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Audio Filters - Working and Classification of Microphones - 3/8

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In the previous tutorials, physical properties of sound waves and acoustic waves were discussed. Sound or Acoustic Waves are in the form of vibration. Sound needs to be converted into electrical signals so that it can be processed by electronic circuits. So, the sound which is a mechanical energy must convert into electrical energy and must be precisely represented as an electrical waveform (analog) for any signal processing operations. 
So, there is need of a device which could sense the audio signals efficiently and convert them into electrical signals. In air medium, the device used for converting sound waves into electrical (audio) signals is called Microphone. 
The term 'Microphone' was first coined in 1875. Micro is extracted from Microscope and is used in sense of enlarging and phone is used to refer speech or sound. So, basically, microphone is a device that converts speech or sound into electrical signals. These have been used in variety of applications like telephone, TV, recording systems, radio, VOIP and many others. 
Working of Microphone
The microphone is a type of transducer which means a device which converts one form of energy into another form. MIC converts the sound energy (Mechanical Energy) into electrical energy. Different microphone have different methods of converting the energy but the common thing in all MIC is Diaphragm. The diaphragm is very thin like a paper or plastic which creates the vibration when sound waves stuck to it. There is a coil in the MIC which is bound on a magnet and this coil is attached to the diaphragm. When diaphragm vibrates, the coil also vibrates. The magnet produces a magnetic field around it and the movement of the coil in the magnetic field creates an electrical current in the coil. That electrical signal comes out of the microphone which is then given to the amplifier (Pre-Amplifier).


Classification of Microphones - 
MIC are categorized on the basis of conversion technology and application. On the basis of conversion technology, the microphones are categorized as follow - 
1) Dynamic MIC
2) Condenser MIC
3) Ribbon MIC
4) Crystal MIC 
Dynamic MIC- These Mics are simplest type of MIC which uses a coil and magnet to convert the sound into an electrical signal. These MIC work on the principle of electromagnetic induction. They have a coil attached to the diaphragm which is surrounded by a permanent magnet. When diaphragm vibrates, the coil attached to it also vibrates and due to electromagnetic induction, an electrical signal proportional to the vibration is generated. 
Pros - These MIC are cheap to design and rugged. 
Cons – Their frequency response to different frequencies does not match compared to a condenser or ribbon MIC.
Construction and Working of Dynamic Microphone

Construction and Working of Dynamic Microphone 

Condenser MIC – This MIC uses two plates of which one is stationary and other one act as the diaphragm. The sound pressure changes the spacing between the two plates which in turn changes the capacitance between the plates. The plates are charged with a charge which is given by the following equation -  
Q = CV
C = capacitance
V = voltage
Q = Charge between the plates
On changing the spacing between the plates, the charge Q is stored between them and this causes a current in a resistance connected in series with this capacitor. 
Pros: This design responds very well to a dynamic range of frequency.  
Cons: Expensive, Requires a power supply to bias the plates. 
 Construction and Working of Condenser Microphone
 Construction and Working of Condenser Microphone 
Ribbon MIC - These MIC have a metallic ribbon suspended between a permanent magnet. When sound waves hit the ribbon, it causes vibration in the ribbon. The vibration caused by the sound pressure cause vibration in the metallic ribbon which is in a magnetic field, so a voltage is induced across the ends of the ribbon. The voltage produced is proportional to the velocity of the ribbon. This voltage is the audio signal received from the Microphone. 
Pros: Add boost to the bass
Cons: Susceptible to wind noise
Construction and Working of Ribbon Microphone

Construction and Working of Ribbon Microphone 

Crystal MIC - This MIC uses a piezoelectric material for generating the voltage. The material is attached to the diaphragm when the diaphragm moves it deforms the material and a voltage is generated at the end of the piezoelectric material. The amount of deformation is proportional to the generated voltage, when stress is removed from the material then voltage gets disappeared. This way, a voltage signal (audio signal) is received by this microphone. 
Pros: Large electrical current
Cons: Frequency response is not good as dynamic MIC
Construction and Working of Crystal Microphone

