Microphone types
Los micrófonos son clasificados según su tipo de transductor, ya sea de condensador o dinámico, y por sus características direccionales. A veces, otras características tales como el tamaño de diafragma, el uso previsto o la orientación de la entrada de sonido principal se utilizan para clasificar el micrófono.
condenser microphone
The "condenser microphone" was invented at Bell Laboratories in 1916 by Edward Christopher Wente). from the transducer: DC bias microphones, and radio frequency (RF) or shortwave condenser microphones.
In a DC bias microphone, the plates are biased with a fixed charge (Q). The voltage between the capacitor plates changes with vibrations in the air (according to the capacitance equation, where Q = charge in coulombs, C = capacitance in farads and V = potential difference in volts). The capacitance of the plates is inversely proportional to the distance between them for a parallel plate capacitor. The assembly of fixed and moving plates is called an "element" or "capsule."
An almost constant charge is maintained on the capacitor. With changes in capacitance, the charge across the capacitor changes very slightly, but at audible frequencies it is sensibly constant. The capsule capacitance (about 5 to 100 pF) and the value of the bias resistance (100 mO at tens of GΩ) form a filter that is high-pass for the audio signal, and low-pass for the bias voltage. Note that the time constant of an RC circuit is equal to the product of resistance and capacitance.
Within the time frame of the capacitance variation (as much as 50 ms at 20 Hz of an audio signal), the charge is virtually 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 sensed through the series resistor. The voltage across the resistor is amplified to improve performance or for recording. In most cases, the electronics of the microphone itself contribute to the voltage gain, so the voltage differential is quite significant, up to several volts for high sound levels. As this is a very high impedance circuit, the current gain is only necessary to modify the constant reference voltage.
They use a comparatively low RF voltage, generated by a low noise oscillator. The oscillator signal can either be modulated in amplitude by changes in capacitance produced by sound waves by moving the diaphragm or capsule, or the capsule can be part of a resonant circuit that modulates the frequency of the oscillator signal. Demodulation produces a low noise audio frequency signal, with a very low source impedance. The absence of a high bias voltage allows the use of a diaphragm with the lowest voltage, which can be used to achieve the widest frequency response due to greater sensitivity. The RF polarization process results in a lower electrical impedance capsule, allowing RF condenser microphones to operate in humid weather conditions, which could create problems in microphones using a DC-reference current with contaminated insulating surfaces. The Sennheiser “MKH” series of microphones uses the RF push technique.
Condenser microphones run the gamut for telephony transmitters as well as other uses, from inexpensive karaoke microphones to high-fidelity recording microphones. They typically produce a high-quality audio signal and are now the common choice of laboratories and recording studios. The inherent suitability of this technology is due to the very small mass that must be moved by the incident sound wave, unlike other types of microphones that require the sound wave to do more mechanical work. They require a power source, either through the microphone inputs on the computer as auxiliary power or from a small battery. This current is necessary for establishing the plate voltage of the power capacitor, and is also necessary 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 that can be electrically connected to provide a range of polar patterns (see below), such as cardioid, omnidirectional, and figure-eight. It is also possible to vary the pattern continuously with some microphones (for example the Røde NT2000 or the CAD M179).
A valve microphone is a condenser microphone that uses a vacuum tube amplifier (valve). They remain popular among vacuum tube processed sound enthusiasts.
An electret microphone is a type of condenser microphone invented by Gerhard Sessler and Jim West at Bell Laboratories in 1962. The application of an external charge described above in condenser microphones is replaced by a permanent charge in an electret material, a ferroelectric material that has been permanently electrically charged or polarized. The name comes from electrostatic and magnet; A static charge is kept attached in an electret by the alignment of the static charges in the material, in the same way that a magnet is made permanent by the alignment of the magnetic domains in a piece of iron.
