 When we started this course, we had made three broad classifications in acoustics. Physical acoustics, which was essentially about how sound propagates in air or in medium in general from point A to point B. Then we had also talked about electro acoustics that was another big category, which concerns itself with transformation of sound into other forms of energy or other forms of energy into sound. Another third big area was psycho acoustics, how the mind, the human brain interprets sound. Then that was the introduction, then we talked in detail about or somewhat in detail about physical acoustics. We went through this plane wave equation, one dimensional equation, one dimensional equation in Cartesian and spherical coordinates, interference of wave, stuff related to physical acoustics. Then we moved into the area of electro acoustics and primarily in area of electro acoustics, what we have talked so far is how is sound generated in engineered systems. In natural systems, sound gets generated. We have not talked about that, but in engineered systems, systems with human brain devices and designs, how is sound generated. So, we have talked about that. Today we will still talk about electro acoustics, but we will focus more on how is sound received or recorded, because that is important to understand from an engineering standpoint for two reasons. If I have the sound of quote and quote of a different of a specific type and we will figure, we will explain what this type means, then what kind of microphones should I use, that understanding has to be there. The second thing is which microphones are good or not so good in a particular application, we have to understand that. So, today we will primarily talk about microphones, we will not talk so much in terms of how microphones can be designed, but how we can intelligently use microphones which meet our needs. So, in that context, we will talk a little bit about the design at a little detail level, but not, we will not go too deep into the detail and then we will also try to map different types of microphones into different types of applications. Microphones essentially measure pressure or in some cases, they measure pressure gradients, in general these are two big broad categories of microphones, pressure microphones and then pressure gradient microphones. And then there is a third category which are hybrid which work on the principle of a combination of both of these, pressure and pressure gradient. So, we will start with pressure microphones, we will start with pressure microphones and within pressure microphones, there are different types or categories of microphones, there could be electro dynamic microphones, there could be condenser or capacitor microphones, there could be piezoelectric microphones, there could be carbon microphones, we will very briefly capture all of these, but let us start understanding how a pressure microphone works. Essentially, what a pressure microphone is, is that I have a casing and then I have a diaphragm which is attached to a suspension system. So, this is my diaphragm, this wiggly line represents the suspension system, so the diaphragm can move in and out. On the back side of the microphone, now this diaphragm is could be metallic or it does not have to be metallic, it is a film, it is a thin film. On the back side, I have a perforated surface, diaphragm and the back perforated surface, they are electrically and then I seal this entire thing. Also, what I have is a small orifice here, which is in green, so I have diaphragm which is member A, I have a perforated screen which is member B, everything into an enclosed cavity and I have a small orifice and my pressure wave when it hits, it gives me a pressure, it hits the diaphragm, so the diaphragm sees some pressure. The front membrane and the back membrane, so what we are talking, this is a particular type of pressure mic, what we are seeing, it is called a condenser mic. The front membrane and the back membrane are connected to a resistance, which is very high in number, very high value of resistor and then I have a battery source, which provides what I call polarizing voltage. The diaphragm and the surface B, they are made up of dielectric material, so they can store charge. Electrical sense, this entire thing looks like, I have a DC supply, a very high value of resistance and then these two flat plates are like capacitors, make a capacitor and what I measure is basically this parameter V 0, which is the voltage across these two plates. Actually, more often I measure the charge Q, so these two plates, they are separated by a distance, some nominal distance, let us say this distance is D. So, my capacitance is C equals area of the surface times permittivity constant over the distance D, now when a pressure wave hits it, hits the diaphragm, the diaphragm moves in or if it is negative pressure, then it moves a little bit out. So, what I am doing is I am sensing changes in this charge Q and then I interpret that change in charge, because my capacitance is changing, my Q is basically C times V 0, when capacitance changes, charge is changing and I sense that change in charge and from that I interpret, try to extract what is the pressure, which the diaphragm is seeing. So, what I have drawn here is basically the