Hello friends, welcome to this first lecture on Acoustics and Noise Control. The title of

the course has this two very important words acoustics and noise control. So,

this introductory lecture is about a brief description of these two words acoustics

and noise control. So, see what acoustic means. Acoustics is obviously as you may be aware is the

science of sound and sound in turn is the cause for which we have the perception of hearing. So,

it is a very important attribute, it is a very important sensory human sense that we are able

to hear different kinds of sound it has great implications in terms of our behavioral as well

as biological existence. So, that part of it is obviously, not engineering, but what is

engineering we will come to it in a moment time, but before we do that let us try to understand

what causes this perception of sound. So, the elementary physiology that we are going to stick

with in terms of that it is very clear that it is some kind of pressure fluctuations which hits your

eardrum. And these pressure fluctuations in turns excite a fluid which is there in your ears which

is called the cochlear fluid that gets into some excitation and those excitations are picked up by

the nervous system present in the auditory canals and it is interpreted as sound by the brains.

So, that complex physiology is something that possibly we are not going to do in this course,

but we are going to non and the same stick to this important idea that the perception

of hearing is because of the fact that there is a certain sound pressure which is fluctuating sound

pressure rather which is causing the eardrum to vibrate. The eardrum is just like a structure

just like the tabla or the drum it is a membrane if it is subjected to some fluctuating pressure

force. Please understand what the difference between fluctuating pressure force and a

standard atmospheric pressure. If it is a standard atmospheric pressure like 101 kilopascal pressure,

then you will not have any sensation of hearing because of that because it is

going to cause only a static deflection. So, let’s make this clearer. So, there are this

term of pressure fluctuation okay, so as the name suggest fluctuation means there is in time axis

if you had to plot the pressure there is a mean pressure and the mean pressure does not change

much in the time scales of our interest. So, let us say the standard atmospheric pressure

is 101 kilopascal okay at some conditions of temperature, humidity and so and so forth;

then usually this does not change in the scale of seconds or minutes or even in an hourly scale.

So, if there is a membrane, this is a membrane structure let’s say and if you have this

pressure acting on to the membrane structure then the resultant of that is going to be a

static deflection. So, the membrane structure is possibly going to deflect like this and that’s it

nothing else will happen. So, it is like as I sit on the chair the chair being spring like,

the chair sorts of deflects into a static deflection position and nothing else right.

But if instead of a standard atmospheric pressure on a static atmospheric pressure, we have a

fluctuating pressure something like this. We don’t quite declare what the type of fluctuations are,

but all that we say is over and above this mean level there is some fluctuating or oscillatory

components if you may like which is acting. So, on top of this what is now going to happen

is that there is going to be an oscillating component and I will show these oscillating

component in this fashion okay, because it is both upward and downward with reference

to a mean which is now not zero, but at 101 kilopascal. The mean has been shifted,

the mean is no longer 0 pascal or in term if you work in terms of gauge pressures,

you can still think that the gauge pressure is zero. But in terms of absolute atmosphere pressure

absolute pressure quantities, the quantity which is the mean pressure is at 101 pascal,

but over and above the 101 kilopascal there is a positive pressure and a negative pressure which

is what it means as a fluctuating pressure. To continue the analogy of me sitting in to this

chair if I just do not move around and nicely comfortably sit statically onto this chair,

I can understand the chair has deflected by a few millimeters probably because of my weight.

But then if I start sort of jumping around in this chair then I can feel that this chair is

also oscillating along with me, this is true for any structure not just for a chair. You can try

this in your car seat you can think about the car itself any structure when it is subjected to a

dynamic, this is what we call a dynamic forces. A static force is that which does not change

with time. So, you will note this down, static quantities are those which do not change with

time. You could have a static weight; you could have other forms of static dead load or so on. So,

static the effect of static load onto a structure causes static deflection.

However, when you have when I say dynamic quantity, I basically mean time varying

quantities. So, dynamic quantities basically are time varying that which is not constant with

time. So, in this course, we will be interested about dynamic quantities, time varying quantities

and as an example a dynamic load acting on a structure causes vibration of the structure.

