RF Condenser Microphones why they are so good in the wet


A question I am often asked when people discover who I work for is “Why are ‘RF condenser’ microphones so good in the wet, being so much less susceptible to humidity problems than the more common ‘AF condenser’ microphones?”

Basically, AF capacitor microphones use the capsule as a capacitor to store charge. With one fixed plate and the other free to vibrate in sympathy with the sound, the capacitance varies, and the charge moves in or out of the capsule accordingly. This is measured by the head preamplifier and an audio signal results. All well and good, but the capsule is inherently in a high impedance circuit (over 1Giga-Ohms) – it has to sit there with stored charge until the diaphragm moves and any changes in the charge are perceived as audio. In a humid atmosphere the stored charge finds it easier to escape on water molecules in the air rather than through the input of the preamplifier, hence noisy and reduced output, and misery all round. The high biasing voltage also attracts dust particles to the diaphragm, reducing its efficiency and linearity.

The RF system (as used in Sennheiser MKH microphones) uses the capsule (a low impedance capsule) in a completely different way: as a tuning capacitor for an RF oscillator – which inherently employs it in a low impedance circuit where a high frequency signal is being passed through the capacitor all the time. Changes in capacitance (caused by sound moving the diaphragm) alter the resonant frequency of the circuit (circa 8MHz) and so its frequency becomes proportional to the audio signal. A simple RF demodulator restores the output to a conventional audio signal. More complex and sophisticated (but still very rugged), this system is highly immune to the effects of humidity and is thus the preferred design to be used out of doors (or when moving from outside to inside on a cold day!).

When condenser microphones were first developed only valve (vacuum tube) technology was available as the active element of the required pre-amplifier / impedance converter. It was simply not a realistic proposition to construct an RF oscillator and demodulator using valve technology if it had to be built into the body of a practical microphone. Thus all of the early condenser microphones were conventional AF condensers. However, by the 1960s transistors had become available...

Sennheiser had only produced dynamic microphones up until that time; but with the advent of the transistor the company began to research ways of producing a condenser microphone using these compact transistors. The problem was that transistors have to operate in relatively low-impedance circuits, but an AF condenser microphone requires a high-impedance environment, which can only be achieved using either a valve or a field-effect transistor (FET). However, as FETs did not exist at the time, Sennheiser’s R&D efforts went into finding an alternative way of producing a condenser microphone using transistors and low impedance circuitry. The solution was the development of the RF Condenser principle.

I should say that at this time other microphone manufacturers were also researching the feasibility of making condenser microphones using RF condenser technology. However, because the field-effect transistor came along very quickly after the bipolar transistor most manufacturers quickly adopted these newer high-impedance devices to produce compact AF condenser microphones. Essentially, they were able to take their existing amplifier circuits, replace the valve with an FET, and immediately produce a condenser microphone with solid-state circuitry. These companies saw no need to spend further time and money developing the technology necessary to manufacture RF condenser microphones when the FET already provided the solution they required. This left only Sennheiser to pursue the RF technology.

As Sennheiser had no history of producing AF condensers they were not bound to existing design traditions, and they could see many fundamental advantages in the RF concept, including the immunity to the effects of humidity, lower self-noise (a valve or FET has higher random noise, especially at low frequencies), and the fact that the RF demodulator circuitry lends itself to generating a balanced audio output by simpler means than having to bolt a large, heavy and expensive transformer onto the back end. It is even simpler than the electronic quasi-balancing circuits used in many modern microphones.
MZH 8000
MZH 8000

Tags: iss025 | sennheiser | microphone | condenser microphone | mkh microphone | fet | N/A
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