electrical component of the circuit, they are also mechanical component of the circuit. You all have now the fundamentals to develop a mechanical component of the circuit, electrical and acoustic component, merge into this thing and you can figure out, when it sees a pressure P, how does Q change, you can figure it out. So, I am not going to go into detail in that aspect, but this is how the pressure microphone works at a very basic level, as you are developing the acoustic component, bear in mind there is a cavity here, so it has some springiness, there is also a mass of the diaphragm, there is also a stiffness of the diaphragm, so all that has to be taken into account, when you develop the entire thing, if you want to. Someone may ask, why do I have this surface B slotted and the essential reason for that is, there could be some frequencies, which may excite the resonance of surface B, so then it does not behave in a very controlled way. So, what the perforations do is, that they act as acoustic resistors, so when you are developing the acoustic sense, you have to include that resistive element and what that resistance does is, it dams out the vibrations. So, this is capacitor mic, this is the capacitor microphone, if my pressure goes up, my charge increases linearly and what we see from this very simple analysis is, that the change in charge does not depend on frequency, there is nothing which relates, which is embedded in the frequency part of, frequency term is not embedded in this ratio, so Q changes regardless, if a pressure wave is hitting at 1000 hertz or a pressure wave is hitting at 50 hertz or pressure wave is hitting at 20000 hertz, if the value of that pressure is same, Q will change by the same amount. So, the response, there is only one caveat, that the size of this device has to be significantly smaller than the wave length of the frequency, wave length of the perturbation. If I plot P d, P d is basically the pressure difference outside and pressure difference inside, inside it is atmospheric pressure, because I have this small orifice, because when you move microphone from location A to location B, you can take it in aircraft, pressure can change. So, I have that small orifice to make sure that, the ambient pressure is same as whatever is the ambient pressure outside, that is the purpose. So, P d is basically P outside minus P inside, which is basically P. So, if I plot P d versus log of omega, as long as my lambda, my size of the object which I can represent by this dimension A is significantly small than lambda over 2 pi, then my response curve is a flat curve. So, a device of this type gives me a fairly straight response curve over, I can make this range as much as I want by just basically shrinking the size of the microphone to whatever value I like. So, this gives a very linear response over an extended frequency, but because I have to have this charging battery in it, this is an expensive device. This is I have to provide a polarization voltage and make that happen that requires lot of money. So, this makes things very expensive. Also the tolerance is required in construction of this kind of equipment are very tight. This particular type of a pressure microphone is expensive to make, the expensive. So, this is one type of a pressure microphone, which is called a capacitor microphone. You said that the charge relation, charge does not depend on the frequency, but the way it changes does depend on the frequency. What do you mean? Because charge is dependent on the capacitance and the capacitance is dependent on the effective distance, which will change with the vibrations. It will change with the vibration and what is it? If I am putting a pressure regardless of whatever frequency it is, let us say it has a value of 5 newtons per meter square, then my member will just move in accordingly by the same amount. It is a pressure wave. So, accordingly it will vibrate and because of that the effective distance between the two membranes will change with the frequency same as that the pressure wave is getting the entire thing. Rate of change of charge will depend on frequency. Rate of change of charge will depend on frequency, but not the charge. I am just asking. But that is why we are measuring charge. So, we are not measuring current, we are measuring the charge, which is an integrated quantity. So, that will not depend on frequency. And sir, the perforation is that you talk about the same thing can happen with a diaphragm also. So, they engineer diaphragm in such a way. You cannot make the diaphragm also perforated. So, you have to engineer diaphragm in such a way that now as you keep on shrinking the size of the diaphragm to make things smaller to meet the frequency, it also automatically starts between more and more stiff. But yes, engineering diaphragms if you want to measure up to 20000 hertz, the full band or audio band, then engineering it is not easy. It is a very difficult thing. But in theory, you can make things as small as possible and that will give you a leader response. Inherently in the mechanics of the system, there is nothing which says that the response will be not straight as frequency change. So, in a very broad sense, this is a pressure microphone, because what we are measuring is changes in pressure. The