Due to their good performance and ease of manufacture, therefore low cost, the vast majority of microphones made today are electret microphones; One semiconductor manufacturer estimates annual production to be more than one billion units. Almost all cell phones, computers, PDAs and headsets are of the electret type. They are used in many applications, from high-quality and lapel recording, to the built-in microphones in small sound recording devices and telephones. Although electret microphones were initially considered of low quality, the best models of these microphones can now compete with traditional condenser models in all aspects and can even offer greater long-term stability and the ultra-flat response necessary for a measurement microphone. Although they do not require bias voltage, like other condenser microphones, they often contain an integrated preamplifier that requires power (often incorrectly called power or bias bias). This preamplifier is frequently phantom powered for sound reinforcement and studio applications. Some mono microphones designed for personal computers (PCs), sometimes called multimedia microphones, use a 3.5 mm connector, as is typically used, without a power jack, for stereo equipment; The connector, instead of carrying the signal to a second channel, carries electrical power through a resistor from (usually) a 5V supply into the computer. Stereo microphones use the same connector; There is no obvious way to determine which system is used by computers and microphones.
Only the best electret microphones can rival other types of quality microphones in terms of noise level and quality. On the contrary, they lend themselves to low-cost mass production with acceptable performance, which has led to their massive use in all types of devices.
dynamic microphone
Dynamic microphones (also known as magneto-dynamic microphones) work through electromagnetic induction. They are robust, relatively cheap and resistant to humidity. This, along with their high gain before feedback potential, makes them ideal for use on stage.
Moving coil microphones use the same dynamic principle that is used in a loudspeaker, but inverted. A small moving induction coil, located in the magnetic field of a permanent magnet, is attached to the membrane. When sound enters through the microphone grille, the sound wave moves the diaphragm, displacing the coil which moves in the magnetic field, which in turn produces a variation of current in the coil through electromagnetic induction. A single dynamic membrane does not respond linearly to all audio frequencies. Some microphones for this reason use multiple membranes for different parts of the audio spectrum and then combine the resulting signals. Correctly combining multiple signals is difficult, and designs capable of doing so are rare and tend to be expensive. On the other hand, there are several designs that more specifically target isolated parts of the audio spectrum. The AKG D 112, for example, is designed to respond to bass sounds rather than treble. In audio engineering, several types of microphones are often used at the same time to obtain the best result.
ribbon microphone
Ribbon microphones use a thin metal ribbon (usually corrugated), suspended in a magnetic field. The ribbon is electrically connected to the microphone output, and its vibration within the magnetic field generates the electrical signal. Ribbon microphones are similar to coil microphones (both produce sound through magnetic induction). They detect sound in a bidirectional pattern (also called a figure-eight, as in the diagram below) because the tape is open on both sides, and because it has little mass, so it responds to air speed rather than sound pressure. Although the symmetrical front and rear pickup can be a nuisance in normal stereo recording, high-side rejection can be used to advantage by placing a horizontal ribbon microphone, for example, above the cymbals of a drum kit, so that the rear lobe picks up only the sound of the cymbals. Crossed figure 8s, or Blumlein pairs, are gaining popularity in stereophonic recording, and the response arrangement of a figure-eight ribbon microphone is ideal for that application.
Other directional patterns can be produced by confining one side of the tape in an acoustic trap or baffle, allowing sound to arrive from only one side. The classic RCA Type 77-DX microphone has several externally adjustable positions of the internal baffle, allowing the selection of various response patterns ranging from "figure eight" to "unidirectional." These older ribbon microphones, some of which still offer high-quality sound reproduction, were once highly valued for this reason, but they could only achieve good low-frequency response when the ribbon was properly suspended, making them relatively fragile. The materials used in the ribbon have been modernized, including new nanomaterials, which has made these microphones more reliable, and even improved their effective dynamic range at low frequencies. Protective wind screens can reduce the danger of damaging an old tape, and also reduce sound explosions when recording. Properly designed windscreens produce negligible treble rolloff. Like other types of dynamic microphones, ribbon microphones do not require auxiliary power; In fact, this voltage can damage some older ribbon microphones. Some new modern ribbon microphone designs incorporate a preamplifier and therefore require auxiliary power. The circuitry of modern passive ribbon microphones, that is, those without the aforementioned preamplifier, are specifically designed to resist damage to the ribbon and auxiliary power transformer. There are also new tape materials available that are immune to wind, blast, and auxiliary power.