How to calculate vibrating structure due to a given applied loading that is the subject matter

of theory of vibration, some of you may have had exposure to that, but even otherwise do not we do

not need too much of theory of vibration at least for this course. But we just need to appreciate

this fact that once you have a dynamic or a time varying load acting onto a structure, the

structure will vibrate. So, again as an example of this we have the physiology of acoustic.

So, let us take this to be our ear and internal to the ear there is a structure as I said which

is the eardrum okay. So, now, if I look at just the eardrum, this is being subjected to

some kind of atmospheric pressure that is for sure. But on top of the atmospheric pressure

the fact that I am talking to you it basically implies that on top of the atmospheric pressure

my voice is able to create some fluctuating components and these fluctuating components

are going to cause the ear membrane to oscillate. And these oscillations will be picked up by some

fluid which is called the cochlear fluid, and it goes in some spiral fashion. And from there

onwards using some complex nervous, I mean sensory pick up it is interpreted by the

brain as a voice or as the sound or whatever you think music or noise and so on and so forth.

So, the basic cause of our perception for hearing is the fact that our ear drums are vibrating and

why are the eardrums vibrating just like any structure would any structure would vibrate

because of the presence of oscillating forces acting onto the structure. So, eardrum is no

different mechanical structure than a plate or this chair on which I gave you the example. So,

accordingly the eardrum vibration is setup by fluctuating pressure components which are caused

because of the sound generation mechanism, this is what we will take up in more details as we go

along. But the moral of the story is this, the perception of sound is due to the oscillatory

pressure forces oscillatory pressures acting on the eardrum causing the eardrum to vibrate.

Again please note that the atmospheric pressure which is a static which is assumed to be static or

constant with time merely causes static deflection of the eardrum structure and is not responsible

for sound perception. So, this is the most important trigger as far as human perception of

sound is concerned. And we will take it up from here that there are some oscillating pressure

forces which we will try and understand how these oscillating pressure forces are generated in the

are taking birth or originating. And then another very important aspect is how they are getting

transmitted from the point of it generation to your eardrums, how is it getting transmitted.

So, accordingly we will have different topics to talk on, but the fundamental point which I

wanted to emphasize is that it is the pressure fluctuations which is acting on the eardrum

which is the basic trigger for the perception of our sense of hearing okay. Again another human

aspect of sound is that sounds could can be pleasing or it can be annoying; and obviously,

there are lots of human subjectivities involved in this criteria there is no hard and fast

rule that what appears as music to a certain individual can appear noisy to the other. So,

in general now sound is not subjective, sound is the perception of hearing, you have sound,

you have it fluctuating pressure, you will you are going to hear sound but whether

that is pleasing her whether that is annoying; obviously, depends upon the individual concern.

So, the discrimination between what is music or what is an intelligible speech in comparison to

what is not is or what is an annoying sound, annoying sound will be called noise. So, this

distinction is actually there are subjectivities involved, but in this course we are primarily

going to be concerned about machine noise. We are not going to worry about music or speech

because that’s basically outside the scope of our discussion here. Being mechanical engineers,

we are interested in machines we are also interested in machines which are quieter, and

I will emphasize why we need quieter machines in a moment time, but usually when it comes to machine

any sound that is emanated from the machine is basically not intelligible. So, it is of no use,

it is definitely not soothing. So, it is best avoided, it is annoying, but in certain instances

as again I will probably emphasize in certain instance, you may like to have a certain kind

of noise, but more often than not all machine noises are annoying and all machine noises in

terms of good machine design perspective. It is an objective that this noise is best got rid of.

So, from sound we come to noise and the objective in this course will be

to understand not only sound, but to be able to at least in some through some examples to be able

to appreciate how all this theory can help us design quieter machines. So, obviously, there

are a lot of machines that are there we won’t talk about all machines. But hopefully at the

back of our mind we will keep this in mind that how different machines make noise and what is it

that one can do to sort of contain this noise or bring this noise to the minimal possible levels.

