When it comes to condenser (capacitor) microphones there are two different technologies involved. These are AF (Audio Frequency) and RF (Radio Frequency).

Conventional Wisdom

To start with I should explain the terms AF and RF in the context of condenser microphones. To put it simply, the normal ‘AF’ or ‘audio frequency’ condenser microphone employs a capsule design which is, in effect, a capacitor that stores a static electric charge. This charge can be created in one of two ways, but in professional studio microphones this is normally generated by a DC polarising voltage applied to the capsule. The amount of charge that can be contained in this capacitor is proportional to various factors, such as the size of the conductive plates (the diaphragm and back plate), and the space between them. These two plates obviously have a fixed size, and the back plate is fixed in position, but the front plate – the diaphragm – is able to vibrate in sympathy with the sound. This vibration back and forth changes the distance between the two plates in direct proportion to the level of the incident sound, and so the value of capacitance varies accordingly. If the capacitance changes, then the size of charge that can be stored changes as well, and a minute current therefore flows in or out of the capsule as that charge ebbs and flows. This current is measured by the head pre-amplifier within the microphone body to generate a more robust audio signal at the microphone’s output.

This is all well and good, and there are a large number of very high-quality AF condenser microphones available from a wide variety of manufacturers. However, this simple system is not without certain inherent problems. One of the most important in the context of this article is that the capsule has to operate in a high-impedance circuit. The charge stored in the capacitor has to be maintained at a constant level until the diaphragm moves in response to incident sound waves, and so the head amplifier must have an extremely high input impedance to avoid drawing current from the capsule. While it is technically possible to achieve this, the stored charge is always looking for other ways to escape and, unfortunately, in a humid atmosphere the stored charge often finds it easier to escape on water molecules in the air. The result is a noisy and reduced output, and misery all round; also, the high biasing voltage, by electrostatic action, attracts dust and smoke particles that can stick to the diaphragm, absorbing moisture, reducing efficiency and linearity and can cause cracking sounds.

Innovative RF Technology

The RF system uses the same basic capacitive capsule design, but in an entirely different way. The principle is to use the capsule as the tuning capacitor in an RF (radio frequency) circuit – where it operates in a low-impedance mode with a high-frequency signal (typically around 8 MHz) passing through the capsule all the time. Changes in the capsule capacitance (caused by sound waves moving the diaphragm) modulates the tuning of an RF oscillator in proportion to the incident sound waves. A simple RF demodulator circuit converts these RF frequency modulations back to a conventional audio signal. This approach is clearly more complex than the simple AF system (although still very rugged and reliable), but the low-impedance nature of the circuitry around the capsule helps to make the system far more tolerant of atmospheric humidity. As a result, this technology has become preferred for microphones intended to be used outdoors, or when moving from outside to inside on a cold day!

When condenser (capacitor) 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 microphones. 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 principle.

AF Traditions

I should say that at this time other microphone manufacturers were also researching the feasibility of making condenser microphones using RF technology. However, because the FET 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 valve amplifier circuits, replace the valve with a 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 RF technology.

As Sennheiser had no history of producing AF microphones, 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.

Another useful advantage of the RF design is that it can, theoretically at least, have a frequency response that extends down to zero Hz. This has been demonstrated with the development of special measurement microphones such as the MKH 110, which boasted a frequency response specified down to 1 Hz. There was also the MKH 110-1 variant which went down to 0.1 Hz! These particular microphones were discontinued many years ago, although quite a number are still in use; the current MKH 8020 omni-directional microphone has a frequency response extending down to 10 Hz – the lower limit is actually determined by the largest physical size of capacitor that can be housed within the microphone body.

Character Or Accuracy

There are, I suppose, two main philosophies in microphone design. One says that a microphone is like a musical instrument with its own particular sonic character. The other says that a microphone should be like an acoustic window, neither adding nor subtracting anything, and able to reproduce the original sound wavefront exactly. Both of these philosophies have validity, but it is the latter that Sennheiser has sought to achieve with its MKH 20 - 70 range of microphones. The latest series, the MKH 8000 microphones, have a natural, very subtle warmth to them, without taking away from the clarity and accuracy.

Digital recording is putting more and more demands on the microphone. Often technical deficiencies, which were previously masked by earlier analogue techniques, can now clearly be heard by the listener using even an average hi-fi system. These deficiencies are mostly inherent noise and distortion caused by non-linearities in the microphone.

If your philosophy says that a microphone should have its own character you may be able to live with some of these deficiencies, and even use them to advantage if their effect is pleasing to your ears. However, if you are striving for the perfect acoustic window, anything that alters the sound waveshape has to be eliminated if at all possible.

Capsule Symmetry

As the diaphragm of a condenser microphone moves in relation to the back-plate, the acoustic impedance is changing continually. The mechanical impedance increases as the diaphragm moves towards the back-plate and decreases as it moves away (Fig.1) – and this lack of symmetry results in quite a large amount of distortion. This is mainly realised as intermodulation distortion, which may become audible as annoying disharmonic effects (Fig.3).

One solution – which, in practice, can only be achieved with RF condenser microphones – is to put a second plate in front of the diaphragm. This is acoustically transparent, but otherwise electrically identical to the back-plate. This arrangement means that whatever the diaphragm’s physical relationship to one plate, it is counteracted by its relationship to the other plate – and the acoustic impedance is thus constant and unchanging. The immediate result is vastly reduced distortion, particularly when relatively large diaphragm movements are incurred, such as when close-miking a sound source (Fig.2).

Figure 3 compares the double-tone distortion of normal AF condenser microphones to the original symmetrical capsule of the MKH 40.

While the older MKH 416 and 816 gun microphones (and the newer MKH 418-S MS-stereo gun mic, which was derived from the 416) use very similar RF techniques, they do not share the symmetrical capsule design of the modern RF condenser microphones. However, as these microphones tend to be used at relatively large distances from the sound source and therefore with lower typical sound pressure levels, the transducer distortions tend to be low anyway.

Level Responses

There is also another important design feature of the symmetrical capsule design. The usual method of maintaining a flat frequency response in a condenser microphone is to use resonators and a highly damped diaphragm. The idea is, in effect, to flatten the response by reducing the output level of the microphone to the level of the lowest common denominator.

In contrast, the method used in the symmetrical capsule design is to only damp the diaphragm lightly. Simple frequency response correction is then integrated into the microphone electronics to give a remarkably flat frequency response, but with a 6 to 10 dB improvement in the overall signal-to-noise ratio (Fig.4).

Inherently, small-diaphragm microphones have a higher self-noise figure than large-diaphragm microphones, but this construction method enables the relatively small-diaphragm symmetrical capsule design to achieve a better noise figure than most current large-diaphragm microphones.

Furthermore, as the capsule is untouched by resonators and excessive damping, the designer is also able to achieve a much more accurate polar pattern, and one that is far more consistent over a wide frequency range. The measured polar responses of Sennheiser’s symmetrical capsule series of microphones are the closest I have seen to theoretical perfection – of any microphone and by any manufacturer (Fig.5).

Put it all together – the RF condenser technology with the symmetrical capsule design – it becomes possible to produce a microphone which has an extremely high immunity to atmospheric moisture, a relatively high output level with very low self-noise, a consistent polar pattern close to the theoretical limits, and a very flat frequency response.