what is the Crystal microphone ? Working principle | Construction | Applications

Crystal Microphone

Principle :

A crystal microphone is based on the principle of “Piezo electric effect” which is defined as “Difference of potential between the opposite faces of some crystals is produced when these are subjected to mechanical pressure”. The crystals which shows this effect are Quartz, Tourmaline, Rochelsalt and Ceramic.

Construction

The constructional details of Crystal Microphone are shown in the above fig. a. The crystal is cut along certain planes to form a slice. Metallic foil electrodes are attached to the two surfaces to carry the potential differences to the output terminals. Two thin crystal slices suitably cut are placed in an insulating holder with an air space between them. A large number of such elements are combined and increase an emf. A diaphragm made of aluminium is attached to the crystal surface through a push rod. The whole unit is encases in a protective case. There is a protective cover over the diaphragm.

Working

When there is a sound waves of compression it compresses the crystal. In case of rare fraction converse takes place and the crystal is extended and is under tension. Due to this compression and extension a varying potential difference is generated which is proportional to the mechanical pressure applied to the crystal by the sound waves.

Specifications


  • Sensitivity = 50 mV ( 26 dB below 1 volt )
  • S/N Ratio = 40 dB
  • Frequency Response: 2 kHz –  8KHz.
  • Distortion = < 1%
  • Directivity = omini directional
  • Output Impedance = 1M ohm.

Applications

These are used for the following purposes -
1. Home recording system
2. Amateur communications
3. Mobile Communications.

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