So, generalize definition of acoustics could be stated as it is the branch of science which deals

with the study of sound. So, we are going to study sound and noise control in a more specialized

branch of acoustic which deals about how to apply this theory behind sound in order to design better

machines in terms of its acoustic performance, and there are good reasons why we should do so I will

have some motivation for you in the following slides. There are different areas within this

very broad field of acoustics. We could talk about how sound is generated in the first place and that

we will do in the part of this course. A more important factor is how sound is

transmitted from the point of generation to the point of reception. So, we could do something

more often than not again the generation is very difficult to tinker with and there are

engineering reasons which I will come to. But the transmission path from the point of generation

of the sound to the point of reception of the sound has lot of scope where in an engineer can

use his or her ingenuity in order to reduce the sound as perceived by the receiver. So,

the transmission path analysis turns out to be very important. Scattering well scattering is

once there is a sound getting transmitted any blockage that happens is called scattering. I

will just probably quickly explain to you these ideas through the writing.

So, generation is one aspect which happens at the source okay. So, for example, as I am talking the

sound is getting generated from my vocal cords, you may wish to understand how is my vocal cord

vibrating. And then how the air around the vocal cords is actually moving such that this pressure

fluctuation is created and then right from my mouth as you will know the mouth is going to

serve as a resonator for the acoustic. So, the sound that is getting generated from my vocal

cords is actually getting amplified at my mouth which is serving is an acoustic resonator; we will

discuss all these in details as we go along and from there on it is getting transmitted right.

So, you could have a source which could be let’s the speaker the speaker could be a human speaker

or it could be the instrument speaker that you are well of both of both the human speaker as well as

the instrument speaker work on the principle of vibration induced noise sorry vibration induced

sound okay. So, there is something which is vibrating in our vocal cords or even in

a speaker there is a membrane which is vibrating and once that vibration happens that sort of. So,

I will make myself a little more clear through this drawing. So, if you have a structure,

which is vibrating. So, let us say this structure vibrate within these envelope

it goes up and down up and down what will happen to the neighboring fluid particles.

Let say there are some fluid particles and I will indicate them in a different color. So,

these are the neighboring fluid particles just some random fluid particles I am drawing right.

So, as the structure is going to vibrate, it is going to throw out these neighboring fluid

particles and it is going to cause motion of the neighboring fluid particles. So,

once the neighboring fluid particle is set into motion or what happens to the fluid particles

which are faraway nothing is going to happen because these fluid particles don’t yet know that

there is a structure which has started vibration. The neighboring fluid particles will immediately

know because they are being pushed around and shafted around by the structure they will

immediately know that the structure is vibrating. So, they will sort of get cramped for space as a

result you will have a region of localized build-up of the pressure within this fluid

particle which is basically called a region of condensation that is you will have a region of

build-up of the local pressure and the local densities. So, this is the source of sound

right. And this is more categorical speaking this is the vibroacoustics source of sound speakers

being one very good example of that right, but there are other sources of sound also and I will

talk of them as about them as we go along. But then when you talk about transmission,

let us understand that you are a little further away from at least my vocal cord right. So, you

do not get to hear what you are exactly my vocal cord is producing, it gets amplified firstly, in

my throat because of the resonance effect which as I said will be detailed out in the later classes,

but what reaches you is the transmitted sound. So, these localized pressures will actually get

transmitted all the way up to the point where the reception is occurring. So, this part is

generation, whereas this part is transmission from the point of generation to the point of

reception whatever changes are taking place is studied under the heading of sound transmission.

Now, here one crucial assumption that we have taken is that in this world there is only the

vibrating structure and nothing else that is not true right, there are obstacles. So, now,

let’s complicate the picture a little bit and say that there is a certain obstacle here it could be

a wall, it could be a fat man standing between me and you, it could be anything right. So,

obviously, the transmission path will now get distorted it cannot have a there is no direct

path of transmission just like in the terms of optics, there is an obstacle between me and you,

you are not going to see me right, but that it is not so very easy in terms of sound. Even if I

play some obstacle and sort of hide myself behind that obstacle, my sound can still reach you. So,

there is a way in which the sound will now travel in some strange manner.

So, any distortion to the path to the direct path would be called as scattering. So, this effect is

that of scattering due to an obstacle okay. We will talk not of all kinds of scatters, but one

very important rather an easy obstacle to talk of is when this is a completely rigid barrier and

it is sort of reflects the sound back. So, even an ideal reflector in that sense is a scatter right,

but typically we will not get into the business of scattering because I don’t think we will have

time allotted to that much details. So, it is just for your awareness that what is the scattering.

