Διαφορά μεταξύ των αναθεωρήσεων του «Μικρόφωνο»

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[[Αρχείο:Microphone U87.jpg|thumb|Ένα πυκνωτικό μικρόφωνο Neumann U87.]]
{{πηγές|9|12|2009}}
 
Το '''μικρόφωνο''' αποτελεί μία διάταξη ηλεκτροακουστικού μορφοτροπέα, που μετατρέπει τα [[ηχητικά κύματα]] σε ηλεκτρικό σήμα. Το πρώτο μικρόφωνο ανακαλύφθηκε το 1876, από τον [[Emile Berliner]]. Τα μικρόφωνα χρησιμοποιούνται σε πολλές εφαρμογές, όπως τηλέφωνα, ακουστικά βαρηκοϊας, συστήματα [[karaoke]], στούντιο ηχογραφήσεων, κινηματογραφικά στούντιο, ραδιοφωνικά & τηλεοπτικά στούντιο, συστήματα αναγνώρισης φωνής, [[Voice over IP|VoIP]], κτλ.
 
== Τύποι μικροφώνων ==
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=== Πυκνωτικά μικρόφωνα ===
[[Αρχείο:Microphone U87.jpg|thumb|Ένα παλαιό μικρόφωνο.]]
[[Image:Oktava319-internal.jpg|thumb|left|upright|Inside the Oktava 319 condenser microphone]]
 
Η λειτουργία του στηρίζεται στις μεταβολές χωρητικότητας ενός ενσωματωμένου [[Πυκνωτής|πυκνωτή]], σύμφωνα με τις μεταβολές της πίεσης που προκαλούνται από τα ηχητικά κύματα. Είναι ο πλέον σύγχρονος και αποδοτικός - από άποψη ποιότητας - τύπος μικροφώνου. Ένα διάφραγμα ενεργεί ως ο ένας οπλισμός του πυκνωτή, η χωρητικότητα του οποίου μεταβάλλεται αναλόγως των ταλαντώσεων. There are two methods of extracting an audio output from the transducer thus formed: DC-biased and radio frequency (RF) or high frequency (HF) condenser microphones. With a DC-biased microphone, the plates are [[Voltage bias|bias]]ed with a fixed charge (''Q''). The [[voltage]] maintained across the capacitor plates changes with the vibrations in the air, according to the capacitance equation (C = Q / V), where Q = charge in [[coulomb]]s, C = capacitance in [[farad]]s and V = potential difference in [[volt]]s. The capacitance of the plates is inversely proportional to the distance between them for a parallel-plate capacitor. (See [[capacitance]] for details.) The assembly of fixed and movable plates is called an "element" or "capsule."
Το '''μικρόφωνο''' είναι συσκευή που μετατρέπει τα [[ηχητικά κύματα]] σε [[ηλεκτρικές ταλαντώσεις]]. H χρησιμότητά του είναι μεγάλη γιατί διαμορφώνει τα ηλεκτρικά σήματα που δέχεται, ανάλογα με την επίδραση των ηχητικών κυμάτων. Οι διαμορφωμένες ηλεκτρικές ταλαντώσεις μεταφέρονται μέσω σύρματος ή κεραίας και μπορούν να μετατραπούν στον αρχικό ήχο.
 
A nearly constant charge is maintained on the capacitor. As the capacitance changes, the charge across the capacitor does change very slightly, but at audible frequencies it is sensibly constant. The capacitance of the capsule (around 5–100 [[Farad|pF]]) and the value of the bias resistor (100 megohms to tens of gigohms) form a filter which is highpass for the audio signal, and lowpass for the bias voltage. Note that the time constant of an [[RC circuit]] equals the product of the resistance and capacitance.
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Within the time-frame of the capacitance change (as much as 50 ms at 20 Hz audio signal), the charge is practically constant and the voltage across the capacitor changes instantaneously to reflect the change in capacitance. The voltage across the capacitor varies above and below the bias voltage. The voltage difference between the bias and the capacitor is seen across the series resistor. The voltage across the resistor is amplified for performance or recording.
* μικρόφωνο άνθρακα: το μικρόφωνο άνθρακα αποτελείται από ένα μεταλλικό σώμα μέσα στο οποίο είναι τοποθετημένο ένα μικρό δοχείο, το οποίο έχει μονωτικές ιδιότητες. Η λειτουργία του μικροφώνου αυτού έχει σχέση με την μεταβολή της ηλεκτρικής αντίστασης των κόκκων του άνθρακα, εξαιτίας της μεταβολής της πίεσης που ασκείται στο διάφραγμα από τα ηχητικά κύματα.
 
RF condenser microphones use a comparatively low RF voltage, generated by a low-noise oscillator. The oscillator may either be amplitude modulated by the capacitance changes produced by the sound waves moving the capsule diaphragm, or the capsule may be part of a [[resonant circuit]] that modulates the frequency of the oscillator signal. Demodulation yields a low-noise audio frequency signal with a very low source impedance. The absence of a high bias voltage permits the use of a diaphragm with looser tension, which may be used to achieve wider frequency response due to higher compliance. The RF biasing process results in a lower electrical impedance capsule, a useful byproduct of which is that RF condenser microphones can be operated in damp weather conditions which could create problems in DC-biased microphones whose insulating surfaces have become contaminated. The [[Sennheiser]] "MKH" series of microphones use the RF biasing technique.
* μικρόφωνο ταινίας: το μικρόφωνο ταινίας αποτελείται από μια λεπτή πτυχωτή ταινία, συνήθως από αλουμίνιο, η οποία μπορεί και πάλλεται ελεύθερα μέσα στο ηλεκτρικό πεδίο που δημιουργεί ένας ισχυρός μαγνήτης. Το μικρόφωνο αυτό έχει κατευθυνόμενη λήψη από δύο αντίθετες κατευθύνσεις και χρησιμοποιείται για ταυτόχρονη εξυπηρέτηση δύο ομιλητών, λόγω της καλής του απόκρισης.
 
Condenser microphones span the range from telephone transmitters through inexpensive karaoke microphones to high-fidelity recording microphones. They generally produce a high-quality audio signal and are now the popular choice in laboratory and studio recording applications. The inherent suitability of this technology is due to the very small mass that must be moved by the incident sound wave, unlike other microphone types which require the sound wave to do more work. They require a power source, provided either via microphone outputs as [[phantom power]] or from a small battery. Power is necessary for establishing the capacitor plate voltage, and is also needed to power the microphone electronics (impedance conversion in the case of electret and DC-polarized microphones, demodulation or detection in the case of RF/HF microphones). Condenser microphones are also available with two diaphragms, the signals from which can be electrically connected such as to provide a range of polar patterns (see below), such as cardioid, omnidirectional and figure-eight. It is also possible to vary the pattern smoothly with some microphones, for example the [[Røde]] NT2000 or CAD M179.
* δυναμικό μικρόφωνο: το δυναμικό μικρόφωνο αποτελείται από έναν ισχυρό μόνιμο μαγνήτη κ' ένα πηνίο τοποθετημένο ανάμεσα στους πόλους του, ώστε να κινείται ελεύθερα. Η λειτουργία του βασίζεται στο φαινόμενο της επαγωγής: όταν ηχητικά κύματα πέφτουν στο διάφραγμα, το πηνίο πάλλεται στο πεδίο του μαγνήτη, τέμνονται οι μαγνητικές γραμμές κι εμφανίζεται στα άκρα του πηνίου επαγωγική τάση. Χρησιμοποιείται κυρίως σε μικροφωνικές εγκαταστάσεις που απαιτούν ιδιαίτερη πιστότητα. Το μικρόφωνο πήρε την ονομασία του από τον Ντέιβιντ Χιουζ, ο οποίος επινόησε μια διάταξη μεταφοράς ήχου που ήταν τόσο ευαίσθητη, που τη θεωρούσε κάτι σαν «μικροσκόπιο ήχου» και την ονόμασε «μικρόφωνο» (microphone).
 
