Microphone

History

The invention of the microphone was determining in the development of the first telephone systems. Emile Berliner invented the first microphone the March 4th 1877, but it is with Alexander Graham Bell that returns the invention of the first microphone really usable. Most of the first developments of the microphones was carried out by the Laboratoires Beautiful.

Technology of the microphone

The microphone is a transducer electroacoustic, following the example animal and human ear: it translates a sound wave into electrical signal using a moving part, the diaphragm or membrane, that the sound waves come to excite (the equivalent in the ear is the tympanum). By a device which depends on the technology of the microphone, these Oscillation S mechanics is converted into a variable electric tension (comparable with the signal of the auditive nerve). This electric tension is forwarded to the system of amplification, preamplification or recording to which the microphone is connected.

As it is known as higher, the moving part of the microphone is arranged according to technology that this one uses: a dynamic microphone comprises a diaphragm fixed at a magnetized mobile reel. The oscillations of the diaphragm are thus equal to that of the reel, whose signal emitted by Inductance will be the aural signal. The professionals of the sound tend to banish the dynamic microphones because of their sound reproduction considered to be too round and dull, in spite of the appearance in the Eighties of magnets to néodymium fidélisant the band-width more.

The electrostatic microphones, although having the defect to saturate with pressure levels acoustics lower than those supported by the dynamic microphones, are largely voted by plebiscite by the professionals because of their fidelity of reproduction. The membrane is not fixed at a winding, but floating, is separated from a plate electrically charged by vacuum. The interior face of the membrane being powdered with a fine layer of gold, a current is created between this one and the plate in the form of static electricity: it is about the aural signal. The combination of gold, metal to the unequalled conductance, and of this technology, make electrostatic microphone a reference as regards transitory answer and band-width. Such characteristics require a phantom food, to provide in simplex or duplex, going from 11 to 52 Vcc, thus named because the generated tension is led via the same channel as the aural signal in connector industry XLR. Moreover, the electrostatic microphones are equipped with electronic preamplifiers with condensers, transistors or lamps, because they often comprise options of treatment of the signal a such modulator of directivity, an attenuator of low frequencies or a limiting device of volume.

A fabric or a grid generally protects the moving part of the microphone, in order to prevent that it is not damaged by a direct contact. One can also have an additional protection against the Vent, in particular in the form of a cap (foam out of plastic) or of a anti-pop filter to attenuate the explosive consonants “p”, “B”, “T” and “D”.

Directivity is an essential characteristic of the microphone.

All the microphones do not collect the sound in the same way. According to the tasks to be achieved, the optimal output comes from directivity. Some perceive the sounds of all the environment and others endeavor to determine distant sound sources. Principal directivities are omnidirectional, ardioid or supercardioïde and bidirectional.

The polar diagram of a microphone represents the limits of the sensitivity of the microphones in space. A dial graduated in degree of 360 units and an ordinate expressed in decibels will indicate to you the space perceived by the microphone according to various frequencies. By analyzing the directional diagram, one realizes that the sound recording can be pre-equalized without resorting to the console of mixing, according to the provision of the microphone vis-a-vis the sound source. In general, directivity applies as well as possible when the diaphragm is perpendicular to the sound source.

Directivity

The directivity of a microphone characterizes its sensitivity according to the source of the sound, according to its central axis. In the table according to the microphone is placed vertically, its end being represented by the red point, and one traces the lines which create the same level of signal at exit of the microphone if one moves a sound source of constant intensity there.

Omnidirectional : no sound source is privileged. The microphone collects the uniform sound of way, in a theoretically perfect sphere. Used to record sounds of environment, the omnidirectional microphone perceives the sounds on 360°, i.e. it collects all the environment. It receives all the sound sources and resonances of this one. It is thus desirable that acoustics of the room lends itself to the recording. It is however less sensitive to the high frequencies coming by its with dimensions and its base camp that in frontal attack. If it is equipped with a broad capsule, one could say that it is practically directional in the high frequencies. It offers very good recordings on whole of chorus, or an instrument soloist with the realistic sound.

