Microphones – Measurement of Sound – Applications

A microphone is a transducer that converts sound into an electrical signal. In other words, a microphone is a special type of pressure transducer converting acoustic energy (pressure waves) into electrical energy.

A microphone is nicknamed as mic or mike. The sensitive transducer element of a microphone is called its element or capsule. The sound is first converted to mechanical motion by means of a diaphragm, the motion of which is then converted to an electrical signal.

Types of Microphone

Several different types of microphone are in use, which employs different methods to convert the air pressure variations of a sound wave to an electrical signal.

  1. Capacitive Microphones
  2. Carbon Microphones
  3. Dynamic Microphones
  4. Inductive Microphones
  5. Piezoelectric Microphones

1. Capacitive Microphones

In this type of microphone, a moving diaphragm is used as one plate of the capacitor. A constant charge is maintained by an external voltage source. The changes in capacitance cause a change in the capacitor voltage. An emitter follower circuit is used to eliminate noise. These are accurate instruments but are expensive.  Capacitive Microphones are used for precision measurements. The frequency range is up to 50kHz. The sensitivity is -50dB.

2. Carbon Microphones

In this type of microphone, moving diaphragm is mounted so as to compress together grains of carbon, thus changing the resistance in a circuit. As resistance changes, current is the circuit changes. Thus, current is a function of the sound pressure. Carbon Microphones finds applications in telephones. The frequency range is up to 5kHz. The sensitivity is about 400 dB.

3. Dynamic Microphones

These microphones use the electromagnetic transduction principle. The most common are the dynamic microphone, which uses a coil of wire suspended in a magnetic field. An A.C. voltage is generated when a coil is moved in a magnetic field. A membrane is mechanically linked to a moving coil surrounded by a permanent magnet. The membrane and the coil are suspended so that they can move back and forth. The sound pressure waves force the membrane towards the magnet, moving the coil across the magnetic field. The voltage developed across the coil terminals is a direct result of the motion of the coil, which clearly depends upon the force of pressure waves. The frequency range is up to 20kHz. The sensitivity is about -80 dB.

 4. Inductive Microphones

In this type of microphone, a diaphragm changes if the magnetic properties of certain material changes. The changes in the dimension can be sensed by the displacement transducers. The frequency range lies in the ultrasonic region. The sensitivity is about -100 dB. The application of Inductive Microphones is in underwater transducers.

5. Piezoelectric Microphones

A crystal microphone or piezo microphone uses the phenomenon of piezoelectricity, the ability of some materials to produce a voltage when subjected to pressure to convert vibrations into an electrical signal. The frequency response lies above the ultrasonic region. The sensitivity lies between -50 to -100 dB.

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