Microphony
Microphony (formerly microphony ) is the property of electronic components to react to mechanical stimulation ( airborne noise , structure-borne noise ) by changing their electrical parameters.
In a narrower sense, the term is understood to be an undesirable effect in sound engineering in which non-microphones work unintentionally as sound transducers and thus cause interference. One also speaks of the "microphonic effect" or "ringing" when a conversion of sound takes place at non-acoustic transducers. The English term for this is "ringing". The microphonic effect is observed quite often with electron tubes ; Here mechanical vibrations of the system housed in the vacuum lead to clearly audible bell-like interference sounds in the output signal. The same applies to the record-stylus system . Other components such as capacitors , coils or transformers also have this effect, albeit to a far lesser extent. Changes in capacitance caused by stepping on cables are also counted as microphones. Like the distortion behavior , the distortion or the noise as well as crackling and crackling, microphony is one of the noise generated or the disturbances that occur.
Mechanisms of Action
Electron tubes
In the case of electron tubes , the control grid is located in the electron-rich space charge zone near the cathode. Moving the grid in relation to the cathode changes the field conditions there and directly influences the electron flow to the anode. With LF amplifiers, this leads directly to an LF signal at the output.
For the pre-amplification of LF signals, low-microphone tubes with a particularly stiff control grid are offered. For example, the AC701, a triode used as an impedance converter in condenser microphones, is specified as microphone-poor.
In principle, all the other electrodes are also responsible for microphones, but the effect is most pronounced with the control grid, especially when it is a tube with high gain .
The change in capacitance of the control grid / cathode, which also takes place in the event of a vibration, plays a role particularly in oscillators in FM receivers and transmitters.
Capacitors
With capacitors , depending on the type, two effects appear: with air capacitors ( rotary capacitors , air trimmers) the capacitance changes due to changes in distance. This can e.g. B. lead to a frequency modulation in an oscillator . If it is the oscillator of an FM receiver or transmitter, the microphony can be heard directly as an audio signal. In ceramic capacitors with a ferroelectric dielectric , microphony occurs due to the piezo effect . Alternating mechanical stresses lead to an alternating voltage on the capacitor. Therefore, such capacitors are avoided as coupling capacitors in LF preamplifiers.
Do the washing up
Inductances , in particular air-core coils that are used in FM receiver and transmitter oscillators, lead to a frequency modulation when the shape changes due to vibration. They are therefore often mechanically dampened by wax or a piece of foam.
Depending on the material, inductances and transformers with a core can cause interference voltages due to the reverse effect of magnetostriction .
electric wire
Microphone cables show microphony in the form of crackling and noise when moving. This is due to the loose contact between the individual shielding wires or to the metal covering.
remedy
Apart from the use of low-microphone components, a remedy can be found by damping the relevant components or the entire device. This is achieved with soft suspension, spring or rubber connections or feet.
Incorrect use of the word microphony
To some extent, the term microphonic falsely microphone designated species, microphone converter principles or microphone arrays. The correct terms for this are microphone technology or miking . A recording with several microphones is called polymicrofoning .
Microphones, on the other hand, interfere with microphone recordings and must be avoided.
literature
- Telefunken: Tube and Semiconductor Pocket Book, 1962
- Telefunken: laboratory book for development, workshop and service, 1962, publisher's edition of the laboratory book for tubes and semiconductors, vol. 2 - published by Telefunken GmbH, Ulm / Donau