So, then there are lots of issues associated with the human perception of sound. And you

know the evolution of the decibel scale for one thing is led from this trigger that it is

the human perception which matters. And once you get into this human perception issues,

you understand that you cannot quantify sound in the usual run of the main type of unit. So,

one needs to evolve the units which are decibel scale and you will have decibel a weighted scale,

b weighted scale and so on and so forth these are all related to human perception of sounds

because finally, what we are interested in is to understand how we can have less annoying

machines less noisy machines right. So, if you have a machine which makes less perceptible

noise then it is all the more better for us in terms of our machine design perspective.

We will also like to touch upon the topics of absorption. There are different materials which

are there which can absorb sound, and what is the basic underline philosophy of sound

absorbing materials if time permits we will try and touch on those things also. We will

talk about sound measurement techniques some of these will be these materials will be uploaded in

the appropriate web page for the course. We will just create some awareness about the different

computational modeling and simulation tools that are available for acoustic. This course is not

about computational acoustics in particular, so we will not talk about detailed formulation aspects

of these computational approaches, we will just create an awareness is to what are the ways and

means by which different computational modeling can be done, so that being slide one.

So, here I again start talking about the sources of sound some of which I

have actually demonstrated in notes that I was writing for you. So, the question was

what triggers perturbations in the ambient medium, what triggers this fluctuations or

perturbations if you may so. By perturbation, I simply means small fluctuations okay. So,

these small fluctuations are triggered by a source and as I said one way in which these perturbations

are these fluctuations could be triggered is the structural vibration. The structural vibration

is a very good example and all sources which creates sound because of vibration are called as

vibro-acoustic or structural acoustic sources. So, examples being the good old bell jar

experiment where well not the bell that we have in our door these days, but the traditional ringing

of a bell, if you say it is just a hammer which is impinging on a plate like structure circular

plate like structure. So, this finally, makes the plate vibrate. And once the plate vibrate,

the ambient air is thrown out is set into motion and that motion of that ambient fluid

particles is communicated from the point of generation to the point of reception,

so that’s how a ringing bell or a tuning fork or my voice or a guitar actually makes sound

okay. So, most musical instruments which are either of percussion type let’s say drums,

tabla all of them are vibro-acoustic instruments. On the other hand, if you have flute in flute

nothing vibrates actually no structural rather vibrates you could have a flute which is

made up of very very rigid structure yet I mean the vibration of the structure could be abysmally

minimal. So, there is nothing that vibrates, but yet you hear the nice sound of a flute,

so that is caused because of the flow right. Another very nice example of flow induced sound,

it’s probably you guys have experienced last month when there was cyclone Vardah striking us

then there was huge noise a shrill noise was like deafening noise was being carried through. So,

the motion of the air as it passes through different structure is able to cause noise also.

And the reason is quite simple because if you recollect the motion of the air

especially at high velocities is when it comes to talking about you know cyclonic weather.

When the wind was really blowing at nearly 100 kilometers per hour then you could expect that

the as the wind this airflow was happening through the buildings through the trees,

there were boundary layers that would be created. And since the velocity were very high,

the Reynolds number was very high and it would go to turbulence right the motion was turbulent. If

you recollect turbulence also has the same idea that there is a mean component and there is a

fluctuating component on top of a mean component. As you know in the turbulent flow example we

will have a mean quantity and associated with the mean quantity is also a fluctuating quantity. So,

what happens is that this fluctuating quantities are transmitted from its point of generation

to far away in points and that process is also process in which sound can get transmitted. So,

the point is this the source of noise this time is the turbulent fluctuations which

happens in the turbulent boundary layer, but then these turbulences induce the fluctuations

the fluctuations are eventually conveyed or transmitted through large distances in the form

of acoustic wave propagation that is the source how the turbulence induced noise can be generated.

This is an example of flow-induced sound. It is the same process if you have heard

engine noise let’s say in an air breathing engine which is basically your aeroplane

engine. Aeroplane engine also makes the sound because of exactly the same process;

it is a very very high velocity flow which is happening through an air breathing engine.

