Microphonics
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Microphonics (also called microphony) describes the phenomenon where certain components in electronic devices transform mechanical vibrations into an undesired electrical signal (noise). The term is derived by analogy to microphones 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 tubes, 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. Certain models or grades of vacuum tubes were made with thicker internal insulating plates to minimize these effects.[1]
With the advent of solid state electronics (transistors), this major source of microphonics was eliminated but smaller sources still remain. The ceramic EIA Class 2 dielectrics used in high-K capacitors ("Z5U" and "X7R") are piezoelectric and will directly transform mechanical vibration into a voltage in exactly the same fashion as a ceramic microphone [2] 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 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 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 sensors to produce the image, but prior to CCDs, vacuum tubes 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.[3]
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, where no current induction occurs.[1]
See also
References
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- ↑ Robert B. Tomer, Getting the most out of vacuum tubes, Howard W. Sams, Indianapolis, USA 1960, Library of Congress card no. 60-13843, available on the Intenet Archive. Chapters 3
- ↑ [2], Capacitors for reduced sound emissions.
- ↑ http://www.michaeldvd.com.au/Articles/VideoArtefacts/VideoArtefactsMicrophony.html