second big category is pressure gradient microphones. These microphones are also called by some people as velocity microphones and we will see that a little later why they are called velocity microphones. In general, the way these mics work is that you have again a tube. So, you have a diaphragm and you have this kind of a tube which is not long. Now, my pressure wave is coming from this side. Let us say this is called P 1. This pressure and this has fairly wide slots. So, unlike a small orifice for equalization of pressure, what you have here is a fairly wide gap which can enter into this tube or a cavity. So, the pressure can sound also travels like this and let us say this length is delta L. The length L, so let us say this pressure on the inside which when it hits the diaphragm is P 2. Sound travels delta L distance more for pressure P 2, because it has to go inside and hit it. So, there is a change in pressure associated with some delta L. So, essentially what we are measuring here is the pressure gradient. So, P D which is the difference in pressure is mathematically we can say gradient of P times delta L. It is a dot product of these two vectors, where P 1 is pressure outside the tube near the diaphragm, P 2 is pressure inside the tube near the diaphragm, delta L is the distance difference in distance traveled by the two waves and it is a dot product, because when you have a gradient it is a vector and it has some directionality associated with it. So, this delta L has to be aligned with that gradient. So, this is essentially del P over del x for instance, if I am only in thinking about x direction times delta L which is a scalar times cosine of theta. So, this delta L is the entire distance from outside to the yes, but we have to take it is dot product. So, in this case if the direction of P 1 is same as this length of this length, then basically literally the delta L will be this length plus this length. So, now what we will do is let us look at a scenario when this whole device is at an angle from P 1 and let us see what it does to us. In this case which is case A, theta is 0, but what we will look at it is another scenario. So, case B, theta is not equal to 0, I will have to make a rather long big drawing just so that things are visible. I have to make a gap that is my P 1, my delta L is associated with this path, this angle is theta, my pressure difference is minus gradient of P times delta L times cosine of theta. Why did I have negative gradient of P? Because I mean if I look at it physically, if P 2 is larger then listen I have to define a coordinate system, this is my coordinate. So, going forward is positive x, if P 2 is larger than P 1 then the displacement in the membrane will be negative x direction. So, for positive gradient P I have negative x, so that is why I have a negative delta P. So, P D is minus delta P times delta L times cosine of theta, again the standard things that the size this dimension A has to be small compared to the wavelengths we are going to measure all that has to be clear. For theta equals 0, P D is maximum for theta equals 90 degrees, P D is minimum which is 0. So, in this kind of a contraption I am measuring the movement of the membrane which is basically a consequence of the difference in pressures attributed to a delta L. So, that is why it is called a pressure gradient microphone. P, we know x omega is P plus E minus j omega x over c, I am assuming in this particular relation that I have a forward traveling planar wave, there is nothing no reflection happening, so there is no P negative term. So, gradient of P is minus P plus omega j over c E minus j omega x over c. So, my P D is minus I am using this relation minus gradient of P, so it gives me minus minus terms become a positive P plus omega j over c E minus j omega x over c times delta L times cosine of theta. So, I will just rewrite this relation P D equals P plus omega j over c E minus j omega x over c times cosine of theta times delta L. So, if there is no reflection then basically P 1 is same as P plus right, if I am trying to plot P D and I do a boat plot. So, I have a log omega here I am plotting decibels for P D, how will the curve look like for a given value of x the question is see at this point x is defined wherever my wave is hitting the microphone x is defined x is not changing theta is not changing for a given value of x and theta. If I change the frequency how does P D change, what will be the curve look like it will be a flat line or positive slope negative slope what will it be? Why will it be? It will be positive slope omega, there is nothing positive negative here, it will be something like this and this will be some I think it will be 20 degree. So, that is my transfer function. So, clearly this is not a, so for different frequencies if I am measuring P D for the same amount of P D same magnitude of P D, if I measure it at 100 hertz the signal measured in terms of voltage when the membrane moves will be 1, if I measure it at 1000 hertz which is a decade apart the signal measured will be 20 decibels less right because of this relation. So, I do not like this and that is coming essentially because of this omega j over c term how do I get rid of this term, what I can do is I can integrate this guy. So, omega j over c goes