And once you have such a very high velocity flow, definitely you are inducing turbulence

at some point or the other. And once you induce turbulence the fluctuations are naturally there,

the fluctuation can be of a different kind between the flow induced noise and the structure induced

noise, but none the same they are fluctuations over the mean value. And once you set up these

fluctuations, you will be able to hear. You have a generation mechanism from the generation mechanism

there is a transmission mechanism which basically is the wave propagation idea and from there on, it

hits your eardrum and your eardrum perceives it as noise, so that is flow acoustics for you right.

So, flow acoustics is another very important branch of acoustics probably more important than

vibro-acoustics in certain applications such as aerospace whereas, vibro-acoustics is all

pervasive. Flow acoustics a lot of research work is being actively done to understand

different mechanisms of noise. Computational flow acoustics is really demanding, you have

to run your CFD simulations very intensively in order to predict the flow acoustics correctly, so

but none the same you should be aware that there are different mechanisms of generating sound.

The other mechanism is combustion-induced noise. So, even if you have observed cooking

right when you poured in your hot oil if you put your vegetables immediately you will hear a

crackling noise that is happening because of some chemical reaction that is happening between the

hot oil and the vegetables right. So, there is the primary factor there is a chemical reaction which

eventually sets of fluid motion that is true even if you are cooking vessel does not vibrate. The

cooking vessel if you have noted is fairly rigid as the cooking process is going on you hardly have

any perception of vibration of the vessel. So, it is not because of the vibration of the vessel that

the cooking is happening. So, this is something you would have definitely seen as you or your

mother probably was cooking in the kitchen. But this believe it or not the same thing is

happening when you are IC engine is undergoing a combustion process. Basically it is hot oil

which is getting spread within the combustion chamber and the same crackling sound is obviously

going to get generated due to the combustion process that is happening in the IC engine

also. The only point is that you don’t sit in inside an IC engine to hear that noise rather

you sit outside your car and that noise will actually if you hear carefully in the exhaust

side of the engine, you would be able to pick up some similarity between those sounds right. So,

this is what the primary driving factor in this case is the combustion process and the chemistry

associated with the chemical reaction of the combustion process which is the driving factors.

Eventually it will trigger of the fluid motion and the once the fluid is set into motion, it will get

conveyed from the point of generation to your ear drums, so that is combustion acoustics for you.

So, we are not going to talk much in details about combustion acoustics or flow acoustics;

we are going to remain fairly generalistic in our study of acoustics persevere. We not going

to specialize towards any of this topic but none the same you should be aware there are different

ways in which sound does get generate. So, from the generation process let us now talk

about the transmission process. After the sound gets generated at its source the question is how

does it reach the receiver. So, as will be shown in very much details is that the transmission from

the origin to the point of reception is through a form of waves. So, waves maybe as a child,

you would have played with different types of you know by throwing stones at pond, you would have

seen some waves. But we will formalize these arguments as to what really a wave is how do

you mathematically describe our wave and there are very nice interesting mathematical tools to study

waves, this is what we will do predominantly in this course. At least the first part of the course

we will try and bolster the foundations and try to understand how waves propagate in a given medium.

So, the reason why we will study waves is because we understand that the transmission of sound from

the origin to our eardrums or to the microphone for that matter is in the form of waves. And

as I said that you know in most cases as a noise control engineer, this is the place where you have

to target. Because again getting back to my IC engines example, the combustion process is sort of

fixed by the engineer who owns the engine in the sense that you know a certain combustion setting

maybe the best in terms of the fuel efficiency of the horsepower rating or the torque rating. So,

as a noise control engineer, you are really not expected to tinker with the engine because that

will offset the design objectives for the engine. But what we will show and in through this course

is that that you could really do a lot in containing that noise in the transmission

path between the source which is the engine and the human ear which is outside your car. So, the

transmission path is through the exhaust pipe. So, what we will do is we will put a silencer or

a muffler in that exhaust pipe and we will try to contain the noise as that the noise

doesn’t reach the outside let it get generated in the engine. You do not want to tinker with

that because that may sort of cut off the best efficiency of designs for the engine

in terms of fuel consumption horsepower rating, torque and so on and so forth.

You don’t want to tinker with these aspects rather you would be better off in controlling the sound

by putting the appropriate muffler within the transmission path between the point of generation

and the point of reception. So, therefore, and the reason why you will be able to design

a good muffler if you understand really how the transmission works right, transmission is in the

form of wave and just to jump the queue. If you can induce some reflections in this transmission

path and make sure that much of the wave actually reflects back to the source rather than get

transmitted from the source to the receiver, you will have a good muffler designs. So, based on

these aspects we will be able to enlighten you as to how muffler design can proceed.

