A microphone is an acoustic to electric transducer or sensor that detects sound signals and converts them into an electrical signal.
Most of the microphones employ light modulation, piezoelectric generation, capacitance change and electromagnetic induction to produce an electrical voltage signal from mechanical vibration.
Different types of microphones use varying methods to convert energy. However, all types include a diaphragm, which is a thin piece of material that vibrates when it is struck by sound waves. Vibration of the diaphragm causes surrounding components of the microphone to vibrate. Conversion of these vibrations is delivered as an audible signal. The two most commonly used microphones are the dynamic and the variable condenser microphones.
Dynamic microphones are operated based on electromagnetic induction. They are versatile and suitable for general-purpose use. In this type, soundwaves trigger the movement of a thin metallic diaphragm and the wire coil attached to it. A magnet within the microphone in turn produces a magnetic field around the coil, and the coil movement in this field enables the current to flow.
In this way, the dynamic microphones produce sound, only in the reverse direction. They have a simple design with less number of moving parts. They are relatively sturdy and resilient to rough handling. They are best-suited for handling high volume levels, such as amplifiers or musical instruments.
Condenser microphones are operated based on the electrostatic field. They require power from a battery or an external source. The resulting audio signal is stronger than that of dynamic microphone. These microphones are sensitive and responsive than the dynamic microscope, which makes them more suitable for capturing subtle nuances in a sound.
A typical condenser microphone will include a capacitor having two plates with a voltage applied between them.
One of these plates tends to be very light and acts as the diaphragm. The diaphragm starts to vibrate when it is struck by sound waves. As a result, the distance between the two plates changes thereby changing the capacitance.
When the plates are close together when the capacitance increases, a electrical current is generated. When the plates are further apart, capacitance decreases and a discharge current is formed. A capacitor supplies the necessary voltage for the microphone. Shure Incorporated demonstrate this process in detail:
Understanding Mic Specifications - Part I - Operating Principle
Operating principle to a condenser microphone. Video courtesy of Shure Incorporated.
The directionality or polar patterns of the microphone denotes the sensitivity of microphone to sound arriving from different angles around its central axis.
- Omnidirectional pattern – The omnidirectional microphones pick up the sound waves, regardless of its origin. They are convenient to use, and have good frequency response.
- Bi-directional pattern – The bi-directional microphones process sound from the front and back of the trasducing element. Most of ribbon microphones are of this pattern. These microphones, principally do not respond to sound pressure, but only to the change in pressure between front and back.
- Cardoid pattern – This pattern is used for sound reinforcement or recording concerts where audience noise is a major problem. These microphones reject sounds from other directions and reduce pick-up from the side and rear, thereby avoiding feedback from the monitors.
- Shotgun pattern – Shotgun microphones are highly directional, and have less sensitivity to the side and rear than other directional microphones. However, they are extremely sensitive along the main axis. They are restricted to the voice range as the frequency responses of these microphones are extremely poor.
The following are the list of applications of microphones:
- Hearing aids
- Tape recorders and karaoke
- Live and recorded audio engineering
- Radio and television broadcasting
- Speech recognition technology.