Peak Freak's guide: Microphones Part 1

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Peak Freak's guide: Microphones Part 1

Postby Peak Freak » 08 Jul 2012 08:57

Microphones: Part 1

At the beginning of a lot of recording chains we have them: Microphones. They are necessary to turn sound waves into electric oscillations. Every microphone has a diaphragm which vibrate mechanically when the sound waves hit them. A transducer then turns this mechanic oscillation into an electric signal.

1 Transducers
Most common for usage in professional audio recordings are electrostatic and electrodynamic transducers. Other transducers, like electromagnetic ones are only seldom used in studios, because of their bad frequency response. These are used in very small microphones or telephones.

1.1 Condenser microphones
These microphones are upscale recording microphones: They record a very clear and natural sound in the whole range of frequencies.
Normally they should be called "electrostatic Microphone" because of their transducer. The diaphragm of one of these transducers is a conductive film, which is only about 1-10 µm thick. This film is affixed to another electrode, with a distance of 5-50 µm. When the film now is vibrating, the capacitance changes.
Both the film and the electrode need a polarization, by applying a DC (clean direct current) of about 40-200 V through a strong resistor. The change of the capacitance between the both electrodes now cause a change of the potential at the resistor: The signal can now be sent through a amplifier inside the microphone.

The size of the diaphragm can vary, however, it has no effect on the maximum sound pressure. Distortions mostly appear not in the capsule, but in the amplifier. Only at extreme intense sound pressures the capsule (the part with the two electrodes) begins to distort. Regardless, the different sized diaphragms are suitable for different purposes. A big diaphragm is for example for close recordings of instruments suitable, but also for voice recordings. Small diaphragms are more suitable for room recordings, or recording a bigger compilation of instruments playing together (aka orchestra).

Condenser microphones need a potential to work (for the amplifier and the electrodes). Batteries are not very suitable though, because they often quit their service. Most common is the usage of Phantom power, you might have seen on some of your hardware a button labeled either "48 V" or "Phantom Power". The potential is send through a symmetric cable, mostly an XLR-Cable, to the microphone, where it can be used. Without this power condenser microphones are not able to work.

A special case are electret-microphones: Their capsules are pre-polarized, so they only need a battery for the amplifier, which is only using a small amount of power. In the end they don't need the external 48V power, and can be used with asymmetric cables. They are also cheaper than normal condenser microphones, but they also have not the same quality of sound. Moreover, they can get "old" and useless, because the polarization is not permanent.

1.2 Electrodynamic microphones
These microphones work with the law of induction. A conductor which moves inside a magnetic field induces a potential.

a) Ribbon microphone
The diaphragm of such a microphone is a small ribbon out of aluminum, 2-4 mm in width and a few cm long. It is clamped between the poles of a permanent magnet and can only oscillate about a few µm. These microphones have a pretty linear frequency response. The quality of the sound is comparable to a condenser microphone. But it is sensitive against wind, commotions and fast movements.

b) Moving coil microphone
In this microphone type, the diaphragm is attached to a moving coil, which is moving in the gap in the middle of a magnet. Because of the high mass of the diaphragm, their frequency response is not linear at all. Moreover, their behavior at impulses isn't that good. Despite these limitations they have a good sound quality, and are often used in studios, on stage or for outdoor recordings. They are more robust then condenser microphones and can record very extreme sound pressures without distortions. Because of their not linear frequency response, most manufacture build them for specific purposes, like only for vocals: the SM58 for example is for example declared as a vocal only microphone.

1.3 Piezoelectric microphones
These microphones use the property of specific crystals, to react on mechanic pressure with a charge shifting. These difference in potential can directly give a signal to a amplifier. Disadvantage of such microphones is the big dependency of the temperature: Changing temperature means a change in frequency response and sensitivity. Therefore they aren't used in studios, only for specific purposes.

2 Directionality
The directionality is defined by the position of the diaphragm, and from which sides it can receive sound pressure. The directionality of a microphone allows us to exclude specific sound sources just by the right positioning of a microphone. Most microphones have a small symbol on it showing their directionality. You can imagine this simple depiction 3 dimensional.
Microphones with omnidirectional characteristic (do I need to go on?) can receive sound from every direction. Most common directionality is the cardioid characteristic.
Basically there are two types of microphones to differ: Pressure microphones and pressure-gradient microphones.

2.1 Pressure microphones
In pressure microphones only one side of the diaphragm receives the sound pressure. The pressure is not dependent to the direction though, so the microphone has an omnidirectional characteristic. However, this directionality is not appliable for every frequency. As long as the microphone is small in comparison with the wavelength the sonic waves can get bend around the microphone. With raising frequency the microphone more and more gets an obstacle for the sound. The directionality gets more and more cardioid with higher frequency.