So, therefore, we will do all that as we go along, but then muffler design proceeds on the

ideas of plane acoustic waves, but that is not the only kind of waves in which the acoustics

wave propagation does take place. We also have cylindrical waves and spherical waves. Cylindrical

waves you would have seen as you throw a stone on the pond, these waves are basically going

on the surface, they go out radically, these are cylindrical way, whereas in the three dimension,

you will be having spherical waves. So, some of these waves will be talking about plane waves and

spherical waves for sure, cylindrical waves could possibly be some assignment for you.

So, we will talk about different forms of wave propagation, plane wave be the easiest and at

least to visualize, and as I said there are interesting applications of plane waves. So,

we will dig a lot deeper in plane waves. We will also see how spherical waves can

be analyzed and we will actually show you that you know there are very interesting cases where

special cases where spherical waves can be approximated in terms of plane waves. So,

in that way the study of plane waves will be very rewarding. So, as I said when there is an

obstacle, sound will be partially reflected or transmitted. So, this is a very crucial idea.

When you don’t want the sound to reach to the human ear or to the point of reception, one idea

would be to make an obstacle which typically is called a noise barrier or what is more technically

called as inducer impedance mismatch, we will talk about all these terms as we go along.

But very loosely speaking this is an obstacle, if you put an obstacle then you are expecting that

the sound will not get transmitted, but rather will get reflected back. So, if you don’t want

to hear me one idea is you put certain kind of barrier just like if you do not want to see me

in front of you, you just put a barrier, I will not be visible to you. Similarly, it the optical

barrier will not work efficiently as an acoustic barrier, but if you know the transmission analysis

carefully and if you are aware of different issues associated with acoustic wave propagation, you

could possibly design a very nice sound barrier, which actually has minimal transmission of my

sound to you. So, how could you design all those things we could possibly think of as we go along.

So, one issue that is definitely there is all the medium will have certain losses.

Associated with the properties of medium other than the process of sound transmission,

there is also a process of sound absorption. Sound absorption happens due to two main features;

one is that of the viscosity of the medium, as you know the fluid viscosities play an important role

in the dynamics of the fluid, and the viscosities are usually treated as a sort of head loss in the

case of mean flow effects. Similarly, even in the case of acoustic propagation, the viscosities,

which are inherent within the fluid do play a role of energy loss mechanisms. So,

these essentially create an effect because of which the sound which gets propagated within the

medium is actually decaying as it is propagating. So, these phenomena is known as sound absorption.

Other than the fact that there is inherent viscosity in the fluid, there is also another

important features that of heat conduction. What happens as we will come to know is that when the

sound wave does travel within a fluid medium, then there are different zones having slightly

different temperatures. It is not a very great difference in temperature which you could possibly

measure with ordinary thermometer, but none the same there are differences in temperature between

different zones in which the sound transmission is taking place. For example, the compression and

refraction zone will have different temperatures. Now, once there is a temperature differential that

is set up within the medium as you know there are heat conductions, which will take place because

of the thermal conductivity of the medium. Now, once this heat conduction takes place,

this is also a sort of energy loss as far as the medium is concerned. So, for this dual effect of

thermal conduction plane within the medium as well as the viscosity effects of the medium,

there is essentially the sound that is getting transmitted through the medium also gets partially

absorbed. And therefore, it slowly decays as it gets transmitted within the media. So, this

phenomena is called sound absorption. Sound does not travel beyond a certain distance is because

the sound essentially gets attenuated, so that is because the medium will have certain losses. So,

all that is getting generated doesn’t reach the point of reception because there is definitely

a natural attenuation to associated with the medium. So, one aspect is that the medium is

inherently having some absorption characteristics, but at times that is not sufficient. You want

better absorption characteristics because you don’t want the noise to reach the point

of reception of or the associated human ear. So, therefore, you would like to design absorbers

which are better than what is naturally inherent in the air. So, therefore, those are called

acoustic absorbing material. So, we could possibly I mean again depending upon how the course goes,

we could branch out to understand how different acoustic absorber materials can be designed such

that we have a better acoustic absorption. So, these are artificial materials which it’s not

like air or water in the sense that this air or water comes with its inherent absorption

characteristics which actually may not suffice for our application needs. We will have acoustic

absorbers usually made of fibrous materials. And once the sound is forced to pass through

this fibrous materials, we will be able to get a lot of absorption out of this material okay,

so that will again be something which will be very useful in terms of our objectives.

