 We use microphones most of the time. Microphones are in AirPods, earphones, even in your mobile phones. And also you might have seen singers or podcasters recording in a studio, singing or speaking to a microphone. Whether you are plucking, drumming, singing or speaking, microphone creates a high quality audio. But how do these microphones work? That is something that we will explore in this video. All right, now there are different types of microphones and one thing common in all of the microphones is that it takes in sound waves which is also called a pressure wave. As it changes air pressure near its source, it takes in a pressure wave and changes it and changes it into electrical energy, something like a voltage signal. It is taking in variations in air pressure and changing it into electricity. All of the microphones do this and there are many types of microphones. There is dynamic, electric, condenser. They carry out the same process but with different methods. Here we will look into how condenser microphones change audio signal to an electrical signal. Let's briefly talk about its structure first and then the physics of it. So here we have a condenser microphone and inside of the microphone there is a metal ring. There is a metal ring with two metal plates attached at both the ends. One plate is called a diaphragm. This is a diaphragm. And the other that you cannot see here, it is on the other side, it is at the back. That is called a back plate. And these plates are mostly coated with gold because gold doesn't corrode. And this is how it looks like. Here you see the metal ring and this is a diaphragm. On the other side there is a back plate which is also a metal plate. Now let's look at a sectional view. Let's look at it from an angle. So for that let me make some space first. Okay, here we have a mic and here we have the metal ring at the top and this right here is the diaphragm. This is the back plate and both of the plates are connected to a battery. In between these plates there is an insulator that is usually air because air is an insulator. This setup now starts acting as a capacitor because you have one positive plate and you have one negative plate and there is an insulator, there's a dielectric between them. So this starts behaving like a capacitor. Now let's go back to the main process, the main function of a microphone. It converts one form of energy into another. Sound energy or variations in air pressure into electrical energy. How do we get electricity out of mechanical motion of air particles which produce sound? One thing that we can be certain of is that since the pressure of the air is varying, it is changing because of someone speaking into the mic. The final electrical signal that we are interested in should also vary and change right. We know that a sound wave can be plotted like you can plot pressure to distance, pressure versus distance and the plot could look somewhat like this. You will have points of very high pressure. These are compressions and you have points of negative pressure which are rear factions. So if the input audio signal is changing, the output electrical signal should also change right. Other than this, you would also want the pitch of your voice to be the same right. If someone has a heavy voice and they are speaking into the mic, when you hear your recording, you should be able to hear the same pitch right. The pitch should match and pitch is defined by the frequency. So that means the frequency of the electrical voltage signal should also be the same as the frequency of the sound wave coming in. We can increase the amplitude of the electrical signal later by running it across an amplifier. So we need not worry about the loudness for now. One thing that we can worry about is keeping the pitch same. So our voltage signal, electrical signal should kind of mimic or copy the audio signal coming in. It should have the same frequency and that means it will also vary just like the audio signal. Now the question is, how do we get the voltage changing? We know that if a capacitor is connected across a battery, it will start storing charge. There will be a plate with a positive charge, in this case the back plate. This is the back plate. This will have a positive charge and the diaphragm, the other plate that will have a negative charge. And in some time, it will get fully charged because of which a potential will develop across the capacitor. So maybe if we change the voltage of the capacitor at a certain frequency which matches the frequency of the audio signal, maybe our job might be done then. Now how can we change the voltage across the capacitor? We could change the amount of charge on these two plates and make the voltage change. But the voltage should mimic or copy the audio signal, right? It should have the same frequency at all times. How do we make the charge move instantaneously from one plate to other, exactly knowing the frequency of the coming sound so that we are able to change the voltage according to that, that seems difficult. That seems quite difficult. Instead of changing charge to change voltage, we can also change the capacitance itself, right? So what are the ways of doing that? Capacitance depends on just the physical characteristics of the plates. So it's area, the material between the plates and also the distance between the plates. Changing the material in between or the area would be a difficult task, right? No one is getting inside a mic to do that all the time. We can, however, change the distance between the plates. And turns out that is what happens. The diaphragm, this diaphragm that you see, it is extremely, extremely thin. So think that when the pressure wave, when the sound waves are coming in and when they strike it, it starts vibrating. When there's a compression, it is pushed inside and when there's a rear faction, it is kind of pulled outside. And how will that look? That could look somewhat like this. So let's say you have sound waves coming in from the left-hand side. And when there's a compression, it pushes the diaphragm inside. And when there's a rear faction, it pulls it outside. So constant sound waves coming in also vibrates a diaphragm according to that. When the diaphragm is pushed inside, the distance decreases. And we know that capacitance is epsilon not A divided by D. So nothing in the top is changing, but D is decreasing, which means the capacitance should increase. Okay, so this is great because now we know that capacitance can change. And it can change totally mimicking or copying the audio signal. But we wanted a varying voltage. Something that can be recorded. So if the capacitance is changing, then the voltage should also change, right? Because we know that Q, this is equal to C into V. There is one problem over here. If the capacitance increases, if this increases, then if there is just a capacitor and a battery in the circuit, the voltage across the capacitor will always be fixed. It is equal to the battery voltage. So VB, this is equal to the voltage across the capacitor. VB is the battery voltage. So when the capacitance increases, more charge gets stored on the plates and there is no change in the voltage. So if we are somehow able to make the charge fixed on both of these plates, then changing the capacitance will lead to the change in voltage, right? So for that, what we can do is we can add a huge amount of resistance over here. Something around one giga ohms or mega ohms. So this resistor, it will keep the charge on the plates constant. There won't be any movement of charge. Now when the capacitor increases, the charge is fixed and the voltage decreases so that their product is equal to the fixed charge on the plates. This means that now VB, this is no longer equal to VC. It is no longer equal to VC because VC has decreased, right? If capacitance has increased, the potential across the capacitor has decreased and if capacitance decreases, then the potential would increase. So if VC is decreasing, if it is decreasing, then there must be one another voltage drop so that the addition is still equal to the battery voltage. And that, and that is across the resistor, that is across the resistor. Then it becomes equal to the battery voltage. So for, for example, if let's say initially the voltage across the capacitor, which was equal to the battery voltage, and that was let's say 48 volts. When the capacitance has increased, some voltage decreased. So now let's say the voltage is 34, less than, less than 48. So the extra voltage drop that is coming from the resistor. There is some voltage drop across this resistance and in this case, that would be 14 specifically. And similarly, if the, if the capacitance decreased, then the voltage would increase, it would become more than 48, maybe, maybe 60. Then the voltage across the resistance would be minus, minus 12, it would switch polarity. But their addition always comes out to be equal to the battery voltage. So as the capacitance oscillates with the same frequency as that of the sound wave, that leads to voltage change again in with the same frequency. And this leads to a voltage drop across the resistor again with the same frequency, completely mimicking the input audio signal. And then this signal, finally, let's see how that looks like. This signal is read across the resistor that is then sent to the amplifier to increase the amplitude. And if there is a phase difference, that is also taken care of. All right, so that is how a condenser, and it is usually called a capacitor, that is how a capacitor microphone changes audio energy into electrical energy using capacitors. Sound waves strike the thin diaphragm, changing the capacitance of the capacitor, which leads to a change in voltage that leads to a voltage drop across the resistor, which is then recorded and sent to other components in the circuit.