Ardioid : directivity forwards, privileges the sound sources placed in front of the microphone. Used for the song, the catch of instruments, the one-way microphone is most widespread. The appearance of its directional diagram the fact of calling Ardioid (in the shape of heart). Although the sounds coming from with dimensions is always collected than the frontals, its design is based on differences in acoustic pressure between the front faces and back. By this calculation, it perceives the sounds in front of the membrane and rejects those coming from behind. Thus the sound is slightly less less realistic than the omnidirectional one. Moreover, they are sensitive to the acoustic pressure. The generated effect is that of a feeling of proximity. It increases also the averages and low frequencies as the distance between the source and the capsule decreases. It is used when sounds except axis are to be avoided, case of reverberation or proximity of other instruments. So the micro ardioids are usually used in wiring for sound.

Hypercardioïde : similar to the ardioid, with a zone before a little narrower and a small back lobe.

Canon : strong directivity forwards, directivitée the ultra ardioid allowing to tighten the collected sound beam. Used on television to record dialogs or with the cinema, and to collect particular sounds in an natural environment. Directivity is here ultra ardioid or and present a field of sensitivity even narrower. The capsule is placed at the bottom of a tubular structure complexes acting as networks of interferences. By their aspect, one calls them also micro rifles or guns. The microphones ultra ardioids do not answer however the low frequencies.

Bidirectional or directivity in 8 : two identical spheres, with before and with the back. Characteristic of the microphones with ribbon. the bidirectional microphone is used only for particular applications such as the sound recording stereophonic during an interview or techniques of studio of recording advanced. This microphone has two capsules assembled in opposition. Each capsule can answer different directivities. However, it is then necessary to count on a Mid-Side matrix (M/S) so that the signal is converted into stereophony. M/s processing or treatment m/s can also convert into stereo signal three microphonic signals by the addition of a third microphone.

According to the type of use for which the microphone is intended (sound environment, Chant, Percussion instrument, Instrument wind, sounds watery…) and according to the conditions of use (studio, scene, in outside…), various types of microphones are found. Most current are: microphone with hand, microphone of studio, micro tie, microphone of surface, micro gun, microphone for instrument.

There does not exist a microphone able to make an optimal recording in all the situations: to each situation of sound recording a microphone corresponds, whose specialized manufacturers propose many models. The principal characteristics of a microphone are thus:

Its standard (with hand, tie,…)
Its technology (dynamic, electrostatic,…)
Its directivity (omnidirectional, ardioid,…)
Its characteristic electroacoustic (sensitivity, maximum acoustic pressure,…)

Technologies of microphone

Four major technologies are presented here. There is the different one, such as for example the Microphone with ribbon, relatively rare, or in Carbone which are less to see more used nowadays, they are the first microphones of the stations radios such as the BBC.

Dynamic microphone with mobile reel

It comprises a diaphragm fixed at a winds mobile magnetized. The latter will become the seat of an induced Running while moving compared to a Aimant fixes, its oscillations being equal to that of the diaphragm. The signal emitted by inductance will be the aural signal. The professionals of the sound tend to banish the dynamic microphones because of their sound reproduction considered to be too round and dull, in spite of the appearance in the Eighties of magnets to the Néodyme fidélisant the band-width more.

Advantages: robustness, passivity (not of external food nor of electronics), capacity to manage strong acoustic pressures.
Disadvantages: miss smoothness in the acute ones making it inapt to take the sound of stamp complexes: cords, acoustic Guitar, cymbals, etc
Some models of references: Shure SM-57, a standard for the resumption of instrument (in particular the clear case and the electric guitar) and Shure SM-58 for the voice (Micro used inter alia by Kennedy at the time of the meeting, Mick Jagger on Voodoo Lounge , Kurt Cobain on Bleach etc). It is interesting to know that these two microphones are identical to the level of construction and that it is only one curved different from equalization which differentiate them. Their higher versions, the BETA57 and BETA58, enjoy a less notoriety, in spite of a definitely higher quality.