There are different fields associated with an acoustic, we will be interested mostly

in linear acoustic because the sound which is not too intense will fall within this domain. However,

if you have very intense sound as a rising in aerospace applications during the launch vehicle,

take off, these intense sound which are like more than 100 dB or so should be analyzed

as non-linear acoustic phenomena; however, this course is only about linear acoustics.

As I said in terms of generation of sound, we have vibro-acoustics, we have aeroacoustics.

We also have a very important field which is underwater acoustics is very important for

navel applications something that navy people are extremely you know for this topic is very

important for design of stealth applications and underwater wall fair devices and so on

and so forth. But we will predominantly not go in the direction of underwater acoustics, but

this is a very important area which is a little advanced from this first course on acoustics.

The methodologies that could be adopted in the different solution processes could be

analytical methods, it could be computational methods, it could be experimental method. So,

my objective in this course would be to give you a brief overview of the computational and the

experimental methods as well. We are predominantly going to fiddle around with some analytical tools,

but understandably the analytical tools are very important to understand the preliminary and the

fundamental concept. The computational tools are important in having an accurate estimate for a

specific application, but the interpretation of the results will be done properly only

if you have a good analytical background. If you have learnt the analytical background

by which you have the interpretation at least for you know simple cases such as 1 D application,

then you should be able to have a very good interpretation of your computational

results also. Analytical results probably cannot be used to accurately predict us

or accidentally estimate certain quantity of interest in three dimension. For that,

we have to resort to computational methods, but the bit fall there is that if you only trust the

computer and lose the insight then it will do more damages than help you solve the problem.

So, therefore, it is very imperative as students who should learn the analytical methods correctly,

computational tools are there. And learning a computational tool could not would not be

that difficult especially most of the commercial packages provide a very good help document using

the help document you could learn your way how to work with those commercial softwares.

But then the interpretation of the result will be based upon how well your theoretical

foundations are how well you have learnt the basic theory for the learning the basics theory

analytical acoustics is very important. And this course will be predominantly focusing on

the analytical techniques certain awareness will be given about computational techniques also.

And experimental acoustics we will talk about the important instruments associated with the

measurement of sound and other associated things some of these experimental techniques

could also be part of your self-study which will be posted in the appropriate webpage.

There are experiments available and I will post them in the course webpage as we go along. So,

predominantly we are going to consult about noise control aspects. The other

important areas could be physical acoustics. In physical acoustics, people are interested to know

different physical processes associated with the acoustic phenomena. For example, in ultrasonics,

we have a completely different physics associated with these wave propagation phenomena as they

happen within elastic waves and so on. Similarly, bioacoustics is concerned about

the biological aspects of acoustics. We are not going to touch upon these topics in this course.

Physiological acoustics and psychoacoustics are again interesting areas in their own

right. There is lots of active research as to understand how exactly we perceive sound what

I told you in the first part of this lecture today is that a basic sketching ideas as to

how this thing is perceived. But even to this day there are lots of open question that how

exactly is the sound perception happening. And what are the physiological ill-effects of over

exposure to noise, what are the psychological ill-effects of over exposure to noise.

So, lots of experiments and lots of data has been gathered over the years which seem to points that

over exposure to noise has its ill-effects in terms of both physiology as well as psychology.

So, but still a lot of active research is going on in this direction. If you pick up any journal

on acoustics, you will see sections devoted to physiological acoustics and psychoacoustics,

but again in this course we are not going to dwell on these topics any further.

Another very active area of research is how to design new acoustic transducer what is

the associated signal processing and so on and so forth. So, there are lots of companies which are

actively involved in making the different acoustic transducers and the associated

acquisition system and the signal processing. So, this is a fairly active area of research. So,

with that brief exposure, I will stop for the day here we will pick it up from here

in the next class. Thank you.