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Introduction A microphone converts air vibrations (acoustic signal) to an electrical signal. First the membrane is set in motion by the air vibration (acoustic signal). The movement or vibration is converted into an electrical signal. How that happens depends on the type of microphone. The better the correspondence between the original air vibration (acoustic signal) and the electrical signal, the better the microphone.(*) The sensitivity of a microphone is specified in electrical voltage per Pascal (pressure), with the unit mV/Pa, 1 Pa corresponding to a sound pressure of 94 dB/SPL.(*) It was briefly touched, the movement or air vibration is converted into an electrical signal, and the greater the similarity the better the microphone. And manufacturers are busy with that. Back to that vibration or air movement. I play a little with those two words. The air movement or air vibration must set the membrane in motion. If that is a system where a magnet or a coil has to move, then it takes a lot of energy to set in motion. There is a certain slowness in it. This is the principle of a dynamic microphone. Because the air has to supply the energy, little comes out of voltage. This microphone is less suitable for our purposes. If you can give that membrane less weight, it can be set in motion more easily. And so systems have come on the market where that membrane is part of an electrical component. We are talking about the capacitor microphones. By the way, it is a misconception that in nature all microphones can be used, many microphones are not able to cope with this extreme environment because they are designed for studio use. This group of capacitor microphones can be divided into three types;
DC-biased capacitor microphone With a DC-biased capacitor microphone, the microphone consists of a plate capacitor in which a side is executed as a membrane sensitive to pressure and a preamplifier that converts that to a usable signal. The plate capacitor is part of a voltage distributor and must be loaded with a very high resistance value and it is also this resistance that ensures the often higher own noise. The capacitor and preamplifier are in the microphone capsule and each have its own power supply. This power supply consists of two parts, one for polarizing the capacitor and one for the preamplifier. Via a high-quality voltage converter, the microphone capsule is powered with a high polarization voltage, often 60 Volts. This whole setup has the disadvantage that the microphone is sensitive to moisture and humid environments. There are manufacturers who do something about this so that a rainstorm no longer has any influence. This type of microphone needs phantom power supply, which may differ from the standard 48Volt.(*) Electret The operation of an electret microphone is the same as that of a condenser microphone, only the polarization voltage is permanently present. The decay of this polarization voltage is very small. Increasingly, the polarization voltage is applied to the rear plate of the capacitor, hence the name "back electret". Because the voltage variations are very small, a chemical substance is applied to one of the plates that increases the electrostatic effect. This allows the electronics to be kept sufficiently simple to be able to produce the microphone at an attractive price. The disadvantage of this construction is a light sound coloration that must be filtered out by the electronics. The electronics in this type of microphone still need power, because it is only the preamplifier, sometimes an AA battery is sufficient, which can be inserted into the housing of the microphone. An electret microphone is also sensitive to moisture. It is therefore advisable to keep the microphone in a dry environment. The microphone is sometimes also called "pre-polarized condenser microphone".(*) RF Capacitor microphone The RF capacitor microphone is not a new invention, although it is often thought that way. The technique has been around since tubes were replaced by transistors. However, there was a problem, direct replacement of the tube with a transistor does not go due to a imbalance between the very high resistance of the capacitor capsule and the much lower input resistance of the transistors. The impedance of a capacitor capsule of 40 pF capacity decreases with an increasing frequency from 132 MΩ at 30 Hz to 199 kΩ at 20 kHz. At very high frequencies, for example at 10 MHz, the impedance is reduced to 398 Ω, a very useful value for steering a transistor. All microphone manufacturers developed different RF circuits. That changed when FETs with properties such as those found in tubes were introduced. All but one of the manufacturers abandoned the RF technique for what it was. The operation of the RF principle is simple, as with other capacitor microphones, sound waves move the membrane of the capacitor capsule and thus change the capacity between the membrane and the fixed electrode (backplate). The occurring capacity variations are not directly converted into audio signals, but modulate a high frequency (radio frequent) signal generated by an oscillator in the microphone. This signal is then immediately demodulated into the microphone and thus gives an audio signal but now with a very low source impedance for controlling a transistor amplifier. There is another important advantage of the RF principle. The low electrical impedance of the capsule provides excellent immunity to harmful effects due to humidity because the leak resistance is much greater than the capsule impedance. The Sennheiser MKH microphones are therefore very suitable for outdoor use. This type of microphone needs phantom power supply, which may differ from the standard 48Volt. Directional characteristic Depending on the construction, a microphone has three basic types of directional characteristics(*);
The directional characteristic is an aspect of the microphone, how that microphone works or is assembled is another. Of course it is possible to record with all these types of microphones, only one microphone does slightly better than the other and why is that so? If you want to make sound recordings in nature, you will soon be confronted with how do I get my sound to be recorded separately from the other unwanted sound. In other words, how do you isolate the source? It may of course be the case that sound of the entire environment is desired, but often it is a more isolated source. There are then two approaches;
What you can or must use is highly dependent on the extent to which you can or must isolate the source from the environment. A parabolic dish generally has the smallest opening angle. The "super-cardioid" or "shotgun" microphone has a slightly larger opening angle. And the more there is less "super", the greater the opening angle. The opening angle is therefore the field that the microphone "hears" or "sees". So if you are not on the ideal line with a parabolic dish for only a few degrees, you will not get the desired result. If you do that with a "super-cardioid" or "shotgun" microphone, you probably won't notice that. Table The tables below contain microphones that are interesting for the use of natural sound recordings. There are more brands and types of microphones whose use for natural sound recordings is not so obvious, but not impossible. The directional characteristics have been brought together in the tables and this is indicated in the title of the table. Apparently microphones may be included in the table which, apart from a different type number, are identical to each other. The differences are then often in the length and/or diameter of the microphone. Arbitrary, but microphones with a sensitivity lower than 20 mV/Pa are not included. The data comes from the manufacturer's specifications; The AT815 is included for reference, which in combination with sE Electronics DM1 has an equivalent sensitivity of 79mV/Pa. The DM1 has a gain of 28dB. Conditions for use is that the microphone itself does not use a phantom power supply and that phantom power of 48V is present for the power supply of the DM1. Battery-powered microphones with a low voltage output can still be used in this way, although this is not stated in so many words in the manual of the DM1. Unfortunately, sE Electronics does not provide an answer for this question. Legend;
Super-cardioid
Omnidirectional
* source https://nl.wikipedia.org/wiki/Microfoon revision April 25, 2021 |