MKH Story

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The MKh STory

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diaphragm

air gap

insulation

backplate

perforation

Sketch of a capacitive transducer

Diaphragm and backplate form a capacitor.

Its capacitance is varied by the diaphragm

excursion. The holes in the backplate provide

the ‘breathing’ of the capsule.

The MKH Story

by Manfred Hibbing

Sennheiser’s professional MKH condenser microphones operate on the radio-frequency

(RF) principle but they are certainly not wireless microphones. This special operating

principle is not obvious from the outside but it clearly distinguishes the MKH microphones

from other condenser microphones and is responsible for their exceptional properties.

High (RF) frequency solves a problem

When the replacement of electronic valves (vacuum tubes) by transistors commenced at the end of the 1950s, this

change was also under consideration for condenser microphones. Reduced size, low supply voltages and low power

consumption were regarded as great benefits, as well as being able to use simpler microphone cables. However,

there was a basic problem: direct replacement of the valve by a transistor was not possible due to the mismatch

between the high impedance of the condenser capsule and the low input impedance of the transistor. Hence, for

optimal matching, the capsule impedance needed to be drastically reduced.

The impedance of a condenser capsule of 40 pF capacitance decreases with

rising frequency from 200 MΩ at 20 Hz to 200 kΩ at 20 kHz. At high (radio)

frequencies, for instance at 10 MHz, the impedance is reduced to 400 Ω

which is a convenient value for driving transistor inputs. So microphone

manufacturers started to develop various RF circuits; but as soon as field-

effect transistors (FETs) with properties similar to valves were available, all

manufacturers but one abandoned the RF technique. At that time Sennheiser

had already solved the crucial problems associated with this technology

and, as a then newcomer, Sennheiser was not bound to existing technical

conceptions but could break new ground.

How does RF microphone technology work? The principle is simple: Sound

waves deflect the diaphragm of the condenser capsule and change the

capacitance between the diaphragm and the nearby back electrode

(backplate). Contrary to the more common low frequency (AF condenser) method, the capacitance variations are

not converted directly into audio signals but modulate a high-frequency (radio-frequency) signal generated by

an oscillator inside the microphone. This signal is then immediately demodulated inside the microphone, thus

recreating the audio signal but with a very low source impedance that is well-suited for driving a transistor

amplifier. Thus an RF condenser microphone is basically comprised of a transmitter and receiver that are directly

wired together. The RF signal is therefore kept inside the microphone; only the audio signal is supplied to its output,

just like all other microphones.

Radio frequency can be modulated in different ways. Most obvious in this case is frequency modulation (FM). If

the resonant circuit of an oscillator is formed by the capsule and a coil, then its frequency is varied directly by

the capsule capacitance variations. For technological reasons Sennheiser preferred the related ‘phase modulation’

because the noise performance of the oscillator could be improved by using a quartz crystal.

Summary of content (10 pages)

PAGE 1
The MKH Story The MKH Story by Manfred Hibbing Sennheiser’s professional MKH condenser microphones operate on the radio-frequency (RF) principle but they are certainly not wireless microphones. This special operating principle is not obvious from the outside but it clearly distinguishes the MKH microphones from other condenser microphones and is responsible for their exceptional properties.
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The MKH Story Floating output and less interference due to AB powering In 1962 Sennheiser launched the omnidirectional MKH 104 as the first RF condenser microphone, which was soon followed by the cardioid MKH 404. These microphones had unbalanced signal outputs. At about the same time 12 V AB powering (“Tonader” T-powering, sometimes called T12 powering) was introduced. It provided a balanced output and shared the two cable leads for both signal conduction and powering.
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The MKH Story No problems with humidity There is another important benefit of the RF principle for practical use. The low electrical impedance of the capsule provides outstanding immunity against detrimental effects due to humidity, because even then the leak resistance is very much larger than the capsule impedance. Thus MKH microphones are well-suited for outdoor use.
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The MKH Story New non-linearity analysis A new challenge for microphone development arose at the beginning of the 1980s when digital recording found its way into the studios, and the LP was, over time, replaced by the CD. Now the music connoisseur, for the first time, could hear the filigrees of sound in the same way that the recording engineer could.
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The MKH Story Push-pull pulls distortion down Theoretical and practical investigations revealed that the source of the distortions is located in the narrow air gap between the diaphragm and the backplate of the condenser capsule. The viscosity of the air trapped inside this gap causes frictional forces that impede the diaphragm movement. With the diaphragm moving outwards the air motion is easier, due to the wider gap, than with an inwards motion.
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The MKH Story A new microphone line is born A new microphone line was designed at the beginning of the 1980s to feature identical technical data as far as the differences were not dictated by the specific directional characteristics. Importantly, the frequency response should be flat for all types. The microphones should be neutral, without adding or omitting sound details. The RF circuitry was completely redesigned, now utilising amplitude modulation (AM) instead of phase modulation.
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The MKH Story The bidirectional MKH 30 (figure-of-eight) The omnidirectional microphone as a pure pressure transducer represents one end of the directivity scale. The other end is occupied by the bidirectional characteristic of a pure pressure-gradient microphone. Therefore the development of a bidirectional microphone seemed reasonable. Moreover, the push-pull transducer was predestined for this application due to its symmetrical design.
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The MKH Story The multi-pattern microphone MKH 800 The MKH 80 was launched in 1993 as the first MKH with switchable patterns (omnidirectional, wide cardioid, cardioid, super-cardioid and figure-of-eight). It was replaced in 2000 by the MKH 800 as the first MKH with an extended frequency response of up to more than 50 kHz. Both microphones incorporate a twin capsule with two symmetrical push-pull transducers. A new design avoided the disadvantages of common dual-diaphragm transducers.
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The MKH Story The versatile MKH 800 TWIN It is a well-known experience of sound engineers that microphone polar patterns may turn out during or after recording to be sub-optimal, but changes are then no longer possible. The MKH 800 TWIN, (launched in 2008) offers a smart solution to this problem. The TWIN microphone is technically based on the MKH 800 and comprises the same twin-transducer with its front and back cardioid polar-patterns.
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The MKH Story The new MKH 8000 line The long MKH 8070 rifle microphone was designed for picking up distant sound sources. It was launched in 2011, together with the MKH 8060 short gun microphone. There were many reasons for the development of the new MKH 8000 line, which was launched in 2007 with the MKH 8020 (omni), MKH 8040 (cardioid) and MKH 8050 (super-cardioid) and has been continuously expanded since then. Most important was an unobtrusive design for video productions etc.