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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING Modern microphones and their applications. Encyclopedia of radio electronics and electrical engineering
Encyclopedia of radio electronics and electrical engineering / Audio equipment A microphone is an indispensable attribute of sound amplification systems, amateur and professional sound recording equipment, radio and television broadcasting studios. With the development of multimedia systems, it has now become a standard external component for many computers. This article tells about the design of microphones, their most important characteristics, how to choose the best microphone for specific application conditions. In this article, we will try to describe a general approach to choosing a microphone, based on its internal structure and purpose, as well as answer some questions that may arise from recording enthusiasts and just anyone who does not have special knowledge in this area. To do this, describing their various designs and types, we will give examples of both domestic and foreign models. What is a microphone? A microphone is an electro-acoustic device that converts acoustic sound vibrations of the air into electrical signals. It is the first link in any path of sound recording, sound amplification, voice communication. Its characteristics and operating conditions largely determine the quality of the signal throughout the path. Many types of distortions of audio signals (nonlinear, transient, features of the transmission of acoustic conditions and perspective) and various interferences (wind, vibration, acoustic) often cannot be eliminated by subsequent signal processing without significant deterioration of useful components. In a microphone, when sound vibrations are converted into electrical signals, various interrelated physical processes occur. In accordance with this, the microphone can be considered as a series of functional links. The first link is acoustic, the receiver of sound waves. The sound (oscillatory) pressure generated by the sound source acts on the acoustic input (or inputs). As a result of the interaction between the receiver and the sound field, a mechanical force is formed, which depends on the frequency of the sound signal, the size and shape of the microphone body and its acoustic inputs, the distance between them, the angle of incidence of the sound wave relative to the acoustic axis of the microphone, and the nature of the sound field. The receiver type determines such an important parameter as the directivity characteristic (CH). The second link is acoustic-mechanical, it serves to match the force generated by the receiver in a given frequency range with the value of the vibrational velocity (for dynamic microphones) or displacement (for condenser ones) of the moving element of the microphone's electromechanical transducer. The properties of this link are determined by the mutual arrangement, size and frequency dependence of the acoustic-mechanical elements included in it, which in a constructive sense represent various gaps, slots, holes, volumes, porous elements located inside the microphone capsule. This link determines the frequency response of the microphone's sensitivity (FCH) and to a large extent helps to form the CL in a wide frequency range. The third link - electromechanical, is an electromechanical transducer operating in a microphone in the generator mode and converting the mechanical vibration of a moving element (its speed or displacement) into an electromotive force (EMF). The efficiency of the converter is characterized by the coefficient of electromechanical coupling. The converter determines the sensitivity of the microphone. The fourth link is electrical. It performs the function of matching the converter with the subsequent amplifying device (for example, in condenser microphones, it matches the large capacitance of the capsule with a relatively low-impedance input of the subsequent amplifying device). In some models of microphones, the electric link also corrects the frequency response of the microphones. Receiver and transducer types are the defining elements of microphones. Acoustic-mechanical and electrical links are matching, the main task of which is to ensure minimal losses of the useful signal and obtain the required frequency response of the output signal. Microphones are usually classified according to three main features: the type of receiver, the type of transducer, and the purpose (operating conditions). How are microphones classified? The type of receiver determines one of the main characteristics of a microphone - the directional characteristic. The directivity characteristic is the dependence of the sensitivity of a microphone at a given frequency on the angle of incidence of the sound wave. By type of receiver, microphones are divided into the following groups. Pressure receivers (non-directional, "zero order", "circular"). In them, the sound affects the moving element (membrane, diaphragm) only from one side. As a result, at low and medium frequencies, where the dimensions of the microphone are small compared to the sound wavelength, the sensitivity of the microphone practically does not change at different angles of sound incidence. Gradient or differential pressure receivers (directional). They are of two types: Differences in the shape of the HH of unidirectional receivers are determined both by the degree of asymmetry of the inputs and by the value of the acoustic-mechanical parameters of the internal structure of the acoustic-mechanical link. The directivity characteristics (diagrams) of these types of receivers are graphically presented in Fig. 1.