Carbon microphone
A carbon microphone, also known as a button microphone, uses a capsule or button containing carbon granules pressed between two metal plates like Berliner and Edison microphones. By applying a voltage across the metal plates, you cause a small electrical current to flow into the carbon. One of the plates, the diaphragm, vibrates in tune with the incident sound waves, applying varying pressure to the carbon granules. The change in 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. Changes in resistance produce a corresponding change in current flow through the microphone, producing the electrical signal. There was a time when carbon microphones were commonly used in telephony; They have extremely low sound reproduction quality and a very limited frequency response range, but they are very robust devices. Boudet's microphone, which uses relatively large carbon balls, was similar to granular carbon button microphones.
Unlike other types of microphones, the carbon microphone can also be used as a type of amplifier, using a small amount of electrical energy. Originally, carbon microphones were used as telephone repeaters, making long-distance calls possible in the era before vacuum tubes. These repeaters work mechanically, coupling a magnetic telephone receiver to the carbon microphone: the weak signal from the receiver was transferred to the microphone, where it was modulated into a strong electrical current, in turn producing a strong electrical signal to send over the line. A consequence of this amplifying effect was feedback oscillation, resulting in an audible screech on early wall telephones when the receiver was placed near the carbon microphone.
Piezoelectric microphone
A crystal microphone or piezo microphone[24] uses the phenomenon of piezoelectricity—the ability of some materials to produce a voltage when subjected to pressure, to convert vibrations into an electrical signal. An example of this is sodium potassium tartrate, which is a piezoelectric crystal that functions as a transducer (in the form of an extra-flat component), either as a microphone or as a speaker. Crystal microphones were commonly supplied with vacuum tube (valve) equipment, such as home recorders. Their high output impedance also matches the high impedance (typically about 10 megohms) of the input stage of the vacuum tubes. They were difficult to match in the early days of transistorized equipment, but were quickly replaced by dynamic microphones for a time, and later by small electret condenser devices. The high impedance of crystal microphones made them very susceptible to stray noise, both from the microphone itself and the connecting cable.
Piezoelectric transducers are often used as contact microphones to amplify the sound of acoustic musical instruments, to detect drum hits, to trigger electronic samples, and to record sound in difficult environments, such as high-pressure underwater. The pickups mounted on acoustic guitars are generally piezoelectric devices in contact with the strings. This type of microphone is different from the magnetic coil pickups commonly seen on typical electric guitars, which use magnetic induction, rather than mechanical coupling, to pick up vibrations.
Fiber optic microphone
A fiber optic microphone converts acoustic waves into electrical signals by detecting changes in light intensity, rather than detecting changes in capacitance or magnetic fields, as with conventional microphones.[25][26].
During operation, light from a laser source travels through an optical fiber to illuminate the surface of a reflective diaphragm. The sound vibrations of the diaphragm modulate the intensity of light reflected by the diaphragm in a specific direction. The modulated light is then transmitted through a second optical fiber to a photodetector, which transforms the intensity modulated light into analog or digital audio for transmission or recording. Fiber optic microphones have a high dynamic and frequency range, similar to that of the best conventional high-fidelity microphones.
Furthermore, they are not influenced by electric, magnetic, electrostatic or radioactive fields (this is called 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 industrial turbines or in the environment of magnetic resonance imaging (MRI) equipment.
They are robust, resistant to environmental changes in temperature and humidity, and can be produced for any directionality or impedance matching. The distance between the microphone's light source and its photodetector can be up to several kilometers without the need for a preamplifier or any other electrical device, making fiber optic microphones suitable for industrial acoustic monitoring and surveillance.
They are used in very specific application areas, such as infrasound detection and noise cancellation. They have proven especially useful in medical applications, allowing radiologists, staff and patients located within the strong magnetic field and noisy environment in MRI rooms, as well as in remote control rooms, to communicate normally.[27] Other uses include industrial equipment monitoring and detection, audio calibration and measurement, high fidelity recording, and compliance with sound levels limited by law.[28]
laser microphone
Laser microphones often appear in movies as spy gadgets, as they can be used to pick up sound at a distance from microphone equipment. A laser beam is directed at the surface of a window or other flat surface that is affected by sound. The vibrations of this surface change the angle at which the beam is reflected, allowing the movement of the laser beam point to be detected, which after returning to the equipment is converted into an audio signal.