away, so I can multiply this by 1 over j omega, so the omega term goes away or in electrical terms they use this word called integrator. So, my input is P D and whatever voltage I am getting is after integrating it, so I get rid of this. So, once I integrate it my response curve, my response curve becomes how will be the response curve look like that omega term goes away. So, what will happen, it will become a flat straight line right shall I introduce you and get it. Sir, but there is x e minus j omega x by c, x is fixed all what I am plotting is the transfer function of P D with I am plotting a transfer function of P D with respect to the incident wave. So, this e minus j x term gets eliminated, so I get a flat straight line. This will be P not P D, you can call it I am being a little sloppy yes, this is voltage. So, I get a flat straight line, so this is how pressure gradient microphones they are also called pressure gradients they are also called velocity microphones because we know that from Newton's law, I think this is the relation. So, they are also measuring basically velocity, especially after you have done the integration in the velocity, that is why they are called velocity microphones, but in the truest sense they measure pressure gradient. This is the response for a plane wave and once I integrate it, this is the response for a plane wave of a pressure gradient one, but in real application suppose I am speaking and it goes to you it may not be a plane wave you may be you do not know whether it is a plane wave or it is a spherical wave. So, let us also see how it responds this kind of a contraption how it works for a spherical wave. So, for a spherical wave the wave front forward going without any reflections wave front is P plus over r e minus j omega r over c and let us say my wave front is such these are spherical waves in this direction and the orientation of the diaphragm is like this, this is my diaphragm. So, my gradient of P is what P plus e minus j omega r over c times minus 1 over r square minus j omega over c r which I can simplify as minus e r omega times 1 over r plus j omega over c. So, my P d which is the difference in pressure is minus gradient of P times delta l is a dot product which is P r omega times 1 over r plus j omega over c cosine of theta delta l. So, I will just rewrite this because I am going to do a boat plot here is P r omega cosine theta 1 over r plus j omega over c delta l. If I plot in decibels because of this particular term how will my boat plot look like this is log omega this is decibels for P d. A boat plot has a low frequency asymptote and high frequency asymptote what is the low frequency asymptote going to be flat line will be flat line and the high frequency asymptote will have a positive slope of 20 degree. And the actual curve will look something like the crossover point will be such that this is log omega naught where omega naught is equal to r c over omega naught equals to the wherever I am placing the microphone. And how far it is from the spherical source that here I have still not integrated now either in a hardware either you integrate it or you do not integrate it. So, in case of plane waves I had decided to integrate it because this is a flat response. So, let us see what it does in this case. So, once I integrate it basically my actual response curves looks like this basically the way you get it is you divide this whole bracketed term by omega j omega. So, this is again my decibels log omega this break point which is going to be somewhere here is going to correspond to omega naught above a certain point it starts between flat I mean this is going to be an asymptote. So, I should be more careful something like it becomes fairly flat, but at low frequencies you see a spike. The other thing we have not talked about is that there is a cosine theta term in both the spherical and as well as the plane wave expressions. What that means is that if my microphone is placed in the direction of the incident wave it will see maximum amplitude if it is placed at 90 degrees it will see 0 value. So, I draw polar plot and on a linear scale the curve looks like this these are supposed to be equal circles if this is a bad drawing, but this is supposed to be equal circles. So, this is 0 degrees the rate of the radius represents the magnitude I do not want to confuse with this length represents the magnitude. So, at 0 degrees the microphone will be very sensitive to incident pressure waves at 90 degrees it will have it will not record anything and at minus at 180 degrees it will again start recording at maximum level. So, the response here is bidirectional that is the first thing. Other thing is that in most of the cases you had unless you are really really far away from radiating you know single source in most of the unless you are really very far from you know point sources. Suppose I am speaking unless you are really far away from me this point source acts as a spherical source we have talked about it earlier. So, in most of the cases a microphone measures sounds emanating for point sources with act as radially propagating sources and what that means is that if I place this microphone very close to the source what will be the measurement it will faithfully record high frequency data because