous sound level. A noise may be a bip and again after some time there
is a bip and again there is a another bip. So, each high level noise stays for how much
and then let us convert that to equivalent continuous sound level, it has been done .
This various specific sound level and the specific time duration point of view. And,
this formula has been given like this is the E L equivalent, which is a time dependent
phenomena is T 1 into 10 to the power L 1 by 10 this 10 to the power L 1 by 10 we have
already experience in the last lecture. And, that has been multiplied by t 1 what is t
1 the t 1 is exposure time of the L 1 sound. So, some source of sound having a L 1 SPL
level, stays for t 1 hours. So, this will be the cumulative effect exposure effect of
that particular sound. This is the cumulative exposure effect of the second source of the
sound, third source of the sound; like that you add it up. After, you add it up log make
the logarithmic of that big number multiply by the 10.
Very usually what we did in the log the this the totaling of the particular noise
level. And finally, after multiplying by this by 10 divide this one happen capital T. And
this capital T is nothing, but the hours' time this is equal to this exposure time the
arithmetic some of the exposure time. So, I have some I have suppose I have suppose
measured some 75 minutes noise sample in a any locations may be industry . So, it has
60 dB and that stay for 30 minutes, this 30 minute may not be continuous 30 minutes, it
may be a 2 minutes and then there are again some gap of then there are some 70 dB or
70 dB for 2 minutes and then 85 dB for 1 minute. And, then again 60 dB for 5 minutes something
like that ups and downs, but totally this 30 60 dB is stay for 30 minutes. And, total
amount of 70 dB sound stays for 20 minutes 85 dB s for 15 and this 90 90 dB is for 10
minutes. So, by I need to find out the, what is the
L average and the L equivalent? . The L average I know I have already done
this in the earlier lecture . So, all this by 10 and then logarithmic sorry 10 to the
power of that big numbers totaling of that big number is a huge number, and this huge
number is divided by 4 because there are 4 the data set or maybe the 4 observations.
And, I am getting around 85.2 dB is the average sound level of that particular situation
which, but I have not taken any any exposure time into account.
So, next what I will do I will go for the l equivalent, which is I am taking the exposure
time into account. So, this particular the that particular table is expanded.
Now, t are also mentioned over here in minutes and t is also mentioned in hours and this
t in hours is multiplied with the respective 10 to the power L. And, then this number are
created and again you have to add it just like this you have to add this t 1 multiplied
by 10 to the power L 1 by 10 this number. So, there is a huge number and this has to
be next is the logarithmic of that into 10 is 83.97 and that has to be divided by capital
T. The capital T is nothing , but this plus this plus this plus this which is 45 minute
I am sorry 75 minute and the 75 minute is nothing, but have to be converted into
the hour which is 1.25 hour. So, finally, this is the result this L
equivalent is 67.1 decibel; that means, if you are exposed to 60 dB with sound level
for 30 minutes 70 dB for 20 minutes, 85 for 15 minutes, 90 for 10 minutes is almost like
you are exposed to that 75 minutes in a sound environment equivalent to 67.1 dB . So,
the noise exposure has been taken into account.
Then, there is a another way people has looking to it is called as a L n concept that
is L n is the 3 L s the levels. And, this is how much noise is the above of that
particular level? So, suppose if I say L is 30 75 dB A, this evaluated scale point of
view . That means, the 75 dB represent this L 30 equal to 75 dB A represent that, the
30 percent of the measurement time 75 dB the noise was more than exceeded the 75 dB.
So, similarly there are 3 such levels has been considered the L 10 amount of sound dB
in dB a that exceeded 10 percent of the sound 10 percent of the time . And L 90 is the 90
percent of the time, how much sound is exceeded of that exceeded of that particular
level more than that level. And, L 50 is the averages this is the the the the median point
of view, the 50 percent of the time how much is the more than that particular level.
So, let us I have 20 noise sample data I have obtained suppose for my bus busy road station
junction, road junction and those are the data number 1 2 3 4 to 20 and those are the
SPL level or of those particular, and I want to find out this 3 levels L 10 L 50 and L
90. So, what I did I did a bar diagram? . So,
those 20 data and this SPL. So, this is first one is 43 81 25 66 like that.
So, some 4 the very high, some of the very low, some of the in the medium category medium
range. So, so, I plotted those particular thing and
I know that this is through I have 20 data. So, 10 percent of the 20 data is 2 and so,
this to the highest 2 or 81 and 83. So, the 10 L 10 is this line, because everything
is more than 81, I mean 10 percent of the time is the sound level of that particular
data is more than 81 decibel. Very similarly 90 percent of those 20 is 18. So, if I draw
a line over here this 2 19 and 17 are below. So, those 18 are above this 99.