==== Μικρόφωνα Electret ====
* πυκνωτικό μικρόφωνο: Η λειτουργία του στηρίζεται στις μεταβολές χωρητικότητας ενός ενσωματωμένου [[Πυκνωτής|πυκνωτή]], σύμφωνα με τις μεταβολές της πίεσης που προκαλούνται από τα ηχητικά κύματα. Είναι ο πλέον σύγχρονος και αποδοτικός - από άποψη ποιότητας - τύπος μικροφώνου.
{{main|Electret microphone}}
[[Image:US Patent 3118022 - Gerhard M. Sessler James E. West - Bell labs - electroacustic transducer - foil electret condenser microphone 1962 1964 - pages 1-3.png|thumb|left|First patent on foil electret microphone by G. M. Sessler et al. (pages 1 to 3)]]
An electret microphone is a relatively new type of capacitor microphone invented at [[Bell laboratories]] in 1962 by [[Gerhard Sessler]] and [[James Edward Maceo West|Jim West]].<ref>{{cite journal
|first=G.M. |last=Sessler |coauthors= West, J.E.
|title=Self-biased condenser microphone with high capacitance
|journal=Journal of the Acoustical Society of America |volume=34 |year=1962 |pages=1787–1788
|doi=10.1121/1.1909130}}</ref>
The externally-applied charge described above under condenser microphones is replaced by a permanent charge in an electret material. An [[electret]] is a [[ferroelectric]] material that has been permanently [[electric charge|electrically charged]] or ''polarized''. The name comes from ''electr''ostatic and magn''et''; a static charge is embedded in an electret by alignment of the static charges in the material, much the way a magnet is made by aligning the magnetic domains in a piece of iron.
 
Due to their good performance and ease of manufacture, hence low cost, the vast majority of microphones made today are electret microphones; a semiconductor manufacturer<ref>http://www.national.com/nationaledge/dec02/article.html</ref> estimates annual production at over one billion units. Nearly all cell-phone, computer, PDA and headset microphones are electret types. They are used in many applications, from high-quality recording and [[lavalier microphone|lavalier]] use to built-in microphones in small [[sound recording]] devices and telephones. Though electret microphones were once considered low quality, the best ones can now rival traditional condenser microphones in every respect and can even offer the long-term stability and ultra-flat response needed for a measurement microphone. Unlike other capacitor microphones, they require no polarizing voltage, but often contain an integrated [[preamplifier]] which does require power (often incorrectly called polarizing power or bias). This preamplifier is frequently [[phantom power]]ed in sound reinforcement and studio applications. Microphones designed for [[Personal Computer]] (PC) use, sometimes called multimedia microphones, use a stereo 3.5&nbsp;mm plug (though a mono source) with the ring receiving power via a resistor from (normally) a 5&nbsp;V supply in the computer; unfortunately, a number of incompatible dynamic microphones are fitted with 3.5&nbsp;mm plugs too. While few electret microphones rival the best DC-polarized units in terms of noise level, this is not due to any inherent limitation of the electret. Rather, mass production techniques needed to produce microphones cheaply don't lend themselves to the precision needed to produce the highest quality microphones, due to the tight tolerances required in internal dimensions. These tolerances are the same for all condenser microphones, whether the DC, RF or electret technology is used.
 
=== Δυναμικά μικρόφωνα ===
[[Image:Patti Smith performing in Finland, 2007.jpg|thumb|[[Patti Smith]] singing into a [[Shure SM58]] (dynamic cardioid type) microphone]]
Dynamic microphones work via [[electromagnetic induction]]. They are robust, relatively inexpensive and resistant to moisture. This, coupled with their potentially high gain before feedback makes them ideal for on-stage use.
 
Το δυναμικό μικρόφωνο αποτελείται από έναν ισχυρό μόνιμο μαγνήτη και ένα πηνίο τοποθετημένο ανάμεσα στους πόλους του, ώστε να κινείται ελεύθερα. Η λειτουργία του βασίζεται στο φαινόμενο της επαγωγής: όταν ηχητικά κύματα πέφτουν στο διάφραγμα, το πηνίο πάλλεται στο πεδίο του μαγνήτη, τέμνονται οι μαγνητικές γραμμές κι εμφανίζεται στα άκρα του πηνίου επαγωγική τάση (αντίστροφη λειτουργία του ηλεκτροδυναμικού μεγαφώνου). Χρησιμοποιείται κυρίως σε μικροφωνικές εγκαταστάσεις που απαιτούν ιδιαίτερη πιστότητα. Το μικρόφωνο πήρε την ονομασία του από τον Ντέιβιντ Χιουζ, ο οποίος επινόησε μια διάταξη μεταφοράς ήχου που ήταν τόσο ευαίσθητη, που τη θεωρούσε κάτι σαν «μικροσκόπιο ήχου» και την ονόμασε «μικρόφωνο» (microphone). A single dynamic membrane will not respond linearly to all audio frequencies. Some microphones for this reason utilize multiple membranes for the different parts of the audio spectrum and then combine the resulting signals. Combining the multiple signals correctly is difficult and designs that do this are rare and tend to be expensive. There are on the other hand several designs that are more specifically aimed towards isolated parts of the audio spectrum. The [[AKG Acoustics|AKG]] D 112, for example, is designed for bass response rather than treble.<ref>[http://www.akg.com/site/products/powerslave,id,261,pid,261,nodeid,2,_language,EN.html "AKG D 112 - Large-diaphragm dynamic microphone for bass instruments]"</ref> In audio engineering several kinds of microphones are often used at the same time to get the best result.
 
=== Μικρόφωνα ταινίας ===
[[Image:Edmund Lowe fsa 8b06653.jpg|thumb|upright|left|[[Edmund Lowe]] using a ribbon microphone]]
Tο μικρόφωνο ταινίας αποτελείται από μια λεπτή πτυχωτή ταινία, συνήθως από αλουμίνιο, η οποία μπορεί και πάλλεται ελεύθερα μέσα στο ηλεκτρικό πεδίο που δημιουργεί ένας ισχυρός μαγνήτης. Το μικρόφωνο αυτό έχει κατευθυνόμενη λήψη από δύο αντίθετες κατευθύνσεις και χρησιμοποιείται για ταυτόχρονη εξυπηρέτηση δύο ομιλητών, λόγω της καλής του απόκρισης. Ribbon microphones are similar to moving coil microphones in the sense that both produce sound by means of magnetic induction. Basic ribbon microphones detect sound in a [[Microphones#Directionality|bidirectional]] (also called figure-eight) pattern because the ribbon, which is open to sound both front and back, responds to the [[pressure gradient]] rather than the [[Sound#Sound pressure|sound pressure]]. Though the symmetrical front and rear pickup can be a nuisance in normal stereo recording, the high side rejection can be used to advantage by positioning a ribbon microphone horizontally, for example above cymbals, so that the rear lobe picks up only sound from the cymbals. Crossed figure 8, or [[Blumlein pair]], stereo recording is gaining in popularity, and the figure 8 response of a ribbon microphone is ideal for that application.
 
