For more than a century, our microphones have changed a lot! Do you know how to recognize the different types of microphones? Discover the microphone history through this article.
The carbon microphone
Through the years, and even ages, we have seen many microphone types. The oldest type is the carbon microphone.
The carbon microphone uses a capsule with carbon granules pressed between two metal plates. The sound vibrations cause one of the thin metal plates to vibrate and presses the carbon granules more or less together.
This will cause a change in resistance between the two metal plates. A disadvantage is that there is always a power source needed to operate a carbon microphone since the carbon microphone doesn’t produce an electrical signal on its own.
The quality of a carbon microphone is poor, but despite the poor quality, the carbon microphone had been in use in large quantities as the microphone in a telephone. In the early years of radio broadcasting, the only microphone that was available was the carbon microphone.
The crystal microphone
Another microphone type that is not very common nowadays, is the crystal microphone.
A crystal microphone uses the phenomenon of piezoelectricity,- the ability of some materials to produce a voltage when subjected to pressure-, to generate an electrical signal that is proportional with the audio vibrations in the air.
The quality of a crystal microphone is usually a lot better than a carbon microphone. To obtain the best quality, it is needed to terminate a crystal microphone with a high impedance.
This, of course, may cause a problem, because high impedance microphone cables can not have a length of tenths of meters, because of the capacity of the cable used.
The cable capacity will cause the loss of high frequencies in a high impedance circuit.
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The ribbon microphone
Another well know microphone type is the ribbon microphone.
A very thin strip of metal foil can move between the poles of a magnet, and this will generate a very small electrical signal. Although the quality of a ribbon microphone can be very good, ribbon microphones are fragile. An unexpected air blast on the ribbon can damage it.
Since the signal generating element in a ribbon microphone is only a single strip of metal foil, the output signal from the ribbon ‘motor’(as it is usually called) is very low and the ribbon element represents a very low electrical impedance. Therefore, usually, a step-up transformer is used in a ribbon microphone, to bring the output signal to a higher level and a more standard impedance, in general, 200 ohms. As a result of the construction, most ribbon microphones have a figure-8 directional characteristic, which may not be ideal in many recording situations.
And even with the step-up transformer inside the ribbon microphone, the output is still low, so you will need a lot of gain to bring the microphone signal to a level that can be used.
The dynamic microphone
Very popular, even nowadays, are dynamic microphones.
A dynamic microphone uses a membrane with an attached coil of very thin wire, that can move over a permanent magnet. Since the mass of the coil is very low, a good dynamic microphone can capture the whole audio range without a problem.
Signal levels are higher than from a ribbon microphone and the capsules can be made in such a way that the impedance is 150 or 200 ohms, so no transformer is needed.
Another important property of a dynamic microphone is that they can be very robust and will (with a bit of luck) keep working if they are incidentally dropped…
The condenser microphone
However, if we want the best quality, we want a condenser microphone.
A condenser is technically: “two conductors separated by an insulator”.
In the case of a condenser microphone capsule, one of the conductors is a perforated disc of (usually) brass. Very close to this perforated backplate is a very thin sheet of PVC, polyester or mylar. This non-conductive substrate has a very thin layer of gold or nickel on it, to form the second ‘conductor’.
If the membrane of a condenser capsule moves close to the backplate, the electrical capacity gets higher and on the contrary, if is mover further away from the backplate, the electrical capacity will get lower. A condenser microphone will never produce any signal on its own. Any condenser microphone will need some electronics, to convert the variations in capacity.
To start with, we have to bring a certain charge on the condenser capsule, to ‘polarize’ it. (An exception is the electret microphone; this type has a permanent charge ‘frozen’ into the capsule and does not need an external polarization voltage.)
From a formula, it can be seen that a change in capacity will result in an electrical output signal.
The formula is Q = C * U. If the charge Q in Coulombs stays the same and the capacity (C) will change, also the voltage (U) will change. And that is what we need from a microphone.
Unfortunately, at the moment we try to load the signal of the condenser capsule, most of its output is gone. This means that we can only use an amplifier with an extremely high impedance, to use the signal from the condenser capsule.
In the past, a tube amplifier was used to convert the capsule signal, but nowadays field effect transistors are used because of their high input impedance.
Because the moving mass in a condenser capsule is extremely low, high frequencies can be captured without a problem. Because the thin membrane can follow air movements very quick, the transient response of a condenser microphone is excellent. There are many types of condenser microphone capsules, we will investigate them next time.
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