Microphone sensitivity characteristics: 1 - omnidirectional (non-directional), 2 - bidirectional, 3-5 - cardioid On fig. 2 schematically depicts the principle of construction of omnidirectional (a), bilaterally directional (b) and unidirectionally directional (c) microphones.
Combined microphones, or microphones with variable XH, are sometimes distinguished into a special group. In these microphones, it is possible to obtain almost any HH from the family (see Fig. 1) by a combination of electrical signals from two receivers - omnidirectional (curve 1) and bilaterally directional (curve 2), or from two cardioid microphones deployed by 180° (electrically combined ), as well as a change in the magnitude of the polarization voltage on the halves of a fixed electrode or membranes in double-membrane condenser microphones. A special group is represented by highly directional microphones, which are used in cases where it is not possible to get close to the source of the useful signal. Acute HN in them is realized in several different ways. "Bigradient" or "bicardioid" (second-order gradients) are microphones consisting of two identical, spatially spaced and coaxially located capsules with a "figure eight" or "cardioid" HN, included in antiphase. The frequency range of such receivers is extremely limited. The most common among highly directional microphones are "travelling wave" (interference) microphones, consisting of a tube with holes or slots, on the rear end of which there is an omnidirectional or unidirectional microphone capsule (Fig. 3).
The holes (slots) in the tube are closed with a cloth or porous material, the acoustic impedance of which increases as it approaches the primer. Exacerbation of CN is achieved due to the interference of partial sound waves passing through the holes of the tube. When the sound front moves parallel to the axis of the tube, all partial waves arrive at the moving element simultaneously, in phase. When sound propagates at an angle to the axis, these waves reach the capsule with a different delay, determined by the distance from the corresponding hole to the capsule, while partial or complete compensation of the pressure acting on the moving element occurs. A noticeable aggravation of HN in such microphones begins at a frequency where the length of the tube is more than half the length of the sound wave; with an increase in the frequency of HN, it becomes even more aggravated. Therefore, even with a significant length of such microphones, which can reach a meter or even more, the frequency response at frequencies below 150 ... 200 Hz is determined only by the capsule and is usually close to a cardioid or supercardioid. The third, actually encountered type of highly directional microphones - reflex. In these microphones, a capsule with an omnidirectional or unidirectional CI is placed at the focus of a parabolic reflector (Fig. 4).
At the same time, due to the properties of the parabola, sound waves after reflection are concentrated at the focus of the parabola, at the location of the movable element of the capsule, and reach it in phase. Sound waves arriving at an angle to the axis of the parabola are scattered by the reflector without reaching the microphone. In the reflex system, the CI is even more frequency dependent than in the interference system, and changes from almost omnidirectional at low frequencies (with a reflector diameter less than the sound wavelength) to a narrow lobe at high frequencies. The frequency response of such microphones has a rise towards high frequencies with a slope of about 6 dB per octave, which is usually compensated either electrically or by a special capsule design. What groups are microphones divided into by type of transducer? According to the type of electromechanical transducer, microphones are divided into carbon, electromagnetic, piezoelectric, electrodynamic (dynamic) and capacitor (electrostatic). Professional microphones (with the exception of microphones for communication and sound in vehicles) usually use the last two types of transducer. Therefore, we will consider them in more detail. Dynamic microphones, in turn, are divided into coil and ribbon. Schematically, their simplest device is shown in Fig. 5 (a and b, respectively). In the first variant, a cylindrical frameless coil (as a rule, two- and, less often, four-layer) is placed in the annular gap of the magnetic circuit, in which a uniform magnetic field of the radial direction is created. The coil is glued to a domed diaphragm with a corrugated collar that acts as a suspension. When the diaphragm (made of polymeric material) oscillates under the action of sound pressure, the coil wire crosses the magnetic field of the gap (the width of which is usually 0,4 ... 0,6 mm) and an EMF is induced in the coil. Permanent microphone magnets are made of special materials with high residual induction and coercive force. The value of the active resistance of such a coil in various models usually ranges from 20 ... 600 Ohms.