Other directional patterns are produced by enclosing one side of the ribbon in an acoustic trap or baffle, allowing sound to reach only one side. The classic [[RCA Type 77-DX microphone]] has several externally-adjustable positions of the internal baffle, allowing the selection of several response patterns ranging from "Figure-8" to "Unidirectional". Such older ribbon microphones, some of which still give very high quality sound reproduction, were once valued for this reason, but a good low-frequency response could only be obtained if the ribbon was suspended very loosely, and this made them fragile. Modern ribbon materials, including new nanomaterials<ref>{{cite journal
|url=http://www.bizjournals.com/masshightech/stories/2008/02/11/story8.html
|title=Local firms strum the chords of real music innovation
|journal=Mass High Tech: the Journal of New England Technology
|date=February 8, 2008
}}</ref> have now been introduced that eliminate those concerns, and even improve the effective dynamic range of ribbon microphones at low frequencies. Protective wind screens can reduce the danger of damaging a vintage ribbon, and also reduce plosive artifacts in the recording. Properly designed wind screens produce negligible treble attenuation. In common with other classes of dynamic microphone, ribbon microphones don't require [[phantom power]]; in fact, this voltage can damage some older ribbon microphones. Some new modern ribbon microphone designs incorporate a preamplifier and, therefore, do require phantom power, and circuits of modern passive ribbon microphones, ''i.e.'', those without the aforementioned preamplifier, are specifically designed to resist damage to the ribbon and transformer by phantom power. Also there are new ribbon materials available that are immune to wind blasts and phantom power.
 
=== Μικρόφωνο άνθρακα ===
Το μικρόφωνο άνθρακα αποτελείται από ένα μεταλλικό σώμα μέσα στο οποίο είναι τοποθετημένο ένα μικρό μονωμένο δοχείο, που περιέχει κόκκους άνθρακα υπό πίεση. Η λειτουργία του μικροφώνου αυτού έχει σχέση με την μεταβολή της ηλεκτρικής αντίστασης των κόκκων του άνθρακα, εξαιτίας της μεταβολής της πίεσης που ασκείται στο διάφραγμα από τα ηχητικά κύματα. Ο τύπος αυτός χρησιμοποιήθηκε από τους [[Emile_Berliner|Berliner]] και [[Thomas_Edison|Edison]]. A voltage is applied across the metal plates, causing a small current to flow through the carbon. One of the plates, the diaphragm, vibrates in sympathy with incident sound waves, applying a varying pressure to the carbon. The changing pressure deforms the granules, causing the contact area between each pair of adjacent granules to change, and this causes the electrical resistance of the mass of granules to change. The changes in resistance cause a corresponding change in the current flowing through the microphone, producing the electrical signal. Carbon microphones were once commonly used in telephones; they have extremely low-quality sound reproduction and a very limited frequency response range, but are very robust devices. The Boudet Microphone of 1880 using carbon balls was a similar invention like the granule carbon button microphones.<ref>[http://www.machine-history.com/Boudet%20Microphone]Boudet Microphone</ref>
 
 
Unlike other microphone types, the carbon microphone can also be used as a type of amplifier, using a small amount of sound energy to produce a larger amount of electrical energy. Carbon microphones found use as early [[repeater|telephone repeaters]], making long distance phone calls possible in the era before vacuum tubes. These repeaters worked by mechanically coupling a magnetic telephone receiver to a carbon microphone: the faint signal from the receiver was transferred to the microphone, with a resulting stronger electrical signal to send down the line. One illustration of this amplifier effect was the oscillation caused by feedback, resulting in an audible squeal from the old "candlestick" telephone if its earphone was placed near the carbon microphone. The Boudet Microphone of 1881 using carbon balls was a offspring of the powdered carbon button microphones.
 
=== Πιεζοηλεκτρικά μικρόφωνα ===
A crystal microphone uses the phenomenon of [[piezoelectricity]]&nbsp;&mdash; the ability of some materials to produce a voltage when subjected to pressure&nbsp;&mdash; to convert vibrations into an electrical signal. An example of this is [[Rochelle salt]] (potassium sodium tartrate), which is a piezoelectric crystal that works as a transducer, both as a microphone and as a slimline loudspeaker component. Crystal microphones were once commonly supplied with [[vacuum tube]] (valve) equipment, such as domestic tape recorders. Their high output impedance matched the high input impedance (typically about 10&nbsp;[[Ohm|megohms]]) of the vacuum tube input stage well. They were difficult to match to early [[transistor]] equipment, and were quickly supplanted by dynamic microphones for a time, and later small electret condenser devices. The high impedance of the crystal microphone made it very susceptible to handling noise, both from the microphone itself and from the connecting cable.
 
Piezoelectric transducers are often used as [[contact microphone]]s to amplify sound from acoustic musical instruments, to sense drum hits, for triggering electronic samples, and to record sound in challenging environments, such as underwater under high pressure. [[Pick up (music technology)#Piezoelectric pickups|Saddle-mounted pickups]] on [[acoustic guitar]]s are generally piezoelectric devices that contact the strings passing over the saddle. This type of microphone is different from [[Pick up (music technology)#Magnetic pickups|magnetic coil pickups]] commonly visible on typical [[electric guitar]]s, which use magnetic induction, rather than mechanical coupling, to pick up vibration.
 
=== Μικρόφωνα οπτικής ίνας ===
[[Image:Optimic1140 fiber optical microphone for wiki.jpg|thumb|right|The [[Optoacoustics]] 1140 fiber optic microphone]]
 
A fiber optic microphone converts acoustic waves into electrical signals by sensing changes in light intensity, instead of sensing changes in capacitance or magnetic fields as with conventional microphones.<ref>{{cite journal |first=Alexander |last=Paritsky |coauthors= Kots, A. |title=Fiber optic microphone as a realization of fiber optic positioning sensors |journal=Proc. of International Society for Optical Engineering (SPIE) |volume= 3110 |year=1997 |pages=408–409}}</ref><ref>{{US patent reference | number = 6462808 | y = 2002 | m = 10 | d = 08 | inventor = Alexander Paritsky and Alexander Kots | title = Small optical microphone/sensor}}</ref>
 
During operation, light from a laser source travels through an optical fiber to illuminate the surface of a tiny, sound-sensitive reflective diaphragm. Sound causes the diaphragm to vibrate, thereby minutely changing the intensity of the light it reflects. The modulated light is then transmitted over a second optical fiber to a photo detector, which transforms the intensity-modulated light into analog or digital audio for transmission or recording. Fiber optic microphones possess high dynamic and frequency range, similar to the best high fidelity conventional microphones.
 
