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INVENTION The word "microphone" ( Greek ''mikros'' "small" and ''phone'' "sound") originally referred to a mechanical Hearing Aid for small sounds. {Link without Title} The invention of a practical microphone was crucial to the early development of the telephone system. Emile Berliner invented the first microphone on March 4 , 1877 , but the first useful microphone was invented by Alexander Graham Bell . Many early developments in microphone design took place in Bell Laboratories . PRINCIPLE OF OPERATION All microphones capture sound waves with a thin, flexible Diaphragm (or ribbon in the case of ribbon microphones). The vibrations of this diaphragm are then converted by various methods into an electrical signal that is an Analog of the original sound. Most microphones in use today use electromagnetic generation (dynamic microphones), capacitance change (condenser microphones) or piezoelectric generation to produce the signal from mechanical vibration. MICROPHONE VARIETIES Capacitor or condenser microphones In a capacitor microphone, also known as a condenser microphone, the diaphragm acts as one plate of a Capacitor , and the vibrations produce changes in the distance between the plates. Since the plates are 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: : 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.): : Capacitor microphones can be expensive and require a Power Supply , commonly provided from mic inputs as Phantom Power , but give a high-quality sound signal and are now the preferred choice in laboratory and studio recording applications. Electret capacitor microphones An electret microphone is a relatively new type of condenser microphone invented at is a Dielectric material that has been permanently Electrically Charged or ''polarised''. The name comes from ''electr''ostatic and magn''et''; a static charge is embdedded 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. Electret microphones have existed since the 1920s but were considered impractical, but they have now become the most common type of all, used in many applications from high-quality recording and Lavalier use to built-in microphones in small Sound Recording devices and telephones. Though electret mics were once considered low-cost and low quality, the best ones can now rival capacitor mics in every respect apart from low noise and can even have the long-term stability and ultra-flat response needed for a measuring microphone. Unlike other condenser microphones they require no polarising voltage, but normally contain an integrated Preamplifier which does require power (often incorrectly called polarizing power or bias). This preamp is frequently Phantom Power ed in sound reinforcement and studio applications. 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 electrets cheaply don't lend themselves to the precision needed to produce the highest quality microphones. Dynamic microphones In a dynamic microphone a small movable Induction Coil , positioned in the Magnetic Field of a Permanent Magnet , is attached to the Diaphragm . When sound enters through the windscreen of the microphone, the sound wave vibrations move the diaphragm, When the diaphragm vibrates, the coil moves in the magnetic field, producing a varying Current in the coil (See Electromagnetic Induction ). The principle is exactly the same as in a Loudspeaker , only reversed. Dynamic microphones are robust, relatively inexpensive, and resistant to moisture, and for this reason they are widely used on-stage by singers. They tend to have a poor low-frequency response, which is advantageous for reducing handling noise as a vocal mic, but tends to exclude them from other uses. Ribbon microphones In Ribbon Microphone s a thin, usually corrugated metal ribbon is suspended in a magnetic field. The ribbon is electrically connected to the microphone's output, and its vibration within the magnetic field generates the electrical signal. Basic ribbon microphones detect sound in a 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 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 mic horizontally, for example above cymbals, so that the rear lobe picks up only sound from the ceiling. 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. Ribbon mics give very high quality sound reproduction, and were once valued for this reason, but a good low-frequency response can only be obtained if the ribbon is suspended very loosely, and this makes them fragile. Protective wind screens can, however, reduce the danger of damaging the ribbon, but will somewhat reduce the bass response at large miking distances. Ribbon microphones don't require Phantom Power ; in fact, this voltage can damage these microphones. Carbon microphones A carbon microphone, formerly used in Telephone handsets, is a Capsule containing Carbon granules pressed between two metal plates. 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 voltage across the two plates, and hence 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. 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 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. Piezo microphones A piezo (pronounced "''pee-ay-zo''" or "''pie-ee-zo''") microphone 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. Piezo transducers are often used as contact microphones to amplify acoustic instruments for live performance, or to record sounds in unusual environments (underwater, for instance.) An example of this is Rochelle Salt (potassium sodium tartrate), which is a piezoelectric crystal that works as a transducer both ways; it is also commonly used as a slimline loudspeaker component. Laser microphones A Laser Microphone is an exotic application of Laser technology. It consists of a laser beam that must be reflected off a glass window or another rigid surface that vibrates in sympathy with nearby sounds. This device essentially turns any vibrating surface near the source of sound into a microphone. It does this by measuring the distance