a) dynamic microphone b) ribbon microphone 1 - domed diaphragm with a corrugated collar, 2 - cylindrical coil, 3 - magnet, 4 - magnetic circuit, 5 - corrugated foil tape, 6 - magnetic gap As a rule, with this type of transducer, microphones are made omnidirectional or with one-way directivity. In the latter case, holes are opened in the case of the magnetic system, sealed with silk or other porous material, which implements active acoustic resistance at the second input. To expand the range towards low frequencies in such microphones, additional closed volumes are usually used, connected inside with a magnet through tubes and holes of different sections. An omnidirectional microphone MD-83, as well as MD-97 and MD-91 microphones with one-way directivity - for speech sound amplification systems, currently produced by Microfon-M LLC (St. Petersburg) can serve as an example of such domestic microphones. . To compensate for electromagnetic interference (AC hum) in coil microphones, an antiphonal coil is usually included in series with the voice coil, which is wound, as a rule, on a magnetic system. The coils are switched on in such a way that the background voltages induced on them, excited in both coils, are mutually compensated. In a tape converter (Fig. 5, b), a corrugated (to ensure greater flexibility) metal (usually aluminum) ribbon several microns thick is used as a moving element, placed in a magnetic field between the pole pieces of a permanent magnet, the gap between which is usually of the order 1,5...2 mm. The ribbon serves both as a current conductor and as a movable transducer system. With this type of transducer, a microphone with an "eight" HN is usually implemented (due to the complete symmetry of the transducer), non-directional (with an acoustic labyrinth covering one side of the ribbon), less often - unilaterally directed. The ribbon, unlike the coil, has an extremely low electrical resistance of the order of 0,1 ... 0,3 Ohm, and the signal voltage at its output is only 20 ... 30 μV at a pressure of 1 Pa, microphone cables. Therefore, the voltage developed by the ribbon is preliminarily increased by means of a step-up transformer placed in the microphone housing in a permalloy screen. Sound engineers note the naturalness, softness, and transparency of the transmission of the timbre of many musical instruments, especially strings and cymbals, which are special for ribbon microphones. This is due to the lightness of the moving element - the ribbon, and, consequently, small transient distortions. Also, in dynamic microphones, it is theoretically possible to use an orthodynamic transducer, but so far it has not found application in mass-produced microphone models. Therefore, it makes no sense to dwell on its design here. Condenser (electrostatic) microphones (CM) have two electrodes - movable and fixed, forming the capacitor plates (Fig. 6). The movable electrode is a membrane made of a metal foil or a polymeric metallized film several microns thick. Under the action of sound pressure, it oscillates relative to a fixed electrode, which leads to a change in the capacitance of the capsule (capacitor) relative to the state of rest. In CM, the value of the capacitance change, and hence the output electrical signal, must correspond to the sound pressure. The degree of compliance of the output voltage with sound pressure in amplitude and frequency determines the frequency response and dynamic range of a particular microphone. An integral part of any CM is a node that matches the electrical impedance of the converter with the subsequent amplifying device. This electric link KM can be of high-frequency and low-frequency types. With a high-frequency type of conversion, the KM capsule is connected to the high-frequency generator circuit (of the order of several MHz). In this case, a frequency modulation of the RF signal is obtained, and only after demodulation is an audio frequency signal formed. This inclusion of the capsule does not require a polarizing voltage, it is characterized by a low level of the microphone's own noise. However, the high-frequency circuit in the microphone has not found wide application, mainly due to the complexity of frequency stabilization, and is rarely found in industrial models of microphones in the audio range. In the further presentation of the principles of operation and varieties of CM, we will mean CM with a low-frequency link, which include most modern CM models. In them, the conversion of sound pressure into an electrical signal occurs with external or internal (electret) polarization. CM in a system with external polarization (Fig. 6) forms a flat capacitor from electrodes with a capacity of 10 ... 100 pF with an air gap of 20 ... 40 μm, which through a resistance of about 0,5 ... 2 GΩ is charged from an external voltage source UP. When the membrane vibrates under the action of sound pressure or pressure difference, the magnitude of the charge of the plates due to the large time constant of the RC chain remains unchanged. The magnitude of the variable component of the voltage resulting from the vibrations of the membrane and the corresponding change in capacitance is proportional to the displacement of the membrane.