Fiber optic microphones do not react to or influence any electrical, magnetic, electrostatic or radioactive fields (this is called [[Electromagnetic interference|EMI/RFI]] immunity). The fiber optic microphone design is therefore ideal for use in areas where conventional microphones are ineffective or dangerous, such as inside [[Gas turbine#Industrial gas turbines for electrical generation|industrial turbines]] or in [[magnetic resonance imaging]] (MRI) equipment environments.
 
Fiber optic microphones are robust, resistant to environmental changes in heat and moisture, and can be produced for any directionality or [[impedance matching]]. The distance between the microphone's light source and its photo detector may be up to several kilometers without need for any preamplifier and/or other electrical device, making fiber optic microphones suitable for industrial and surveillance acoustic monitoring.
 
Fiber optic microphones are used in very specific application areas such as for [[infrasound]] monitoring and [[Noise-canceling microphone|noise-canceling]]. They have proven especially useful in medical applications, such as allowing radiologists, staff and patients within the powerful and noisy magnetic field to converse normally, inside the MRI suites as well as in remote control rooms.<ref>{{cite web |url=http://www.rt-image.com/Case_Study_Can_You_Hear_Me_Now_Technology_for_better_communication_in_the_MRI_su/content=9004J05E48B6A686407698724488A0441 |title=Case Study: Can You Hear Me Now? |work=[[rt image]] |publisher=[[Valley Forge Publishing]] |pages=30–31 |accessdate=2009-08-23 }}</ref>) Other uses include industrial equipment monitoring and sensing, audio calibration and measurement, high-fidelity recording and law enforcement.
 
=== Μικρόφωνα Laser ===
[[Laser microphone]]s are often portrayed in movies as spy gadgets. A laser beam is aimed at the surface of a window or other plane surface that is affected by sound. The slight vibrations of this surface displace the returned beam, causing it to trace the sound wave. The vibrating laser spot is then converted back to sound. In a more robust and expensive implementation, the returned light is split and fed to an [[interferometer]], which detects frequency changes due to the [[Doppler effect]]. The former implementation is a tabletop experiment; the latter requires an extremely stable laser and precise optics.
 
A new type of laser microphone is a device that uses a laser beam and smoke or vapor to detect [[sound]] [[vibration]]s in free air. On 25 August 2009, U.S. patent 7,580,533 issued for a Particulate Flow Detection Microphone based on a laser-photocell pair with a moving stream of smoke or vapor in the laser beam's path. Sound pressure waves cause disturbances in the smoke that in turn cause variations in the amount of laser light reaching the photo detector. A prototype of the device was demonstrated at the 127th Audio Engineering Society convention in New York City from 9 through 12 October 2009.
 
=== Μικρόφωνα υγρού ===
{{main|Water microphone}}
Early microphones did not produce intelligible speech, until Alexander Graham Bell made improvements including a variable resistance microphone/transmitter. Bell's liquid transmitter consisted of a metal cup filled with water with a small amount of sulfuric acid added. A sound wave caused the diaphragm to move, forcing a needle to move up and down in the water. The electrical resistance between the wire and the cup was then inversely proportional to the size of the water meniscus around the submerged needle. Elisha Gray filed a [[Patent caveat|caveat]] for a version using a brass rod instead of the needle. Other minor variations and improvements were made to the liquid microphone by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray, and one version was patented by Reginald Fessenden in 1903. These were the first working microphones, but they were not practical for commercial application. The famous first phone conversation between Bell and Watson took place using a liquid microphone.
 
=== Μικρόφωνα MEMS ===
The [[Microelectromechanical systems|MEMS]] (MicroElectrical-Mechanical System) microphone is also called a microphone chip or silicon microphone. The pressure-sensitive diaphragm is etched directly into a silicon chip by MEMS techniques, and is usually accompanied with integrated preamplifier. Most MEMS microphones are variants of the condenser microphone design. Often MEMS microphones have built in analog-to-digital converter (ADC) circuits on the same CMOS chip making the chip a digital microphone and so more readily integrated with modern digital products. Major manufacturers producing MEMS silicon microphones are Wolfson Microelectronics (WM7xxx), Analog Devices, Akustica (AKU200x), Infineon (SMM310 product), Knowles Electronics, Memstech (MSMx), Sonion MEMS, AAC Acoustic Technologies,<ref>{{cite web |url=http://seekingalpha.com/article/157790-mems-microphone-will-be-hurt-by-downturn-in-smartphone-market |title=MEMS Microphone Will Be Hurt by Downturn in Smartphone Market |publisher=[[Seeking Alpha]] |accessdate=2009-08-23 }}</ref> and Omron.<ref>{{cite web |url=http://www.omron.com/media/press/2009/11/c1125.html |title=OMRON to Launch Mass-production and Supply of MEMS Acoustic Sensor Chip -World's first MEMS sensor capable of detecting the lower limit of human audible frequencies- |accessdate=2009-11-24 }}</ref>
 
=== Μεγάφωνα ως μικρόφωνα ===
A [[loudspeaker]], a transducer that turns an electrical signal into sound waves, is the functional opposite of a microphone. Since a conventional speaker is constructed much like a dynamic microphone (with a diaphragm, coil and magnet), speakers can actually work "in reverse" as microphones. The result, though, is a microphone with poor quality, limited frequency response (particularly at the high end), and poor [[sensitivity (electronics)|sensitivity]]. In practical use, speakers are sometimes used as microphones in applications where high quality and sensitivity are not needed such as [[intercom]]s, [[walkie-talkie]]s or [[Xbox Live]] chat peripherals.
 
However, there is at least one other practical application of this principle: Using a medium-size [[woofer]] placed closely in front of a "kick" ([[bass drum]]) in a [[drum set]] to act as a microphone. The use of relatively large speakers to transduce low frequency sound sources, especially in music production, is becoming fairly common. Since a relatively massive membrane is unable to transduce high frequencies, placing a speaker in front of a kick drum is often ideal for reducing cymbal and snare bleed into the kick drum sound. Less commonly, microphones themselves can be used as speakers, almost always as [[tweeter]]s. This is less common, since microphones are not designed to handle the power that speaker components are routinely required to cope with. One instance of such an application was the [[Standard Telephones and Cables|STC]] microphone-derived 4001 super-tweeter, which was successfully used in a number of high quality loudspeaker systems from the late 1960s to the mid-70s. A well-known example of this use was the [[Bowers & Wilkins]] DM2a model.
 
