What is a Microphone? |Types of Microphones

What is a Microphone? |Types of Microphones

What is a Microphone?

MICROPHONES

The microphone is a transducer that converts acoustic energy into electrical energy.

 Microphones may be classified:

According to Mode of operation:

i) Pressure operated

ii) Velocity operated

 Pressure operated microphones

Pressure-operated microphones employ a diaphragm with only one surface exposed to the sound source. 

The displacement of the diaphragm is proportional to the instantaneous pressure of the sound wave. 

At lower frequencies, such microphones generally cause a resonant response, giving rise to a peak that may reach 6 to 8 dB with reference to 1000 Hz. 

The pressure-operated microphones are carbon, crystal, dynamic and capacitor microphones.

 Velocity operated microphones

A velocity microphone is one in which the electrical output substantially corresponds to the instantaneous particle velocity in the addressed sound wave. 

A velocity microphone is also referred to as a gradient microphone. A gradient microphone is a microphone in which the output corresponds to the gradient of the sound pressure. 

The velocity-operated microphones are ribbon microphones.

Types of microphones

 Carbon Microphone

In a carbon microphone, small carbon granules are held in close contact in a brass cup called a "button" which is attached to the centre of a metallic diaphragm. 

Sound waves, striking the surface of the diaphragm, disturb the carbon granules changing the contact resistance between their surfaces. 

The change in contact resistance causes a current from a battery connected in series with the carbon button and the primary of a transformer to vary in amplitude, resulting in a current waveform similar to the acoustic waveform striking the diaphragm. 

After leaving the secondary of the transformer, the minute changes of current through the transformer

primary are amplified and reproduced in a conventional manner. 

The circuit diagram as well as the construction of a single button carbon microphone is shown in Fig. below.

The output voltage from a carbon or pressure microphone is proportional to the displacement of the diaphragm.

The field pattern is circular.

One of the disadvantages of the carbon microphone is that it has continuous high-frequency

hiss caused by the changing contact resistance between the carbon granules. In addition, the

frequency response is limited and the distortion is rather high. 

Carbon microphones used in communications i.e., telephone networks are designed to be

moved, with the current flowing through them.

2. Crystal Microphone:

A crystal microphone employs one or more Rochelle salt crystals placed in such a manner that

when their surfaces are struck by a pressure wave, they are bent or twisted out of shape. This

action results in the generation of an electrical current because of the piezoelectric effect of

such crystals.

When a crystal is subjected to strain, electrical polarization takes place. The polarization is

proportional to the mechanical strain. The inverse effect is produced when electrical current is

applied to the crystal. The mechanical movement in this case is proportional to the applied

current.

 

 

3. Dynamic Microphone:

The dynamic or moving coil microphone employs a voice coil attached to a diaphragm. The

sound pressure on the diaphragm makes to move the coil in a strong magnetic field, which

generates a voltage proportional to the sound pressure at the diaphragm. This microphone is

also referred to as a pressure microphone.

 

 

Microphones of this type do not require output transformers. The output voltage is taken directly

from the voice coil. The output impedance is approximately 200 Ohms. A low impedance of this

nature permits the microphone to be placed at a considerable distance from the preamplifier

without affecting the microphone characteristics. This microphone, like other pressure-operated

microphones has a circular field pattern. This is the most widely used microphone in these days.

4. Condenser Microphone

This is based on the principle of variable capacitance. It is polarised by a battery. The

appearance and basic functioning is shown in fig.

. The diaphragm of this microphone is a thin membrane of nickel, which is spaced about 0.001 inches (25 meters) from the fixed back

plate keeping air as the insulating medium. As long as the diaphragm is not exposed to the

sound pressure, the capacitance remains constant and the AC output voltage will be zero.

 

Whenever the sound waves strike the diaphragm, it undergoes compressions and ratification

and the capacitance across the microphone varies. The capacitance is inversely proportional to

the distance or space between the two plates. Q = CV. Since the voltage remains constant the

variations in capacitance varies the charge. The current in the circuit varies making a variable

voltage drop across resistance 'R'. AC output voltage is taken through a capacitor and

immediately fed to a pre-amplifier.

The function of a polarising voltage or its equivalent is to translate the diaphragm’s motion into a

linearly related audio output voltage, which is amplified by a very high impedance FET which

must be located close to the capacitor. Special measures must be taken to prevent the space or

distance between the capacitor plates from changing because of temperature and humidity.

Use of foam windscreens for protection in damp or corrosive environments is recommended.

Frequency response is fair over the entire audio spectrum. It requires a polarising battery and a

preamplifier. It is basically an omni directional microphone; it is best for indoor and outdoor use

5. Ribbon Microphone:

The ribbon (velocity) microphone is a microphone in which a very light metallic ribbon is

suspended in a strong magnetic field. Pressure waves cause the ribbon to vibrate in the

magnetic field generating voltage corresponding to the particle velocity of the pressure wave.

Velocity microphones may be designed to have a wide frequency range, good sensitivity, low

distortion, and low internal noise

 

 

6. Wireless or cordless microphone:

Wireless microphones are classified as of two types. These are handheld and collar type. The

specific application of handheld mic is that, a person giving a performance in between a

programme can hold this mic in his hand and give the announcements as and when desired.

Whereas the collar type mic can be used by a person gives any demonstration or lecture by

permanently fixing the mic to his collar of his shirt, so that he can freely move. A typical example

of a wireless microphone system consists of a lapel microphone concealed in the clothing of the

user

 

Wireless mic works on the principle of VHF radio transceiver system. It consists of two units.

The microphone along with transmitter is one unit and receiver the other. The microphone

output is fed to the input of the transmitter’s modulator circuit as an AF signal. A crystal

oscillator will generate the necessary carrier frequency and fed to modulator. The AF generated

by the mic is modulated with the carrier signal and transmitted as RF signal. The transmitter

circuit requires an operating dc voltage of 9V, which will be provided by dry cell inside the

casing of the microphone unit.

The receiver consists of a collapsible antenna and a demodulator circuit with a crystal oscillator.

The transmitted signal is received by a receiver located at a remote point (near the audio

amplifier) and converts back to audio signal. This AF signal is connected to the input of an audio

amplifier through jacks. The receiver circuit requires operating voltage of 12V dc.

The maximum transmitting distance under normal conditions is about 200 feet. When several

wireless microphones are used on the same set, each microphone transmitter must be operated

on a different frequency to avoid interference.