In a more robust and expensive application, the returned light is split and fed to an interferometer, which detects surface motion by changes in the optical path length of the reflected beam. This is an experimental development; since it requires an extremely stable laser and very precise optics.
A new type of laser microphone is a device that uses a laser beam and smoke or vapor to detect sound vibrations outdoors. On August 25, 2009, US Patent 7,580,533 issued for a flow particle detection microphone based on laser and photocell coupling, with a moving stream of smoke or vapor in the path of the laser beam. Sound pressure waves cause disturbances in the smoke, which in turn cause variations in the amount of laser light reaching the photodetector. A prototype of the device was demonstrated at the 127th Audio Engineering Society convention in New York from October 9 to 12, 2009.
liquid microphone
The first microphones did not allow speech to be reproduced intelligibly, until Alexander Graham Bell made improvements including a variable resistance between microphone and transmitter. Bell's liquid transmitter consisted of a metal container 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 container was then inversely proportional to the size of the water meniscus around the submerged needle. Elisha Gray presented the advertisement for a version with a brass rod instead of the needle. Other variants and minor improvements to the liquid microphone (devised by Majoranna, Chambers, Vanni, Sykes, and Elisha Gray) were presented, and Reginald Fessenden patented his own version in 1903. These were the first microphones, but they were not practical for commercial application. The famous first telephone conversation between Bell and Watson was carried out using a liquid microphone.
Microelectromechanical Microphone (MEMS)
MEMS type microphones ("Microelectromechanical systems" in English), are also called microphone chips or silicon microphones. A pressure-sensitive diaphragm is etched directly into a silicon wafer using MEMS processing techniques, and is typically accompanied with an integrated preamplifier. Most MEMS microphones are variants of the condenser microphone design. Digital MEMS have been built into integrated analog-to-digital (ADC) circuits on the same CMOS chip, making the chip a complete digital microphone, more easily incorporated into modern digital products. The main manufacturers producing silicon MEMS microphones are Wolfson Microelectronics (WM7xxx) now Cirrus Logic,[29] Analog Devices,[30] Akustica (AKU200x), Infineon (product SMM310), Knowles Electronics, MemsTech (MSMx), NXP Semiconductors (division purchased by Knowles[31]), Sonion MEMS, Vesper, Acoustic Technologies AAC[32] and Omron.[33].
More recently, there has been increased interest and research in the fabrication of piezoelectric MEMS, which represent a significant architectural and material change from existing MEMS designs based on capacitor technology.[34][35].
Speakers as microphones
A speaker is a transducer that converts an electrical signal into sound waves. Functionally, it is the opposite of a microphone; since conventional speakers are built much like a dynamic microphone (with a diaphragm, coil and magnet), speakers can actually work "in reverse" as microphones. The result, however, is a microphone with poor quality, limited frequency response (especially at the high end), and poor sensitivity. In practice, loudspeakers are sometimes used as microphones in applications where high quality and sensitivity are not needed, such as intercoms, walkie-talkies or video game voice chat peripherals, or where conventional microphones are in short supply.
However, there is at least one other practical application of this principle: the use of a medium-sized speaker placed closely in front of the bass drum pedal of a drum kit to act as a microphone. The use of relatively large speakers to transduce low-frequency sound sources, especially in music production, is becoming quite common. An example of a product of this type of device is the Yamaha SUBKICK, a 6.5-inch (170 mm) subwoofer mounted in front of percussion instruments. Having a relatively heavy membrane, it is not capable of transducing high frequencies, so placing a speaker in front of a kick drum is often ideal for capturing the sound of the kick drum. Less commonly, the microphones themselves can be used as speakers, almost always to reproduce high-pitched sounds. Microphones, however, are not designed to handle the required powers typically used to drive speakers. An example of such an application was the STC 4001 “super tweeter”, derived from a microphone. This device was used successfully in a number of high-quality speaker systems from the 1960s to the mid-1970s.