== Capsule design and directivity ==
The inner elements of a microphone are the primary source of differences in directivity. A pressure microphone uses a [[Diaphragm (mechanical device)|diaphragm]] between a fixed internal volume of air and the environment, and responds uniformly to pressure from all directions, so it is said to be omnidirectional. A pressure-gradient microphone uses a diaphragm which is at least partially open on both sides; the pressure difference between the two sides produces its directional characteristics. Other elements such as the external shape of the microphone and external devices such as interference tubes can also alter a microphone's directional response. A pure pressure-gradient microphone is equally sensitive to sounds arriving from front or back, but insensitive to sounds arriving from the side because sound arriving at the front and back at the same time creates no gradient between the two. The characteristic directional pattern of a pure pressure-gradient microphone is like a figure-8. Other polar patterns are derived by creating a capsule that combines these two effects in different ways. The cardioid, for instance, features a partially closed backside, so its response is a combination of pressure and pressure-gradient characteristics.<ref>{{cite web | last = Bartlett | first = Bruce | title = How A Cardioid Microphone Works | url = http://www.prosoundweb.com/install/spotlight/cardioid/cardioidmics.shtml | accessdate = 8/11/2008 }}</ref>
 
== Microphone polar patterns ==
(Microphone facing top of page in diagram, parallel to page):
<gallery heights="80px">
Image:Polar pattern omnidirectional.png|<center>Omnidirectional</center>
Image:Polar pattern subcardioid.png|<center>Subcardioid</center>
Image:Polar pattern cardioid.png|<center>[[Cardioid]]</center>
Image:Polar pattern supercardioid.png|<center>Supercardioid</center>
Image:Polar pattern hypercardioid.png|<center>Hypercardioid</center>
Image:Polar pattern figure eight.png|<center>Bi-directional or Figure of 8</center>
Image:Polar pattern directional.png|<center>Shotgun</center>
</gallery>
 
A microphone's directionality or polar pattern indicates how sensitive it is to sounds arriving at different angles about its central axis. The above polar patterns represent the [[locus (mathematics)|locus]] of points that produce the same signal level output in the microphone if a given [[sound pressure level]] is generated from that point. How the physical body of the microphone is oriented relative to the diagrams depends on the microphone design. For large-membrane microphones such as in the Oktava (pictured above), the upward direction in the polar diagram is usually [[perpendicular]] to the microphone body, commonly known as "side fire" or "side address". For small diaphragm microphones such as the Shure (also pictured above), it usually extends from the axis of the microphone commonly known as "end fire" or "top/end address".<br>
Some microphone designs combine several principles in creating the desired polar pattern. This ranges from shielding (meaning diffraction/dissipation/absorption) by the housing itself to electronically combining dual membranes.
 
===Omnidirectional===
An omnidirectional (or nondirectional) microphone's response is generally considered to be a perfect sphere in three dimensions. In the real world, this is not the case. As with directional microphones, the polar pattern for an "omnidirectional" microphone is a function of frequency. The body of the microphone is not infinitely small and, as a consequence, it tends to get in its own way with respect to sounds arriving from the rear, causing a slight flattening of the polar response. This flattening increases as the diameter of the microphone (assuming it's cylindrical) reaches the wavelength of the frequency in question. Therefore, the smallest diameter microphone will give the best omnidirectional characteristics at high frequencies.
 
The wavelength of sound at 10&nbsp;kHz is little over an inch (3.4&nbsp;cm) so the smallest measuring microphones are often 1/4" (6&nbsp;mm) in diameter, which practically eliminates directionality even up to the highest frequencies. Omnidirectional microphones, unlike cardioids, do not employ resonant cavities as delays, and so can be considered the "purest" microphones in terms of low coloration; they add very little to the original sound. Being pressure-sensitive they can also have a very flat low-frequency response down to 20&nbsp;Hz or below. Pressure-sensitive microphones also respond much less to wind noise than directional (velocity sensitive) microphones.
 
An example of a nondirectional microphone is the round black ''eight ball''.<ref>[http://lloydmicrophoneclassics.com/mic_history.html History & Development of Microphone.] Lloyd Microphone Classics.</ref>
 
===Unidirectional===
A unidirectional microphone is sensitive to sounds from only one direction. The diagram above illustrates a number of these patterns. The microphone faces upwards in each diagram. The sound intensity for a particular frequency is plotted for angles radially from 0 to 360°. (Professional diagrams show these scales and include multiple plots at different frequencies. The diagrams given here provide only an overview of typical pattern shapes, and their names.)
 
===Cardioids===
[[Image:Us664a microphone.jpg|thumb|right|US664A University Sound Dynamic Supercardioid Microphone]]
The most common unidirectional microphone is a [[cardioid]] microphone, so named because the sensitivity pattern is heart-shaped. A hyper-cardioid microphone is similar but with a tighter area of front sensitivity and a smaller lobe of rear sensitivity. A super-cardioid microphone is similar to a hyper-cardioid, except there is more front pickup and less rear pickup. These three patterns are commonly used as vocal or speech microphones, since they are good at rejecting sounds from other directions.
 
A cardioid microphone is effectively a superposition of an omnidirectional and a figure-8 microphone; for sound waves coming from the back, the negative signal from the figure-8 cancels the positive signal from the omnidirectional element, whereas for sound waves coming from the front, the two add to each other. A hypercardioid microphone is similar, but with a slightly larger figure-8 contribution. Since pressure gradient [[transducer]] microphones are directional, putting them very close to the sound source (at distances of a few centimeters) results in a bass boost. This is known as the [[Proximity effect (audio)|proximity effect]]<ref>[http://www.tonmeister.ca/main/textbook/node473.html Proximity Effect.] Geoff Martin, ''Introduction to Sound Recording''.</ref>
 
===Bi-directional===
"Figure 8" or bi-directional microphones receive sound from both the front and back of the element. Most ribbon microphones are of this pattern.
 
===Shotgun===
[[Image:shotgun microphone.jpg|thumb|left|An Audio-Technica shotgun microphone]]
 
"Shotgun" microphones are the most highly directional. They have small lobes of sensitivity to the left, right, and rear but are significantly less sensitive to the side and rear than other directional microphones are. This results from placing the element at the end of a tube with slots cut along the side; wave cancellation eliminates much of the off-axis sound. Due to the narrowness of their sensitivity area, shotgun microphones are commonly used on television and film sets, in stadiums, and for field recording of wildlife.
 
===Boundary or "PZM"===
Several approaches have been developed for effectively using a microphone in less-than-ideal acoustic spaces, which often suffer from excessive reflections from one or more of the surfaces (boundaries) that make up the space. If the microphone is placed in, or in very close proximity to, one of these boundaries, the reflections from that surface are not sensed by the microphone. Initially this was done by placing an ordinary microphone adjacent to the surface, sometimes in a block of acoustically transparent foam. Sound engineers Ed Long and Ron Wickersham developed the concept of placing the diaphgram parallel to and facing the boundary.<ref>({{Cite patent|US|4361736}})</ref> While the patent has expired, "Pressure Zone Microphone" and "PZM" are still active trademarks of [[Crown International]], and the generic term "boundary microphone" is preferred. While a boundary microphone was initially implemented using an omnidirectional element, it is also possible to mount a directional microphone close enough to the surface to gain some of the benefits of this technique while retaining the directional properties of the element. Crown's trademark on this approach is "Phase Coherent Cardioid" or "PCC," but there are other makers who employ this technique as well.
 