a) omnidirectional microphone: b) a microphone with two-way directivity 1 - metallized film, 2 - calibrated insulating gasket, 3 - fixed electrode Approximately twenty years ago, abroad and in our country, the industrial production of electret condenser microphones began, which do not require an external source of polarizing voltage; they use a polymeric electret film as a membrane, metallized from the outside. This film is polarized by one of the known methods and has the property of maintaining a constant surface charge for a long time. Thus, instead of an external source, an internal source is used. Otherwise, the operation of such a converter is fundamentally no different from a conventional CM. In the early 80s, NIIRPA developed a number of unidirectional and non-directional condenser microphones, but at present most of them have been discontinued for various reasons. Recently, when developing new models of microphones, electret material is applied in one way or another to a fixed electrode, which makes it possible to use thinner metal and polymer films as a membrane, which have significantly higher mechanical parameters compared to an electret film. This allows, with the same sensitivity of the capsule, to have a wider nominal frequency range of directional reception, extended both towards low (due to a decrease in the thickness, and hence the flexural rigidity of the membrane), and towards high (due to a decrease in the mass of the membrane) sound frequencies. An example of such professional microphones is the cardioid single-membrane electret microphone MKE-13M ("Microphone-M") and the omnidirectional lavalier MKE-400 ("Nevaton") produced by St. Petersburg enterprises, which are not inferior in their characteristics to the best models of foreign companies (in including KM with an external voltage source) and are more popular in Western European studios than in Russia.
a) single membrane microphone: b) double membrane microphone 1 - membrane 2 - fixed electrode 3 - air gap 4-5 - openings of acoustic channels 6 - insulating ring 7 - calibrated gaskets A simplified design of KM capsules is shown in Fig. 7. It can be seen from the figures that a single-membrane condenser microphone (small diaphragm), with an appropriate choice of design parameters, can be unidirectional (Fig. 7, a), non-directional (in this case, slot 7 must be closed), as well as bilateral ( Fig. 7b). In a double-membrane microphone (DKM or large twin diaphragm), both membranes can be electrically active (Fig. 7b). Without going into detail about the physics of the processes occurring in the DKM, which can be found in the specialized literature, we can say that each half of the DKM capsule represents, in acoustic and mechanical terms, a separate microphone with a cardioid directivity characteristic, the second acoustic input of which is not through a slit, as in a single-membrane microphone, and through the second (opposite) membrane, and the sensitivity maxima of these microphones are rotated by 180o. Such a microphone is also called acoustically combined. In addition to acoustic, DKM also implements electrical combination. So, by applying a polarizing voltage to one of the membranes (active), and the second (passive) shorting to a fixed electrode, it is possible to obtain, with the right choice of design parameters, a microphone with a one-sided characteristic curve close to a cardioid. When a polarizing voltage of equal magnitude and sign is applied to the second membrane, we obtain an omnidirectional microphone. When a polarizing voltage of equal magnitude and opposite in sign is applied to the second membrane, we obtain a two-way directivity ("eight"). In intermediate cases, if necessary, any CN can be obtained (see Fig. 1). As an example of such microphones with switchable XH, one can cite the C414B-ULS (AKG), U87i and U89i (Neumann), as well as the domestic MK51 (Nevaton). What are the main characteristics and parameters of microphones that serve as criteria for choosing them and why? When choosing microphones for certain working conditions, it is necessary to take into account the entire set of technical and operational requirements, based on the specific features of their use. In this regard, it is necessary to clearly understand what the technical characteristics of microphones determine. The main technical characteristics to be considered when choosing microphones are as follows: 1. The nominal frequency range, which, together with the uneven frequency response of the sensitivity, measured in dB, serves as a criterion for the correct transmission of the useful signal spectrum. 