what is the carbon microphone ? working principle ? advantages and disadvantages |

Carbon Microphone

 Basic Concept :

The basic concept behind the carbon microphone is the fact that when carbon granules are compressed their resistance decreases. This occurs because the granules come into better contact with each other when they are pushed together by the higher pressure.The carbon microphone comprises carbon granules that are contained within a small contained that is covered with a thin metal diaphragm. A battery is also required to cause a current to flow through the microphone.
When sound waves strike the carbon microphone diaphragm it vibrates, exerting a varying pressure onto the carbon. These varying pressure levels are translated into varying levels of resistance, which in turn vary the current passing through the microphone.The varying current can be passed through a transformer or a capacitor to enable it to be used within a telephone, or by some form of amplifier.
The frequency response of the carbon microphone, however, is limited to a narrow range, and the device produces significant electrical noise. Often the microphone would produce a form of crackling noise which could be eliminated by shaking it or giving it a small sharp knock. This would shake the carbon granules and enable them to produce a more steady current.

Construction


The construction of a carbon microphone is shown in below figure a. Fine carbon granules are enclosed between two metal plates. The upper plate is attached to a movable metal diaphragm through a metal piston or plunger. The lower metal plate is fixed and is insulated from the diaphragm. A protective cover with holes is used to protect the unit A battery is connected between two metal plates. When load is connected current flow through the carbon granules and the load. The path of current is shown by arrows. The output is obtained through a transformer to eliminate d.c.content of the microphone output.

working-

  • When sound waves strike the diaphragm, it moves to and fro.
  • During compression condition, it presses the carbon granules and during rarefaction, it loosens them, when carbon granules are pressed, the resistance decreases and hence the current through the circuit increases.
  •  When carbon granules loosen, the resistance increases, decreasing the current through the circuit. In the absence of sound, a steady current flows. Thus, sound waves superimpose a varying current, or audio current on the steady DC current. The net resistance of the carbon granules is given by Eq. 
                                              Ro +  δr 

where,

r - Net resistance in ohms. -
R0 - Steady resistance in ohms for no sound
δr - Variation of resistance due to sound pressure (it will have positive as well as negative value)
        The development of a voltage V across a load resistance RL is illustrated in Fig.

Characteristics of a carbon microphone

  • The Sensitivity will be Very high.
  • The output of a carbon microphone is about 20 dB below 1V i.e., about 100 mV).
  • Signal-to-noise Ratio Poor random variation of resistance of carbon granules generates a continuous hiss.

  • Frequency Response Carbon microphones have a frequency response of 200 to 5000 Hz, and therefore are unsuitable for high fidelity work.

  •  The resonance peak is at 2000 Hz and overall frequency bandwidth is usually tip to 5 kHz.
  • Distortion is high. The content is rich in harmonics unless variation in resistance (δr) is a very small percentage of steady resistance R. Distort ion is of the order of 10%. Also, carbon granules have a tendency to stick to each other which further increases the distortion. Directivity of carbon microphone is substantially omni-directional. However, high frequency response over 300 Hz falls beyond an angle of 400 from the front of the microphone. Output Impedance It is about 100Ω.

Other feature :


  •  It is mechanically very rigid
  •  It is prone to moisture and heat
  •  It is small in dimensions
  •  Cost of the microphone is the lowest of all other microphones.

Advantages


  •  Very rugged
  •  Small size
  •  Very cheap
  •  Good sensitivity.

Disadvantages


  •  High distortion
  •  Limited frequency response
  •  Not suitable for high fidelity work.

Applications :


  •  Due to limited frequency range, it is useful only in telephones. It is also sometimes used in         portable radio communication sets.

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