== Application-specific designs ==
A [[lavalier microphone]] is made for hands-free operation. These small microphones are worn on the body. Originally, they were held in place with a lanyard worn around the neck, but more often they are fastened to clothing with a clip, pin, tape or magnet. The lavalier cord may be hidden by clothes and either run to an RF transmitter in a pocket or clipped to a belt (for mobile use), or run directly to the mixer (for stationary applications).
 
A [[wireless microphone]] is one in which the artist is not limited by a cable. It usually sends its signal using a small FM radio transmitter to a nearby receiver connected to the sound system, but it can also use infrared light if the transmitter and receiver are within sight of each other.
 
A [[contact microphone]] is designed to pick up vibrations directly from a solid surface or object, as opposed to sound vibrations carried through air. One use for this is to detect sounds of a very low level, such as those from small objects or [[insect]]s. The microphone commonly consists of a magnetic (moving coil) transducer, contact plate and contact pin. The contact plate is placed against the object from which vibrations are to be picked up; the contact pin transfers these vibrations to the coil of the transducer. Contact microphones have been used to pick up the sound of a snail's heartbeat and the footsteps of ants. A portable version of this microphone has recently been developed. A [[throat microphone]] is a variant of the contact microphone, used to pick up speech directly from the throat, around which it is strapped. This allows the device to be used in areas with ambient sounds that would otherwise make the speaker inaudible.
 
A [[parabolic microphone]] uses a [[parabolic reflector]] to collect and focus sound waves onto a microphone receiver, in much the same way that a [[parabolic antenna]] (e.g. [[satellite dish]]) does with radio waves. Typical uses of this microphone, which has unusually focused front sensitivity and can pick up sounds from many meters away, include nature recording, outdoor sporting events, [[eavesdropping]], [[Police|law enforcement]], and even [[espionage]]. Parabolic microphones are not typically used for standard recording applications, because they tend to have poor low-frequency response as a side effect of their design.
 
A stereo microphone integrates two microphones in one unit to produce a stereophonic signal. A stereo microphone is often used for [[broadcast]] applications or [[field recording]] where it would be impractical to configure two separate condenser microphones in a classic X-Y configuration (see [[microphone practice]]) for stereophonic recording. Some such microphones have an adjustable angle of coverage between the two channels.
 
A [[noise-canceling microphone]] is a highly directional design intended for noisy environments. One such use is in [[aircraft]] cockpits where they are normally installed as boom microphones on headsets. Another use is on loud concert stages for vocalists. Many noise-canceling microphones combine signals received from two diaphragms that are in opposite electrical polarity or are processed electronically. In dual diaphragm designs, the main diaphragm is mounted closest to the intended source and the second is positioned farther away from the source so that it can pick up environmental sounds to be subtracted from the main diaphragm's signal. After the two signals have been combined, sounds other than the intended source are greatly reduced, substantially increasing intelligibility. Other noise-canceling designs use one diaphragm that is affected by ports open to the sides and rear of the microphone, with the sum being a 16 dB rejection of sounds that are farther away. One noise-canceling headset design using a single diaphragm has been used prominently by vocal artists such as [[Garth Brooks]] and [[Janet Jackson]].<ref>[http://www.crownaudio.com/pdf/mics/136368.pdf Crown Audio. Tech Made Simple. ''The Crown Differoid Microphone'']</ref> A few noise-canceling microphones are throat microphones.
 
== Connectors ==
[[Image:Mic-IEC-Symbol.svg|right|thumb|[[Electronic symbol]] for a microphone.]]
 
The most common connectors used by microphones are:
*Male [[XLR connector]] on professional microphones
*¼&nbsp;inch (sometimes referred to as 6.5&nbsp;mm) [[jack plug]] also known as 1/4&nbsp;inch [[TRS connector]] on less expensive consumer microphones. Many consumer microphones use an unbalanced 1/4&nbsp;inch [[phone jack]]. Harmonica microphones commonly use a high impedance 1/4&nbsp;inch TS connection to be run through guitar amplifiers.
*3.5&nbsp;mm (sometimes referred to as 1/8&nbsp;inch mini) stereo (wired as mono) mini phone plug on very inexpensive and computer microphones
 
Some microphones use other connectors, such as a 5-pin XLR, or mini XLR for connection to portable equipment. Some lavalier (or 'lapel', from the days of attaching the microphone to the news reporters suit lapel) microphones use a proprietary connector for connection to a wireless transmitter. Since 2005, professional-quality microphones with [[USB]] connections have begun to appear, designed for direct recording into computer-based software.
 
=== Impedance-matching ===
Microphones have an electrical characteristic called [[electrical impedance|impedance]], measured in [[ohm]]s&nbsp;(Ω), that depends on the design. Typically, the ''rated impedance'' is stated.<ref name="autogenerated1">International Standard IEC 60268-4</ref> Low impedance is considered under 600&nbsp;Ω. Medium impedance is considered between 600&nbsp;Ω and 10&nbsp;kΩ. High impedance is above 10&nbsp;kΩ. Condenser microphones typically have an output impedance between 50 and 200&nbsp;ohms.<ref name=Eargle2002>{{cite book |title=Audio Engineering for Sound Reinforcement |last=Eargle |first=John |authorlink= |coauthors=Chris Foreman |year=2002 |publisher=Hal Leonard Corporation |location=Milwaukee |isbn=0634043552 |page=66 |url=http://books.google.com/books?id=YWzZe6z4xdAC }}</ref>
 
The output of a given microphone delivers the same [[Power (physics)|power]] whether it is low or high impedance. If a microphone is made in high and low impedance versions, the high impedance version will have a higher output voltage for a given sound pressure input, and is suitable for use with vacuum-tube guitar amplifiers, for instance, which have a high input impedance and require a relatively high signal input voltage to overcome the tubes' inherent noise. Most professional microphones are low impedance, about 200&nbsp;Ω or lower. Professional vacuum-tube sound equipment incorporates a [[transformer]] that steps up the impedance of the microphone circuit to the high impedance and voltage needed to drive the input tube; the impedance conversion inherently creates voltage gain as well. External matching transformers are also available that can be used in-line between a low impedance microphone and a high impedance input.
 
Low-impedance microphones are preferred over high impedance for two reasons: one is that using a high-impedance microphone with a long cable will result in loss of high frequency signal due to the capacitance of the cable which forms a low-pass filter with the microphone output impedance; the other is that long high-impedance cables tend to pick up more [[mains hum|hum]] (and possibly [[Radio frequency interference|radio-frequency interference]] (RFI) as well). Nothing will be damaged if the impedance between microphone and other equipment is mismatched; the worst that will happen is a reduction in signal or change in frequency response.
 
Most microphones are designed ''not'' to have their impedance matched by the load to which they are connected;<ref>http://www.shure.com/ProAudio/Products/us_pro_ea_imepdance</ref> doing so can alter their frequency response and cause distortion, especially at high sound pressure levels. Certain ribbon and dynamic microphones are exceptions, due to the designers' assumption of a certain load impedance being part of the internal electro-acoustical damping circuit of the microphone.<ref>Robertson, A. E.: "Microphones" Illiffe Press for BBC, 1951-1963</ref>
 
=== Digital microphone interface ===
The AES 42 standard, published by the [[Audio Engineering Society]], defines a digital interface for microphones. Microphones conforming to this standard directly output a digital audio stream through an XLR male connector, rather than producing an analog output. Digital microphones may be used either with new equipment which has the appropriate input connections conforming to the AES 42 standard, or else by use of a suitable interface box. Studio-quality microphones which operate in accordance with the AES 42 standard are now appearing from a number of microphone manufacturers.
 
