Microphone: Is a transducer which converts sound signal into an electrical signal.
• Employed for sound amplification systems or sound measurement system
• In sound amplification systems, electricalsignal of the microphone is amplified before it is fed to the loudspeaker which convert electrical energy into acoustical energy
Technical specifications of microphone
• Frequency response • Dimensions • Equivalent Noise (db) –Based on the network• Dynamic rangE Sensitivity • Open circuit sensitivity (pressure field sensitivity) • Loaded sensitivity • Phase response • Type of polarization-externally polarized, pre-polarized • Directivity • Stability of transducer • Calibration chart 
Types of Microphone 
Carbon granule microphone
• The sound field acts upon on electro conductive diaphragm that develops pressure on a capsule of compressed carbon powder, This capsule or carbon button, as it is called
contains carbon granules between two carbon discs
• The sound wave incident on the diaphragm cause it to vibrate, transmitting the displacement to the carbon particles and changing their pressure dependent contact
resistance, The front and rear contacts are insulated and brought out to terminals
• The diaphragm motion compresses the carbon granules, the electrical resistance of which there by changes, This change in electrical resistance is used to get a suitable electrical signal which is a measure of the sound impinging of the carbon microphone
• When the DC voltage is applied across the capsule, the alternating resistance produces an AC voltage drop, which is proportional to the displacement of the diaphragm.                                             Condenser microphone
• Perfected as a much needed substitute for the carbon microphone which was very
noisy
• So called because it makes use of the principle of condenser (capacitor) to convert acoustic signals to an output voltage
• Consists of metal housing with a thin metallic membrane or diaphragm
• A back plate is mounted in parallel behind highly tensioned diaphragm (which is designed to have high resonant frequency) and form the other plate of the condenser
• The fixed plate is called the polarization electrode and is maintained at a potential of about 200 V. Electret Capacitor Microphone(ECM)
• Also known as pre-polarized microphone is a type of condenser microphone
• Overcome all the inconveniences of the traditional capacitor microphone, like the need for a polarization power supply
• Almost applications like telephones and consumer electronics and has replaced the carbon microphone for general purpose applications
• Excellent, easy to use and inexpensive device which can be operated on a 5 V supply voltage
• Normally require integrated pre-amplifier  
MOVING COIL  -Often found in recording studios, broadcast and motion picture production,
home hi-fi and video recording systems and on stages of live sound
reinforcement
• The principle is known in 1877, when bell developed the telephone, but lack of
electronic amplification made it impossible to use. Dynamic microphones are
similar to conventional loudspeakers in most respects
• Both have a diaphragm (or cone) with a voice coil attached near the apex
• Both have a magnetic system with the coil in its gap, This microphone operates
like a speaker  reverse
PIEZOELECTRIC MICROPHONE                                                                                                                    Employ crystals that can become electrically polarized and produce
voltages proportional to the strain
• Since the piezoelectric is reversible, a piezoelectric microphone will
function as a source when an alternating voltage is applied to its terminals
• Rochelle salt is one of the first materials to be used in acoustic transducers
• Signal crystals of rochelle salt have been employed widely in microphones
• Unfortunately, the crystals deteriorate in the presence of moisture and get
permanently damaged if the internal temperature exceeds 460 c
These are the type of microphones used