So, I can say this L 19 is 21 decibel, because 90 percent of the all the data or the all
the time I mean involve the this things the exposure or more than 21 .
Similarly, I have drawn another line over here 50, because 50 percent of 20 is 10. So,
if you see above this particular line there are 10 dots and below this particular line
is 10 dots. So, I can say the L 50 is 38, because 50 percent of the time the noise is
more than 38 38 are more than that. So, L 50 is 38 L 90 is 21, because 90 percent of
the time is is either 21 or more and more than that and L 10 is 81.
Because, the the 10 percent of the time the total of the total the it is more than
81 81 or more than 81 so, 81 83 or above that . So, how can you easily do that, if you have
this data in sure that data in the descending order. So, from the highest to lowest.
So, you know this is the the sample is 10 sorry 20 sample. So, this 2 so, this
81 will be the take to a part. So, 81 will be your L 10 this 38, because this is the
total 10 total 10 numbers dots above that 38 is the L 50. And, this 2 or bellows
21 22 or below. So, this 21 is this one, I think this line has to be shifted little
bit here. So, this it is more than 21 is more than that is the 90 percent of the level also.
So, from this point of view the traffic noise index is has been derived, which is given
as the T N I traffic noise index is this this formula 4 times L, 10 and L minus L 90, what
is this L 10 and L 90 the the upper limit and the lower limit. This is the between this
particular upper limit and the lower limit, this range is gives you the range between
the L 10 and the L 90. The fluctuating noise between these 2 particular point plus
L 90 minus 30 and this total has to be expressed in dB a view why it is L 90 minus L 30?
Because, L 90 what L 90 is not the upper limit, L 90 is the lower limit. So, L 90 is referred
as the represent the the background noise. So; that means, the L 90 is the lower 10 percent
and the all 90 percent of those are above that. So, we can say that this particular
noise level always going to be there. So, that is the background noise .
So, this is the second part of this equation confirm the background plus. The first part
of the equation confirms the range and that is give the T N I index of a traffic noise
index. Very similarly, you also there is in index
called the noise climate. It is the relation is is difference between this is range in
L 10 by the minus L 90. So, if there is a kind of a time line is SPL given for a particular
the noise source or may be a mix noise source . So, you have to find out what is
the L 10 and what is the L 90 and the difference between them L 10 minus L 90 is your the noise
climate . So, you can actually understand what is the
the the the you ta you remove the the top 10 percent, you remove the bottom 10 percent.
So, the middle 80 percent is within what zone it is gives you kind of a indication?
What is the fluctuation of the the noise climate or the the noise you are expose too or a particular
area is expose too. Then, next is the noise dosing this is also
very important sometimes it is very important from the industrial noise point of view.
It is says that it is as it is the the the the the it is the defined as are
the noise dose is how much noise and how much duration, what is the time duration of that
noise, both are taken together for a particular the level.
So, what we have seen in the equivalent L equivalent. It is the totaling it is the totaling
of if you have a n number of SPL level and dose n number of SPL level have t 1 t
2 t 3 t n. Then, it is gives you a kind of a average scenario what should be the actual
equivalent level of the noise for dose timeline , but here it is not for the different
noise level, it is for a particular single noise level and single noise level is exposed
for that particular time. So, it is particularly very much important
for industry industrial noise calculations also. It is what you have given here is that
or what is the benchmark is that? A time weighted noise level for 85 dB A for 8 hours period
is taken as a 100 percent dose . That means, if I am exposed to 85 dB a sound for 8 hours
morning 8 o clock to evening 4 o clock 8 hours, I mean exposed to that particular noise
85 dB noise. And; that means, that particular total amount of the noise exposure to me
is almost over 100 d 100 percent. And, suppose next day I am exposed to 90
dB or may be 95 dB noise, but for only for 1 hour. So, definitely what will be the corresponding
change in the noise dozing, it will not be definitely going to be 100 percent, it
may be less or may be more because of the ninety dB is more and time then again the
time duration is less t. So, how that it can be calculated it can be
calculated by the simple equation 100 into t by 8 the t has to be mention in hours. And,
this is L equivalent of that particular noise level minus 85 divided by 10. And, that will
give you a percentage dozing suppose if you just put I mean, I think, I have it in
the yeah I have a it in this particular format I mean this particular slide.
Suppose, I have SPL levels 79 82 98 like that and all are having suppose expose time of
8 hours duration . So, the suppose a case of suppose I have taken 85 and 8 hours . So,
I put T capital T as 8 and this t if you just go through the earlier slide is T and
this L E q has to be changed. So, here this is 85 minus 85 by 10 is 0 . So, 10 to the
power 0 is 1 and this 8 by 8 is also 1. So, it is 100 and so, that is 85 dB and for
a time of 8 hours is equivalent to 100 percent of the dosing.