== Measurements and specifications ==
[[Image:Oktava319vsshuresm58.png|thumb|left|A comparison of the far field on-axis frequency response of the Oktava 319 and the [[SM58|Shure SM58]]]]
 
Because of differences in their construction, microphones have their own characteristic responses to sound. This difference in response produces non-uniform [[phase (waves)|phase]] and [[frequency]] responses. In addition, microphones are not uniformly sensitive to sound pressure, and can accept differing levels without distorting. Although for scientific applications microphones with a more uniform response are desirable, this is often not the case for music recording, as the non-uniform response of a microphone can produce a desirable coloration of the sound. There is an international standard for microphone specifications,<ref name="autogenerated1" /> but few manufacturers adhere to it. As a result, comparison of published data from different manufacturers is difficult because different measurement techniques are used. The Microphone Data Website has collated the technical specifications complete with pictures, response curves and technical data from the microphone manufacturers for every currently listed microphone, and even a few obsolete models, and shows the data for them all in one common format for ease of comparison.[http://www.microphone-data.com/]. Caution should be used in drawing any solid conclusions from this or any other published data, however, unless it is known that the manufacturer has supplied specifications in accordance with IEC&nbsp;60268-4.
 
A [[frequency response]] diagram plots the microphone sensitivity in [[decibel]]s over a range of frequencies (typically at least 0–20&nbsp;kHz), generally for perfectly on-axis sound (sound arriving at 0° to the capsule). Frequency response may be less informatively stated textually like so: "30&nbsp;Hz–16&nbsp;kHz&nbsp;±3&nbsp;dB". This is interpreted as a (mostly) linear plot between the stated frequencies, with variations in amplitude of no more than plus or minus 3 dB. However, one cannot determine from this information how ''smooth'' the variations are, nor in what parts of the spectrum they occur. Note that commonly-made statements such as "20&nbsp;Hz–20&nbsp;kHz" are meaningless without a decibel measure of tolerance. Directional microphones' frequency response varies greatly with distance from the sound source, and with the geometry of the sound source. IEC&nbsp;60268-4 specifies that frequency response should be measured in ''plane progressive wave'' conditions (very far away from the source) but this is seldom practical. ''Close talking'' microphones may be measured with different sound sources and distances, but there is no standard and therefore no way to compare data from different models unless the measurement technique is described.
 
The self-noise or equivalent noise level is the sound level that creates the same output voltage as the microphone does in the absence of sound. This represents the lowest point of the microphone's dynamic range, and is particularly important should you wish to record sounds that are quiet. The measure is often stated in [[dB(A)]], which is the equivalent loudness of the noise on a decibel scale frequency-weighted for how the ear hears, for example: "15&nbsp;dBA SPL" (SPL means [[sound pressure]] level relative to 20&nbsp;[[micropascal]]s). The lower the number the better. Some microphone manufacturers state the noise level using [[ITU-R 468 noise weighting]], which more accurately represents the way we hear noise, but gives a figure some 11–14&nbsp;dB higher. A quiet microphone will measure typically 20&nbsp;dBA SPL or 32&nbsp;dB SPL 468-weighted. Very quiet microphones have existed for years for special applications, such the Brüel & Kjaer 4179, with a noise level around 0&nbsp;dB SPL. Recently some microphones with low noise specifications have been introduced in the studio/entertainment market, such as models from [[Georg Neumann GmbH|Neumann]] and [[Røde]] that advertise noise levels between 5–7&nbsp;dBA. Typically this is achieved by altering the frequency response of the capsule and electronics to result in lower noise within the A-weighting curve while broadband noise may be increased.
 
The maximum SPL ([[sound pressure level]]) the microphone can accept is measured for particular values of [[total harmonic distortion]] (THD), typically 0.5%. This is generally inaudible, so one can safely use the microphone at this level without harming the recording. Example: "142&nbsp;[[dB SPL]] peak (at 0.5%&nbsp;THD)". The higher the value, the better, although microphones with a very high maximum SPL also have a higher self-noise.
 
The clipping level is perhaps a better indicator of maximum usable level,{{Citation needed|date=February 2009}} as the 1%&nbsp;THD figure usually quoted under max SPL is really a very mild level of distortion, quite inaudible especially on brief high peaks. Harmonic distortion from microphones is usually of low-order (mostly third harmonic) type, and hence not very audible even at 3-5%. Clipping, on the other hand, usually caused by the diaphragm reaching its absolute displacement limit (or by the preamplifier), will produce a very harsh sound on peaks, and should be avoided if at all possible. For some microphones the clipping level may be much higher than the max SPL.
 
The dynamic range of a microphone is the difference in SPL between the noise floor and the maximum SPL. If stated on its own, for example "120&nbsp;dB", it conveys significantly less information than having the self-noise and maximum SPL figures individually.
 
[[Sensitivity (electronics)|Sensitivity]] indicates how well the microphone converts acoustic pressure to output voltage. A high sensitivity microphone creates more voltage and so will need less amplification at the mixer or recording device. This is a practical concern but is not directly an indication of the mic's quality, and in fact the term sensitivity is something of a misnomer, 'transduction gain' being perhaps more meaningful, (or just "output level") because true sensitivity will generally be set by the noise floor, and too much "sensitivity" in terms of output level will compromise the clipping level. There are two common measures. The (preferred) international standard is made in millivolts per pascal at 1&nbsp;kHz. A higher value indicates greater sensitivity. The older American method is referred to a 1&nbsp;V/Pa standard and measured in plain decibels, resulting in a negative value. Again, a higher value indicates greater sensitivity, so −60&nbsp; dB is more sensitive than −70&nbsp;dB.
 
{{clear}}
 
== Measurement microphones ==
Some microphones are intended for testing speakers, measuring noise levels and otherwise quantifying an acoustic experience. These are calibrated transducers and will usually be supplied with a calibration certificate stating absolute sensitivity against frequency. The quality of measurement microphones is often referred to using the designations "Class 1," "Type 2" etc., which are references not to microphone specifications but to sound level meters.<ref>IEC Standard 61672 and/or ANSI S1.4</ref> A more comprehensive standard<ref> IEC 61094</ref> for the description of measurement microphone performance was recently adopted.
 
Measurement microphones are generally scalar sensors of [[pressure]]; they exhibit an omnidirectional response, limited only by the scattering profile of their physical dimensions. [[Sound intensity]] or sound power measurements require pressure-gradient measurements, which are typically made using arrays of at least two microphones, or with [[Anemometer#Hot-wire anemometers|hot-wire anemometers]].
 