Electrostatic condenser microphone

The electrostatic microphones, although having the defect to be sensitive to handling and to saturate on levels with acoustic Pressure lower than those supported by the dynamic microphones, are largely voted by plebiscite by the professionals because of their fidelity of reproduction. The membrane is not fixed at a winding, but floating, is separated from a plate electrically charged by Vide. The interior face of the membrane being powdered with a fine layer of Gold, metal very conducting, a current is created between this one and the plate in the form of static electricity: it is about the aural signal itself. The technology of the electrostatic microphone is a reference as regards transitory answer and band-width. Such characteristics require a phantom Alimentation, to provide in Duplex, going from 11 to 52 Vcc (standardized to 48 Vcc), thus named because the generated tension is conveyed via the same channel as the aural signal in connector industry XLR. Moreover, the electrostatic microphones are equipped with electronic preamplifiers on floors of Condensateur S, Transistor S or Lampe S, because they often comprise options of treatment of the signal a such modulator of directivity, an attenuator of low frequencies or a limiting device of volume.

Advantages: sensitivity, definition.
Disadvantages: brittleness, need for a food external, forced employment, inapt to take again too high acoustic pressures. Sensitive to handling, it is generally fixed on a mounting at suspension made elastic wire, generally in zigzag, intended to absorb the shocks and the vibrations. It is very rare that it is used as microphone with hand, except certain models which incorporate a suspension intern.
Some models of reference: Neumann U87ai and U89i, Shure SM81 and KSM44, AKG C3000 and C414.

Electrostatic microphone with électret

The microphone with électret is in its principle close to the condenser microphone but has the effect of having a component to permanent polarization: the électret. The problem, it is that the load of polarization decreases in time, which results in a loss of sensitivity of the microphone with the passing of years.

Generally, a pile food of the microphone with électret is necessary for the field-effect transistor supply transformer aerial matching placed immediately behind the capsule. Indeed the very high impedance of this one is incompatible with the low entry or average impedance (200 Ohm S or 47kohms) of the apparatuses current ized Transistor. Moreover this direct connection would be prone to many parasitic inductions and a fall acute the very strong one on long cable.

Cetains models current semi-professionals use indifferently an internal pile (1.5 volt) or the phantom Alimentation normally planned for the condenser microphones (48 volts on the tables of mixing) thanks to a switch. This adds to the risk of confusion between 2 technologies “électret” and “electrostatics pure”

Easily able to be miniaturized, the microphone with electret is very much used in the audio-visual field (micro tie, micro helmet, etc) where one appreciates it for his report/ratio cuts/sensitivity. The best models even manage to compete with certain electrostatic microphones in term of sensitivity.

FOOT-NOTE BUCKET: The current électrets profit from a construction mitigating this annoying limited life expectancy that the électret knows since the Seventies.

Advantages: possibility of miniaturization extreme, sensitivity.
Disadvantages: diminution of the sensitivity to the wire of time.
Some models of references: AKG C1000, Shure KSM32, Grinds Videomic, Sony ECM.

Magnetic microphone

It is used mainly on the low electric guitars, the electric pianos or the electric violins, to collect the vibration of the metal cords. There exist several types of micro guitar (see guitar). The latter are selected according to the instrument, of the style of played music, and the sound coloring which one wishes to have. In the beginning, the microphone was with simple winding, known as individual coil , being composed of a Aimant surrounded by a winds. The Magnetic field of the magnet crosses in particular the reel, which is subjected to the variations of this field induced by the cords moving - they play the part of a mobile diaphragm which varies the counter electromotive force traversing the reel. To the reel two electric wires are in general connected: that which will convey the sound in the shape of electrical signals is called the “hot spot”, the other is quite simply connected to the mass.

The problem of the simple microphones is that it génénèrent parasitic sounds. This problem was solved in the years 1950 by the guitarist the Paul, employed by the Gibson firm, by using two simple microphones whose polarities were reversed: it is the double microphone also called Humbucker .

The micro doubles can be:

splittés (one uses one of two windings to give a simple sonority of microphone) on certain guitars.
appairés, in series or parallel, phase or opposition of phase, according to the electric assembly installed on the guitar, thus creating additional combinations of sonorities.

There exists also micro the guitar " actifs" mainly of the mark EMG, Inc which have many advantages in particular on scene and in studio.

For a few years, rests integrating of the piezoelectric sensors (those of the guitars electroacoustics) have made their appearance, which one can combine with the microphones already present on the guitar.

Some photographs of microphones

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