the response curve is flat in this zone, but it will record low frequency information at a higher decibel level. Now, when I play the same sound track in reality suppose I am speaking my low frequency sound or you know information gets recorded at a higher decibel level high frequency get record gets recorded at quote unquote normal level when I play the same sound track again I will sound like a person who has more bass this is a trick a lot of singers use that if they want to have a lot of their heavy voice they use this they use these kind of microphones and place microphone very close to there now you may have seen it like that. So, it is a base enhancing technique also a lot of discussions when you hear on radio you will see that we will feel that the person who is speaking he has a very heavy voice it is because of this artifact. So, this is the polar plot in absolute terms and if I plot the same thing in decibel terms the polar plot looks a little steeper it looks like a butterfly because the shape of these pictures is like a figure of eight these microphones are also called figure eight microphones there is some general. So, we have talked about microphones which measure pressure microphones which measure pressure gradients now there is a third category which rely on both. So, these are called some guys call it pressure gradient microphones type two, but that is not a very popular term, but it is pressure and pressure gradient microphones. So, the construction looks something like this again I have a diaphragm. So, I have a suspension system suspension stiffness here in these two wiggly things is a fairly rigid diaphragm and then there is a cavity of volume V naught which is filled with regular air and on the other side I have a perforated screen which offers an acoustic resistance R e my pressure wave is here in the pressure p 1 and the same pressure wave hits on the other side at the level p 2 and that length is delta l. So, I can construct a simple four point network for this for just the electrical acoustic portion of this thing. So, this is called Z A T I will explain what these terms mean this is C A this is R A pressure. So, Z A D is the overall impedance offered by the diaphragm C A is associated with the compliance of this cavity of volume V naught R A we have already defined its acoustic resistance p 1 is in pressure outside the diaphragm p 2 is pressure just outside the screen and this is what kind of model is this impedance model or mobility model because my pressure is the across variable. So, let us say my volume velocity which the diaphragm is seen is V V D and volume velocity which the screen is seen is V V naught two relations here V V D I am basically considering this loop loop 1 and then there is another loop loop 2. So, my p 1 is V V D times Z A D plus V V D times 1 over C A minus V V O times C A right. So, I will write that relation V V D times Z A D plus 1 over S C A minus V V naught times 1 over S C A minus V V naught equals P 1 S is my complex J omega and similarly for the second loop my relation is V V naught V V D times 1 over S C A plus V V naught acoustic resistance plus 1 over S C A equals minus V 2. So, let us say this is my relation 1. So, if I know p 1 and p 2 then I can figure out V V D and V V naught I know everything else in these two relations. So, what I will do is in next equation set of equations I will try to compute what is p 1 and p 2. So, p 1 is my incident wave p 0 e minus j omega x over C I am assuming again flat one dimensional wave without any reflections and p 2 equals p 1 plus del over del x plus del over del del theta p naught e minus j omega x over C times del L cosine theta. So, this becomes p 1 1 minus j omega over C del L cosine theta. So, now I know p 1 and I know p 2 I can plug these in one this is equation two. So, if I do so I can from two I can figure out what is V D and V V naught and I also know that p D equals p 2 minus p 1 pressure difference between the two sides of the system and so this is there. So, if I do mathematics with all these three equations at the end of the day I mean this is just regular math I can find the ratio E D over E 1 is this is a long term Z A D times R A plus del L cosine theta over velocity of sound times acoustic compliance. Then, that thing divided by I can find the ratio Z A D times R A minus j R A plus Z A D divided by omega C A. So, you can play with these three relations to get E D over p 1 and you end up getting this. Now, just to make things look a little simple I define a quantity B such that B is delta L over R A times C times C A. So, my p D over p 1 becomes Z A D times R A 1 plus B cosine theta over cosine theta alpha times 1 plus B cosine theta where alpha is this entire term besides 1 plus because. So, in case of a pressure differential mic we had just cosine theta in the term here we have 1 plus some constant times cosine theta. So, pressure differential mic was bi-directional just because of cosine theta terms. Here I have the flexibility to play with B B is basically delta L R A I cannot change C velocity of sound, but I can change all other parameters delta L R A and C A and depending on how I change it I can change the directional pattern of this pressure pressure and pressure this combination mic in whatever way I want it to