Sennheiser MKH 416 The MKH Story

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 M

Ω

at 20 Hz to 200 k

Ω

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.