Construction and Working of Crystal Microphone

The Microphones can also be categorized on the basis of their application. The MIC have different directional properties, frequency response and impedance. Microphones can pick up sound from one direction or from all directions (Omni-directional). The way they pick up sound is called as pick up pattern of MIC. On the basis of directionality or pick up pattern, microphones can be categorized as follow - 
1) Omni-directional
2) Bi-directional
3) Unidirectional 
Omni-directional - These MIC can pick the signal from all the directions. 
Bidirectional - These Mic can pick the sound from front and the rear of the MIC but not from the sides.
Unidirectional - These Mic can pick up sound from only one direction which is the front of the MIC. 
Apart from directionality, Microphones have distinct Polar patterns. All the polar patterns fall within one of the three directionalities. The polar pattern is a visual graph indicating the sensitivity of the diaphragm at different angles. On combining the directionality and polar pattern of the MIC, the pick up pattern of the MIC is obtained. The pick up pattern indicates the sensitivity of the diaphragm in different directions at different angles from the MIC. This actually indicates that how well the MIC will pick up sound from different directions and angles from it. On the basis of pick up pattern, Microphones can be categorized as follow - 
1) Omnidirectional 
2) Figure of Eight / Bidirectional
3) Cardioid
4) Hyper Cardioid/Mini Shotgun
5) Half Cardioid
6) Super Cardioid /Shotgun
7) Lobar/Unidirectional
Omnidirectional - The MIC with Omnidirectional pick up pattern are sensitive to all directions at all angles equally. 
Symbol of Omni-directional Microphone

Symbol of Omni-directional Microphone 

Bidirectional or Figure of Eight - These MIC are sensitive to only two directions opposite to each other. They are either sensitive to front and rear side or sensitive to left and right side. The sensitivity in both the directions is equal at all angles. 
Symbol of Bidirectional Microphone (Figure of Eight)

Symbol of Bidirectional Microphone (Figure of Eight)

Cardioid – These MIC are very sensitive from the front but least sensitive from the back. They give isolation from the unwanted ambient noise and are best for loud sound. So these MIC are most sensitive in the front direction with decreasing sensitivity at the sides and least sensitive at the rear side. 
Symbol of Cardioid Microphone

Symbol of Cardioid Microphone 

Hyper Cardioid/Mini Shotgun - These MIC are sensitive in both front and rear side but they are sensitive to a narrow range of angles on front side
Symbol of Hyper Cardioid Microphone or Mini Shotgun

Symbol of Hyper Cardioid Microphone or Mini Shotgun

Half Cardioid / Sub Cardioid - These MIC are well sensitive at the front direction with some sensitivity in all other directions. They are somewhat a crossover of Cardioid and Omni directional MIC. They are best suited when source of audio is moving and never stationary. 
Symbol of Half Cardioid or Sub Cardioid Microphone

Symbol of Half Cardioid or Sub Cardioid Microphone 

Super Cardioid /Shotgun - These MIC have sensitivity at very narrow angles at front and even narrower at the rear. So, these MIC are good at picking sound precisely from front as well as rear of the MIC. 
Symbol of Cardioid Microphone

Symbol of Super Cardioid Microphone or Shotgun

Lobar/Unidirectional - These are highly directional MIC with very narrow pick up pattern. They pick up the sound slightly from the sides. They are very particular in picking the sound so they will provide what exactly is needed. These are mainly used in video production. 
Symbol of Lobar or Unidirectional Microphone

Symbol of Lobar or Unidirectional Microphone 

Frequency response and impedance of Microphone 
Impedance - As MIC consists of a coil (inductor) or some kind of capacitor inside it, so they are not purely resistive in nature. As the resistance of inductor and capacitor varies as per the frequency. Due to this, the impedance of MIC changes for different frequencies. Different MIC shows different impedance for the different frequency range.
Frequency response- The frequency response of MIC depends on the technology of MIC conversion. As seen, condenser MIC has a good frequency response compared to dynamic MIC.
MIC Level and Line level 
The output of MIC level is very weak signal (-60 dBm), so for further processing of the signal, its strength needs to be increased. The strength of the microphone output can be increased by using a Preamplifier (preamp). The preamp sets the input signal to the line level. The line level is the standard strength of the audio signal to transmit the signal between audio sources to amplifiers, radios, TV, DVD players. The nominal line level voltage is 0.316 V - 1.228 V. Thus a preamp circuit is needed in MIC circuitry for converting the MIC level to line level. 
Learn more about preamplifiers from the following tutorials - 
In the next tutorial, another important building block of an audio system which is the speaker will be discussed. The next tutorial will take through the working principle and classification of the speakers.