be within the ream of feasibility. Let us assume that let us say that B is 1 we can assume we can construct a mic with specific parameters such that B comes to 1. So, in that case my polar pattern looks something like this. Someday I have to take a class in drawing this is called a cardoid shape. So, you get different form factors for this cardoid shape based on how you play with B essentially what this shows is that if I have mic pointing towards you it will sense your sound and if I am opposite you behind the mic it will not sense my sound. So, this is also directional mic, but I can change the shape of this by playing with B in different I mean to a certain extent. So, we have seen a pressure mic how it operates and what kind of features it has then we have seen pressure gradient mic and then combination of pressure and pressure gradient mic. I forgot to mention one thing that why is this called a combination mic or a pressure plus pressure gradient mic and the reason for that is that in the process when we are developing this entire relation somewhere in the middle we will come across a term we will come across a term where P d will be something a longish expression Z A d P 1 R j plus P 1 minus P 2 over j omega C a basically if you the way you get this is if you solve for V and V naught and then find P d from there you get this relation and what this shows is that pressure differential is a function of depends on P 1 minus P 2 and it also depends on P 1 that is why it is combination. So, from a mathematical standpoint this is what I wanted to cover today. So, now in next 25, 20, 25 minutes what we will do is we will just in a subjective sense we will go over we now understand at a basic level how different types of mics work, how based on their designs they can be directional in specific directions. So, based on that what we will do is we will discuss different type of microphones and how they are relevant for specific application. We have pressure mics and there are 3, 4 broad categories one is electrodynamic mic and electrodynamic. So, in pressure mic there are several categories first one is electrodynamic think about this if you have a transducer a speaker which we discussed in last several lecture instead of exciting it with a voltage if I do not excited with a voltage, but I just so sound energy at it. So, pressure ways hit the diaphragm what will happen the diaphragm is going to move and that will induce current in the voice coil that is how an electrodynamic mic works it is basically an inverted acoustic some people also call it moving coil microphone because the coil is moving and electrodynamic mic can have one membrane, but a membrane if it is this large it will have its own modes which will be which can be below a certain threshold. So, some mics some more fancy mics of electrodynamic category they have several membranes to get an extended frequency response for the system. So, this itself can be single membrane and then multi membrane is a fairly commonly used mic because it is a pressure mic is it omnidirectional or is it bidirectional or in one direction it is it is an omnidirectional senses pressure does not matter how the mic is oriented it will just sense the pressure at that point it is not sensing pressure differential. Sir, but it has a cost atom it is it has a cost atom in certain theta it will not sense, but other way it will sense. In theoretical from purely theoretical standpoint it should not have sensitivity to direction see you have a microphone like this and all it is sensing is pressure acting on this membrane. So, if I pressure acts in all directions at a point right pressure acts on all directions pressure gradients are direction specific if I make it like this pressure is still acting inside. The electrodynamic mics are fairly omnidirectional so if you place only omnidirectional mic and there is a crowd they can does not matter where the person is sitting if you have round table discussion you put a microphone in center people all around it their sounds would be picked up by the microphone. Also by nature of their construction these microphones are resistant to moisture which is an important thing. So, and they are fairly inexpensive to make. So, they are very widely used the second one is capacitor mic capacitor condenser we discussed this earlier today right where we are actually measuring the charge on the capacitor. Now, within this there are two types polarizing and non polarizing or others also call it electorate. So, in polarizing we have seen in the earlier part of today's lecture you have to put an external battery to provide an initial charge polarizing charge across the two capacitive plates that makes things very expensive. The polarizing type of mics are fairly very accurate and also they have a very flat frequency response because when we when we did a very basic analysis we saw that the relationship between pressure and omega is of a flat line right. By the way electrodynamics might may not have a flat frequency response as flat as capacitor slash condenser mic because as we saw the response of a regular speaker it is not flat it has different bands. So, electrodynamic mics behave in terms of frequency response similar to speakers we were on polarizing mics. Now electorate mics they do not