between itself and the surface extremely accurately; the tiny fluctuations in this distance become the electrical signal of the sounds picked up. Laser microphones are new, very rare and expensive, and are most commonly portrayed in the movies as spying devices. OTHER MICROPHONE TYPES A pressure gradient microphone is a microphone in which both sides of the diaphragm are exposed to the incident sound and the microphone is therefore responsive to the pressure differential (gradient) between the two sides of the membrane. Sound incident parallel to the plane of the diaphragm produces no pressure differential, giving pressure-gradient microphones their characteristic figure-eight directional patterns. They are also called "velocity microphones", since the output voltage is proportional to the air particle velocity. A Lavalier Microphone is made for hands-free operation. These small microphones are worn on the body and held in place either with a lanyard worn around the neck or a clip fastened to clothing. The 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 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 , 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. CONNECTIVITY Connectors The most common connectors used by microphones are:
Some microphones use other connectors, such as TRS, 5-pin XLR, or stereo mini phone plug on some stereo microphones. Some lavaliers have a proprietary connector to connect them to their transmitter. Impedance Matching Microphones have an electrical characteristic called Impedance , measured in Ohms (Ω) that depends on the design. Low impedance is considered under 600 Ω. Medium impedance is considered between 600 Ω and 10 kΩ. High impedance is above 10 kΩ. Most professional microphones are low impedance, about 200 Ω. Less expensive models have an impedance of at least 600 Ω. Low-impedance microphones are preferred over high impedance on long-run cables for two reasons: one is that using a high-impedance mike with a long cable is likely to result in loss of high frequency signal; the other is that long high-impedance cables tend to pick up more Hum (and possibly Radio-frequency Interference (RFI) as well). To get the best sound, the impedances of the microphone and the equipment to which it is connected must match. There are transformers (called matching transformers) that adapt impedances, such as DI Unit s. In general, any XLR microphone can be connected to any mixer with XLR inputs, and any plug microphone can be connected to any jack plug that is marked as a microphone input, but it can't be connected to a line input. DIRECTIONALITY A microphone's directionality or polar pattern indicates how sensitive it is to sounds arriving at different angles about its central axis. The polar pattern represents the Locus of points that produce the same signal level output in the microphone if a given Sound Pressure Level is generated from that point. An Omnidirectional 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 kHz is about an inch (2.5 cm) so the smallest measuring microphones are often 1/4" (6 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" mikes 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 Hz or below. Pressure-sensitive mikes also respond much less to wind noise than directional (velocity sensitive) mikes. A unidirectional microphone is sensitive to sounds from only one direction. The diagram above illustrates a number of these patterns, with the microphone capsule being represented as a red dot. The top of the diagram is the front of the mic. 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. These diagrams just provide an overview of the typical shapes and their names.) The most common unidirectional mike is a Cardioid microphone, so named because the sensitivity pattern is heart-shaped (see Cardioid ). A ''' Hyper-cardioid ''' is similar but with a tighter area of front sensitivity and a tiny lobe of rear sensitivity. These two patterns are commonly used as vocal or speech mikes, since they are good at rejecting sounds from other directions. Because they employ internal cavities to provide front-back delay, directional mikes tend to have more coloration than omnis, and they also suffer from low-frequency roll-off. These problems are overcome to a large extent by careful design, but only the best cardioids can begin to approach the performance of a tiny low-cost omni in terms of absolute accuracy. This is not always recognised, but is the price paid for directionality, often needed to exclude ambient reverberation wherever very close placement is impossible. Figure 8 or ''' Bi-directional ''' mikes receive sound from both the front and back of the element. Most ribbon microphones are of this pattern. Shotgun Microphone s are the most highly directional. They have small lobes of sensitivity to the left, right, and rear but are significantly more sensitive to the front. This results from placing the element inside a tube with slots cut along the side; wave-cancellation eliminates most of the off-axis noise. Shotgun microphones are commonly used on TV and film sets, and for location recording of wildlife. An omnidirectional microphone is a pressure transducer; the output voltage is proportional to the air pressure at a given time. On the other hand, a figure-8 pattern is a pressure ''gradient'' transducer; the output voltage is proportional to the difference in pressure on the front and on the back side. A sound wave arriving from the back will lead to a signal with a Polarity opposite to that of an identical sound wave from the front. Moreover, shorter wavelengths (higher frequencies) are picked up more effectively than lower frequencies. 