=== Microphone calibration techniques ===
Like most manufactured products there can be variations, which may change over the lifetime of the device. Accordingly, it is regularly necessary to test the test microphones. This service is offered by some microphone manufacturers and by independent certified testing labs. Microphone calibration is ultimately traceable to primary standards at one of the national laboratories such as [[Physikalisch-Technische Bundesanstalt|PTB]] in Germany and [[NIST]] in the USA. Some test enough microphones to justify an in-house calibration lab. Depending on the application, measurement microphones must be tested periodically (every year or several months, typically) and after any potentially damaging event, such as being dropped (most such mikes come in foam-padded cases to reduce this risk) or exposed to sounds beyond the acceptable level.
==== Pistonphone apparatus ====
A pistonphone is an acoustical calibrator (sound source) using a closed coupler to generate a precise sound pressure for the calibration of instrumentation microphones. The principle relies on a piston mechanically driven to move at a specified cyclic rate, on a fixed volume of air to which the microphone under test is exposed. The air is assumed to be compressed [[adiabatic]]ally and the [[Sound pressure|sound pressure level]] in the chamber can be calculated from internal physical dimensions of the device and the adiabatic gas law, which requires that the product of the pressure P with V raised to the power gamma be constant; here gamma is the ratio of the specific heat of air at constant pressure to its specific heat at constant volume.
The pistonphone method only works at low frequencies, but it can be accurate and yields an easily calculable sound pressure level. The standard test frequency is usually around 250&nbsp;Hz.
 
==== Reciprocal method ====
This method relies on the reciprocity of one or more microphones in a group of 3 to be calibrated. It can be performed in a closed coupler or in the free field. Only one of the microphones need be reciprocal (exhibits equal response when used as a microphone or as a loudspeaker).
 
== Microphone array and array microphones ==
{{main|Microphone array}}
 
A microphone array is any number of microphones operating in [[tandem]]. There are many applications:
 
*Systems for extracting voice input from [[ambient noise level|ambient noise]] (notably [[telephone]]s, [[speech recognition]] systems, [[hearing aid]]s)
*[[Surround sound]] and related technologies
*Locating objects by sound: [[acoustic source localization]], e.g. military use to locate the source(s) of artillery fire. Aircraft location and tracking.
*[[High fidelity]] original recordings
*3D spatial [[beamforming]] for localized acoustic detection of [[subcutaneous]] sounds
 
Typically, an array is made up of omnidirectional microphones distributed about the [[perimeter]] of a space, linked to a [[computer]] that records and interprets the results into a coherent form.
 
== Microphone windscreens ==
Windscreens are used to protect microphones that would otherwise be buffeted by wind or vocal [[stop consonant|plosives]] from consonants such as "P", "B", etc. Most microphones have an integral windscreen built around the microphone diaphragm. A screen of plastic, wire mesh or a metal cage is held at a distance from the microphone diaphragm, to shield it. This cage provides a first line of defense against the mechanical impact of objects or wind. Some microphones, such as the [[Shure SM58]], may have an additional layer of foam inside the cage to further enhance the protective properties of the shield. Beyond integral microphone windscreens, there are three broad classes of additional wind protection.
 
One disadvantage of all windscreen types is that the microphone's high frequency response is attenuated by a small amount, depending on the density of the protective layer.
 
===Microphone covers===
Microphone covers are often made of soft open-cell polyester or polyurethane foam because of the inexpensive, disposable nature of the foam. Optional windscreens are often available from the manufacturer and third parties. A very visible example of an optional accessory windscreen is the A2WS from Shure, one of which is fitted over each of the two [[Shure SM57]] microphones used on the United States president's lectern.<ref>[http://www.shure.com/ProAudio/Products/Accessories/us_pro_A2WS-BLK_content Shure - Accessories - A2WS Microphone Windscreens<!-- Bot generated title -->]</ref> One disadvantage of polyurethane foam microphone covers is that they can deteriorate over time. Windscreens also tend to collect dirt and moisture in their open cells and must be cleaned to prevent high frequency loss, bad odor and unhealthy conditions for the person using the microphone. On the other hand, a major advantage of concert vocalist windscreens is that one can quickly change to a clean windscreen between users, reducing the chance of transferring germs. Windscreens of various colors can be used to distinguish one microphone from another on a busy, active stage.
 
===Pop filters===
[[Pop filter]]s or pop screens are used in controlled studio environments to minimize [[Stop consonant|plosive]]s when recording. A typical pop filter is composed of one or more layers of acoustically transparent [[gauze]]-like material, such as woven nylon stretched over a circular frame and a clamp and a flexible mounting bracket to attach to the microphone stand. The pop shield is placed between the vocalist and the microphone. The need for a pop filter increases the closer a vocalist brings his lips the microphone. Singers can be trained either to soften their plosives or direct the air blast away from the microphone, in which cases they don't need a pop filter.
 
Pop filters also keep spittle off the microphone. Most condenser microphones can be damaged by spittle.
 
===Blimps===
Blimps (also known as Zeppelins) are large, hollow windscreens used to surround microphones for outdoor location audio, such as nature recording, [[electronic news gathering]], and for film and video shoots. They can cut wind noise by as much as 25&nbsp;dB, especially low-frequency noise. The blimp is essentially a hollow cage or basket with acoustically transparent material stretched over the outer frame. The blimp works by creating a volume of still air around the microphone. The microphone is often further isolated from the blimp by an elastic suspension inside the basket. This reduces wind vibrations and handling noise transmitted from the cage. To extend the range of wind speed conditions in which the blimp will remain effective, many have the option of fitting a secondary cover over the outer shell. This is usually an acoustically transparent, synthetic fur material with long, soft hairs. The hairs act as shock absorbers to any wind turbulence hitting the blimp. A synthetic fur cover can reduce wind noise by an additional 12&nbsp;dB.<ref>[http://www.rycote.com/products/full_windshield_system Full Windshield System.] Rycote Microphones.</ref>
 
 
 
== Αναφορές ==
{{reflist}}
 
== Εξωτερικοί δεσμοί ==
{{commonscat|Microphones}}
*[http://www.coutant.org/contents.html Info, Pictures and Soundbytes from vintage microphones]
*[http://recordinghacks.com/microphones Searchable database of specs and component info from 600+ microphones]
*[http://arts.ucsc.edu/EMS/Music/tech_background/TE-20/teces_20.html Microphone construction and basic placement advice]
*[http://users.belgacom.net/gc391665/microphone_history.htm History of the Microphone]
*[http://www.sengpielaudio.com/calculator-transferfactor.htm Microphone sensitivity conversion — dB re 1 V/Pa and transfer factor mV/Pa]
*[http://www.dpamicrophones.com/en/Microphone-University/Technology-Guide/Large%20Diaphragm.aspx Large vs. Small Diaphragms in Omnidirectional Microphones]
*[http://www.wikimusician.org/Stereo_Microphone Stereo Microphones, including sound demos]
*[http://www.wikirecording.org/Condenser Guide to Condenser Microphones]
*[http://www.wikimusician.org/How_to_Mic_a_Bass_Speaker_Cabinet How to Mic a Bass Speaker Cabinet]
*[http://www.gearwire.com/schwartzengineeringanddesign-smokemic-aes2009.html ''Smoke & Laser Microphone''], Gearwire
*[http://www.pcb.com/Linked_Documents/Vibration/Microphone_Handbook.pdf Measurement/Engineering Grade Microphone Basics]
 
 
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