have a charge provided by an external battery source rather what they have is that the two parallel plates are essentially made of dielectric materials and they have charge deposited on them to begin with. How do they do it? What they do is that when the thing is being made you take a dielectric material and then it is heated up when things are in heated state above a certain threshold the polarizing polar molecules in that dielectric materials they can move back and forth. So, they heat it the polar molecules have a propensity or freedom to move and orient themselves and in such a state they apply the manufacturing process such that an external electrostatic field is applied. So, you have a positive and negative and electrostatic field is passing through between the field you place this material which is made of dielectric material in a heated state. So, these polar molecules orient themselves accordingly in that state then the thing is cooled. So, then the molecules do not revert back to whatever other orientation. So, then you have a permanent charge built up on both the plates you have a positive charge and negative charge built up on both the plates. So, that makes things less expensive. So, because of this electric my probably today are the most popular microphones in the world. So, they are really cheap and this can be mass produced because basically a lot of these films are like pieces of plastic, cheap plastic and they deposit charge on them through this process. A drawback of these is that over a period of time this charge starts dissipated because of heat. But again these are also have these also by from a theoretical standpoint they have a fairly flat frequency response. If you make them small enough you can enhance the frequency spectrum the microphone to as much as you want. They are very reliable they are still not as good as the polarizing type of microphones and it is not that you cannot make these microphones as good as the polarizing microphones. But when you are doing a large scale production processes you have to maintain bigger tolerances because production process have white tolerance they want to have as white tolerances possible. So, once you make things in very large volumes the quality or the fidelity of an instrument goes down. If you make one piece at a time and pay a lot of attention to every single piece you will do a better job. So, because of that reason because they are mass produced in general they do not have as high a quality as the polarizing microphones. But again they are only directional and these guys are used in a lot of applications. There is a microphone here sitting here in this tablet in your cell phones you have an electric most likely you have an electric my computers almost all places where you have you need very small microphones these things are sitting here. So, that is condenser the third one is piezoelectric again this is also a pressure my but what it does is it senses pressure by due to when pressure gets exerted the crystal generates a voltage that voltage is sent. So, that is the third category the fourth one is carbon resistance these are kind of but earlier especially in on in a lot of analog telephones you would have these mics and the way they operate is there is a small button in the device which is packed with carbon granules as the pressure wave hits a surface of this it presses. So, the resistance of the carbon granule changes accordingly in some predefined way that is resistance changes sense by the electronics and that gets recorded or converted. So, it was used a lot in old type of type of telephones really those black telephones you may still see some and also in recording industry. So, this is these are four major types of pressure microphones electro dynamic capacitor piezoelectric and carbon let us look at pressure gradient mics these are directional we have seen through the mathematics specifically they are bi-directional. So, at 0 degrees and 180 degrees they have maximum response at 90 degrees the response is virtually 0 given that if you have a public address system and the requirement is that the microphone should be able to pick up the voice of let us say the speaker and the voice of the crowd which is on front side people tend to use these type of microphone. So, it depends on the nature of the requirement while these microphones are used you have to pay more attention to the frequency spectrum you have to pay attention even in pressure mics I mean what is the frequency band, but here we saw that the frequency spectrum is booming at low frequencies and flat at high frequencies. So, one has to be careful as I mentioned earlier lot of singers also use it for the special feature that these microphones tend to produce more boomy sound at low frequency. And then the third category we talked about is pressure and gradient mics combo. So, again here you have the advantage of directionality and these mics as well as pressure gradient mics again both these are very frequently used in broadcasting system because of the directionality feature. In auditoriums suppose you have play happening on the stage or you have a bunch of singers you do not want this noise or the sound of the audience to get picked up by the microphone. So, you will use a combination of some of these.