I'm in Slovenia.
Now that *is* a nice part of the world to be. I'll be round the corner in Slovakia at Christmas.
To answer your questions......
You will find many interesting articles on the
Microphone Data website - their
Library is extremely useful.
Dynamic Mics: - basically a loudspeaker in reverse - a coil of wire wriggling around in a magnetic field and producing a voltage when the diaphragm moves.
Condense mics (AF and RF):AFTo 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 electricity charge.
This charge can be created in one of two ways, but in professional studio mics usually this is 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 is that the capacitor 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 others 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.
RFThe 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 8MHz) passing through the capacitor 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 condenser 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 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...
With the advent of the transistor, microphone companies 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 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.
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.
Another useful advantage of the RF condenser 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 boasts 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 a large number are still in use, and the modern MKH 20 omnidirectional microphone has a frequency response extending down to 12Hz. In fact, with a simple modification it can capture sounds as low as 5Hz – the lower limit is actually determined by the largest physical size of capacitor that can be housed within the microphone body.
I hope this helps - this is, in fact, part of an article I wrote for a magazine a few years back; but I hope it helps in some way to answer you question.
Topic: Microphones 101 (Read 2297 times)