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Microphonics describes the phenomenon where certain components in electronic
Electronics is the branch of science, engineering and technology that deals with electrical circuits involving active electrical components such as vacuum tubes, transistors, diodes and integrated circuits, and associated passive interconnection technologies...

 devices transform mechanical vibration
Vibration refers to mechanical oscillations about an equilibrium point. The oscillations may be periodic such as the motion of a pendulum or random such as the movement of a tire on a gravel road.Vibration is occasionally "desirable"...

s into an undesired electrical signal (noise). The term is derived by analogy to microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...

s where that behavior is inherent in the design, while with modern electronics it is sometimes an intentionally added effect but usually undesired.

When electronic equipment was built using vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

s, microphonics were often a serious design problem. The charged elements in the vacuum tubes can vibrate and the motion would change the distance between the elements, producing charge flows in and out of the tube in a manner identical to a capacitor microphone. A system sufficiently susceptible to microphonics could experience feedback
Audio feedback
Audio feedback is a special kind of positive feedback which occurs when a sound loop exists between an audio input and an audio output...

. Certain models or grades of vacuum tubes were made with thicker internal insulating plates to minimize these effects.

With the advent of solid state electronics (transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...

s), this major source of microphonics was eliminated but smaller sources still remain. The ceramic
A ceramic is an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous...

 EIA Class 2 dielectric
EIA Class 2 dielectric
The EIA Class 2 dielectric materials are ceramic dielectric materials used in ceramic capacitors.The EIA Class 2 dielectrics in general are usually based on formulas with high content of barium titanate , possibly mixed with other dielectric electroceramics. Due to its piezoelectric properties,...

s used in high-K capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...

s ("Z5U" and "X7R") are piezoelectric
Piezoelectricity is the charge which accumulates in certain solid materials in response to applied mechanical stress. The word piezoelectricity means electricity resulting from pressure...

 and will directly transform mechanical vibration into a voltage in exactly the same fashion as a ceramic microphone does. Film capacitors using soft dielectric materials can also be microphonic due to vibrational energy physically moving the plates of the capacitor. Wiring and cables can also exhibit microphonics as charged conductors move around, and various materials can develop triboelectric ("static") charges that couple to the electronic circuits. Glass capacitors, while quite expensive, are essentially nonmicrophonic.

The sound of guitar amplifiers that incorporate the electronic chassis
A chassis consists of an internal framework that supports a man-made object. It is analogous to an animal's skeleton. An example of a chassis is the underpart of a motor vehicle, consisting of the frame with the wheels and machinery.- Vehicles :In the case of vehicles, the term chassis means the...

 into the same cabinet as the speaker are susceptible to microphonics. Though a guitar amplifier's microphonics distortion is sometimes appreciated as part of the "special sound" of a guitar amplifier, a faulty vacuum tube
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...

 or other component can cause out of control feedback. Unwanted microphonics-related audible distortions can often be alleviated by using commercially available vacuum tube dampers.

The term may also be used to describe a video artefact common in older video cameras. Today's cameras use CCD
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...

 sensors to produce the image, but prior to CCDs, vacuum tubes
Video camera tube
In older video cameras, before the mid to late 1980s, a video camera tube or pickup tube was used instead of a charge-coupled device for converting an optical image into an electrical signal. Several types were in use from the 1930s to the 1980s...

 performed this task. Loud noises in the studio, such as rock bands or gunshot sounds would cause the tubes to vibrate. This would produce a characteristic and undesirable horizontal banding in the image. CCDs are not susceptible to this problem.

Other uses

The term is sometimes misused to describe sounds transmitted to the ear physically by mechanical vibrations or knocks on the wires of a headphone or in-ear monitor
In-ear monitor
In-ear monitors are devices used by musicians, audio engineers and audiophiles to listen to music or to hear a custom crafted mix of vocals and stage instrumentation for live performance or recording studio mixing...

, where no current induction occurs.http://www.macworld.com/article/55152/2007/01/canalphones.html