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 directional microphones are (partially) pressure gradient Transducer s, their sensitivity is dependent on the distance to the sound source. This is known as the proximity effect, a bass boost at distances of a few centimeters. MEASUREMENTS AND SPECIFICATIONS ]] Because of differences in their construction, microphones have their own characteristic responses to sound. This difference in response produces non-uniform Phase and Frequency responses. In addition, mics 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 (IEC 60268-4), but very few manufacturers adhere to it. A frequency response diagram plots the microphone sensitivity in Decibel s over a range of frequencies (typically at least 0–20 kHz), generally for perfectly on-axis sound (sound arriving at 0° to the capsule). Frequency response may be less informatively stated textually like so: "20 Hz–20 kHz ±3 dB". This is interpreted as a (mostly) linear plot between the stated frequencies, with variations in amplitude of no more than 3 dB plus or minus. 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 Hz–20 kHz" are meaningless without a decibel measure. The self-noise or equivalent noise level is the sound level that creates the same output voltage as the inherent noise of the microphone. 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 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 to 14 dB higher. A quiet microphone will measure typically 20 dBA SPL or 32 dB SPL 468-weighted. The maximum SPL ( peak (<1% THD)". The higher the value, the better. The clipping level is perhaps a better indicator of maximum useable level as the 1% 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 diaphram 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 mikes the clipping level may be much higher than the max SPL. The dynamic range of a mike is the difference in SPL between the noise floor and the maximum SPL. If stated on its own, for example "120 dB", it conveys significantly less information than having the self-noise and maximum SPL figures individually. Sensitivity indicates how well the mike converts acoustic pressure to output voltage. A high sensitivity mike creates more voltage and so will need less amplification at the mixer or recording device. This is a practical concern but not directly an indication of the mike'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 mV per pascal at 1 kHz. A higher value indicates greater sensitivity. The older American method is referred to a 1 V/Pa standard and measured in plain dB, resulting in a negative value. Again, a higher value indicates greater sensitivity, so −60 dB is more sensitive than −70 dB. MEASUREMENT MICROPHONES Some microphones are intended for use as standard measuring microphones for the testing of speakers and checking noise levels etc. These are calibrated transducers and will usually be supplied with a calibration certificate stating absolute sensitivity against frequency. Microphone calibration techniques 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 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 SPL in the chamber can be calculated from PV = const. The Piston phone method only works at low frequencies but can be accurate and is related to an easily calculable sound pressure level. The standard test frequency is usually around 250 Hz. Reciprocal method This method relies on the reciprocity of one or more microphones in a group of 3 to be calibrated. It can still be used when only one of the microphones is reciprocal (exhibits equal response when used as a microphone or as a loudspeaker). MICROPHONE TECHNIQUES There exist a number of well-developed microphone techniques used for miking musical, film, or voice sources. Choice of technique depends on a number of factors, including:
Basic techniques There are several classes of microphone placement for recording and amplification.
Stereo recording techniques There are two essential components that the stereo loudspeakers need to place objects (phantom sources) in the stereo sound-field between the loudspeakers. These are Level difference ''Δ L'', the relative loudness, and time-delay difference ''Δ t'', the difference in arrival time. The "interaural" signals (binaural ''ILD'' and ''ITD'') at the ears are not the stereo microphone technique signals which are coming from the loudspeakers, and are called "interchannel" signals (''Δ L'' and ''Δ t''). These signals are normally not mixed. Loudspeaker signals are different from the sound arriving at the ear. See the section " Binaural Recording For Earphones ". Conventional stereo recording for loudspeakers The following microphone "techniques can be used to capture the live "soundstage":
Surround Microphone Techniques
Binaural recording for earphones Binaural Recording is a highly specific attempt to recreate the conditions of human hearing, reproducing the full three-dimensional sound-field with earphones. Most binaural recordings use model of a human head, with microphones placed where the Ear Canal could be. A sound source is then recorded with all of the stereo and spatial cues produced by the head and human pinnae with frequency dependent ''ILD'' (interaural level difference) and ''ITD'' (interaural time difference, max. (Δ''t'') = 630 µs = 0.63 ms) ear signals. A binaural recording is usually only somewhat successful, in addition to being highly inconvenient. For one thing, it tends to work well '''only''' when played back directly into the ear canal, via '''headphones''' (no speakers), as other methods of playback add additional spatial cues. Furthermore, as all heads and pinnae are different, a recording from one "pair of ears" will not always sound correct to another person. Also, headphones have a frequency response that compensates for the fact that the reflections from the pinnae, head and shoulders strongly affect the frequency spectrum, with the assumption that a recording is taken with a flat frequency spectrum. Introducing the spectral distortion already in the binaural recording results in an unnatural frequency spectrum, even when played through headphones. Finally, as visual cues are generally much more powerful than auditory cues when determining the source of a sound, binaural recordings are not always convincing to listeners. SEE ALSO
MICROPHONE MANUFACTURERS
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