2.2 Pressure-gradient microphones
Both sides of the diaphragm can receive sound pressure. The difference of the pressure on both sides creates the signal.

a) Pressure-gradient microphones with bidirectional characteristic
An ideal pressure-gradient has a directionality shaped like the number 8. The maximum of pressure difference can be achieved from the direction of 0° and 180°. The signal from the opposite however is also opposite in phase. If the sound comes askance, 90° and/or 270°, there is no difference in pressure and no signal.
The maximum of receivable frequency of such microphones are only about 4-10kHz. To be able to record higher frequencies with pressure-gradients changes in constructions are needed, so that they work as pressure microphones (see 2.1) only at higher frequencies. By this a somewhat linear record spectrum is possible.

b) Pressure-gradient microphones with cardioid characteristic
Most common characteristic. To give such a mic a cardioid characteristic, the way of the sound has to be redirected, so that the difference in pressure is the most from the front (0°) and no difference from behind (180°). Most microphones have vents on the sides. If these are concealed with the hands, the directivity changes and it can lead to instant feedback.
Because the cardioid characteristic ceases when reaching deeper frequencies, most microphones have 2 capsules so a somewhat linear directionality for all frequencies is available.

c) Proximity effect of pressure-gradient microphones
A distance of about 1 m (~ 3 feet) leads to a raising of deeper frequencies. This appearance is called proximity effect. To compensate this effect a lot of pressure-gradient microphones have an inbuilt filter, which can be turned on or of. Surely this effect can also be used for sound purposes, for example vocalists.

2.3 Interference-recievers
For another directionality microphones can be modified with a pipe (shotgun microphones). These mostly have a shotgun characteristic/directional characteristic.

2.4 Microphones with switchable directivity
These microphones have two capsules with cardioid characteristics, one diaphragm on each side of the electrode. The potential of the second capsule can be changed, allowing varieties of the directionality, from omnidirectional to cardiodid (when the second capsule has no potential) and then even to bidirectional (when the second capsule has the opposite polarity).

3 Special microphones

3.1 Stereo microphones
Stereo microphones have two capsules, which are separated from each other. Both have mostly the option to have their directivity switched separately. These microphones are mostly used to make stereo recordings.

3.2 Sphere microphones
These are also microphones for stereo recordings. It also has two separated capsules, which are mostly pressure-gradient receivers (2.2) and built in a sphere with an angle of 180° to each other. These microphones are an attempt to make recordings as close as possible to human hearing.

3.3 Pressure zone microphones
Sometimes it occurs that when recording from a little distance of the sound source, the reflections from the floor make the sound "hollow". Either you can move the microphone near to the sound source, or integrate the microphone into the floor, so that these reflections don't occur anymore. This second principe is the basement for pressure zone microphones. These are mostly flat, and can be simply laid on floors or attached to walls.

4 Additional properties of microphones

4.1 Transmission factor and transmission amount
Both describe the sensitivity of a microphone. Condenser microphones(1.1) have a factor of 5-12 mV/Pa, thats about -66…-58 dB for the transmission amount. Dynamic microphones(1.2) have a factor of 1-3 mV/Pa, leading to a transmission amount of about -80… -70 dB. Condenser microphones need as a result a less gain then Dynamic ones.

4.2 Signal-to-noise ratio
It is the difference between the potential a microphone creates, when no sound is applied, and the potential when a sonic pressure of 1Pa at a frequency of 1kHz is applied. For dynamic microphones this value is not denoted by the manufacturer.

4.3 High maximum sound pressure level
The value of sound pressure, which leads the microphone to distort. For condenser microphones it is around 120-126 dB, for dynamic microphones somewhere above 140dB.

4.4 Impulse behavior
It shows the microphones ability to exactly transfer short loud impulses, like drums or percussion instruments generate. In opposite to the previous parameters, the impulse behavior is not definable objectively. However it is a very important factor for a good recording quality. The different impulse behaviors can make a big different in sound of microphones, when these are overall similar in the other properties.
The mass of the diaphragm is mainly affecting the impulse behavior. A condenser microphone is able to reproduce impulses very natural and clear. Dynamic microphones however give an unclearer signal and oscillate longer, even when the sound already disappeared in real. But this doesn't need to be a disadvantage. This property can be used for a special "sound" of instruments or voices.

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Well, this far with part one of the microphone guide, was a lot of work to describe this all in English (thanks dictionary). However mistakes happen, be it grammar/typos or textual ones. If there are questions, misunderstandings, whatever: Post or pm me.
Next part will be about practical use of microphones.
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