So, this 100 and this 100 and that particular point in this graph is 100 85 of 8 hours and
this is 100. And, next is suppose I have exposed to a 91 dB sound and also I am expose for
8 hours for 91 dB next day. So, what is my equivalence equivalent dosing the the on
that particular 85 100 percent dosing. So, I as put 91 over here and minus 85, which
gives me 10 to the power 0.6, because 91 minus 85 this 6 and this 8 by 8 remain 1 and this
100. So, it is gives me almost 300 and 98 almost 400 almost 400.
So, if I expose to 91 dB sound for 8 hours. My noise dozing is almost 4 fold 4 times . So,
this is over here 400 . So, this graph shows a from a 79 to 98, I have drawn this blue
colour and shoot up it is this the fold is shoot up in it is a logarithmic increasing
kind of a scale it is exponential kind of a scale . So, even that gives a particular
understanding or that a high level noise. And, for a minimum time or a
small time is almost equivalent to a low level noise of a high amount of time exposure.
Next, what I did is that I have kept this SPL level same so, just reverse of this table
where this SPL changes time has been kept. So, equal here this fill is kept 85, it
iself and time is now I am varying from 16 hours to almost 1 hour. And, as you know
the 85 with 8 hours is 100, which is the correspondingly is the the benchmark.
So, let us calculate if it is 85 dB and for 4 hours. So, then this T become 4 and this
85 minus 85 this will be 0 10 to the power 0 is 1 this is 0.5 so, 50. So, it is from
8 hours to 4 hours is drastically dropped by half and if you see it is T is a linear,
if T is does not come on the 10 to the power. So, we will get a very linear graph very linear
graph if it is 16 hours, which is twice of the 8 hours your noise dozing is twice of
100 that is 200. And, if it is 1 hour that is the 1 8th of your duration of 8 hour, which
is 1 8th of 100, which is 11 12.5 I have just make it as 13 12 .5.
So, it is a very linear kind of a graph we will get . So, that is a noise dose which
is 1 of the another index noise index . The the noise dose is now let us have a
noise dosing of 2 different noise having 2 different time. Suppose L equivalent
if it is for first 4 hours is I am exposed to 80 dB sound and the last 4 hour I am exposed
to 90 dB sound. So, with this I can find out this first 4
hours of 80 dB and then 90 dB and then I can calculate this is all 0.5 this is 10 to
the power minus 0.5 and 10 to the power plus 0.5. And finally, I can find out this noise
dosing is 100 and 74 100 and 7 4 that is the dosing
So, ambient aircraft noise is also one of the issue nowadays and this particular
noise is monitored by the civil the the the airport authority of India. And those noise
can be carried out where this particular airport is as per hour C P C V at 5 50,000 aircraft
per an year. And, that particular aircra air strip or may be this airport has
to be taken under consideration . And, for that a D N L that is the the day
night average sound level has to be considered for that particular thing, where the L equivalent
has to be measured for the 24 hours period with the 10 dB penalty for the night time
I will explain that in the next slide . So, this time duration night time duration
is the 10 PM to I mean the this 6 AM this night.
So, this D N L total is given by this formula, where this equivalent noise level for the
day, which is this the equivalent a weighted sound level for 1 second in a day equivalent
for 1 second in a day the impact is of 1 seconds, because in a day for 6 A M to 10 P M in dB
. So, it is taken care of the impact of the sound impact of the time and also the E
equivalent of the night is also taken care of and that is also taken care of and that
time duration is 10 PM to 6 AM. Because, as you know the airport which airport
are actually taking care of the 50,000 aircraft passenger aircraft per day is definitely
the whole day, and night there are some takeoff and the the the um landing is operation
is there. And, as it is for the night it has to be taken a penalty I mean we have to take
a 10 dB plus with this particular number, which is actual number, which is actual
the measurement from this airport plus 10 the 10 dB penalty applied for the night time
only this no penalty for the daytime. And, this total period of the time in consideration
in second that how much second we have consider for this particular time data, that
has to be taken into accounts L equivalent for how much period of the the time duration
has to be taken care of. So, at this end of this particular lecture
number 2 of our environmental acoustics again I have some homework for you this some
theoretical questions, that is what is the differentiate with the example the various
forms of ambient noise with example. And, the second one is that the estimate the
noise climate and the traffic noise index that is T N I with this following data
if the L 10 is 80 L 50 is 60 and L 90 is 40 Db. Yeah.
So, find out the what is the noise climate and what is the traffic noise index? for this
particular lecture also I have the consulted the similar books.
Ah. With some publications available in the net from the central pollution control
board New Delhi and our national building code. And, those one books of the acoustics
in built environment. So, thank you for joining in this lecture lecture number 37 ends here.
Thank you.