2. Free field sensitivity, which is usually normalized at a frequency of 1000 Hz and measured in mV / Pa, as well as the parameter associated with this value - the equivalent sound pressure level (for CM), due to the microphone's own noise and normalized in dB relative to zero level: ro = 2x10-5 Pa. Since any signal conversion and amplification system always contains its own noise, and the microphone is the initial link of such a system, the value of the useful signal it creates determines the signal-to-own noise ratio of the entire system. Therefore, reducing the sensitivity of the microphone is an undesirable factor. It should also be borne in mind that the desire to increase the width of the frequency range reproduced by the microphone leads to a decrease in the absolute value of its sensitivity. On the other hand, the wider the frequency range of the microphone, the more difficult it is to obtain a stable frequency response within it. 3. The directivity characteristic determines the spatial selectivity, i.e. the width of the solid angle in which the useful acoustic signal does not have significant amplitude non-uniformity. XN at a fixed distance from the useful signal source determines the useful signal/acoustic noise ratio at a relatively close distance from the useful signal source, i.e., within the boom radius. Closely related to XH is the concept of directivity coefficient, which determines the directional properties of a microphone in the far (relative to the source) field. Its sensitivity to a useful sound source located along the microphone axis is several times higher than to noise sources distributed around the microphone (to a diffuse field), or, in other words, with the same signal-to-noise ratio at the microphone input, a directional microphone can be located in times farther from a useful source than an omnidirectional one. In some approximation, we can assume that an omnidirectional microphone of small (compared to the sound wavelength) transverse dimensions quite accurately perceives the useful signal in a solid angle of 150...180°. With larger dimensions of an omnidirectional microphone, its characteristic curve strongly depends on frequency, narrowing noticeably at high frequencies, so the coverage angle in this case cannot be considered greater than 90°. For a cardioid microphone with a constant frequency HH, the coverage angle is 120°, for a supercardioid microphone - 90°, a hypercardioid microphone - 60°, a two-way directional microphone (with a "figure of eight" HH) the coverage angle is 60° on each side. It is also useful (for example, for calculating sound amplification systems) to know that the directivity coefficient (of a microphone with "circle" and "eight" HN is 1, with "hypercardioid" HN - 4, "supercardioid" - 3,7, "cardioid" - 3 , and for highly directional microphones, on average over the range, it can reach 5-7. 4. The level of limiting sound pressure, expressed in dB relative to ro = 2x10-5 Pa, is the level at which the harmonic distortion coefficient does not exceed 0,5% or another value established in the technical documentation. This parameter shows the limits of the linearity of the amplitude characteristic of the microphone and, together with the level of self-noise, determines the dynamic range of the microphone, and hence the path as a whole. 5. The module of total electrical resistance (impedance), in ohms, usually normalized at a frequency of 1000 Hz, determines the amount of load (input resistance of the amplifier or remote control) that the microphone operates on. As a rule, in order to avoid loss of a useful signal, the load value should exceed the microphone impedance by 5-10 times over the entire frequency range. 6. Overall dimensions, weight, connector type, and other design features make it possible to judge the possibility of using the microphone in certain conditions. The whole set of requirements for a particular microphone is determined by its purpose. What groups are microphones divided into by purpose? By appointment, microphones are divided into three large groups:
Professional microphones also differ significantly in purpose:
In addition, microphones differ greatly in their design, depending on the conditions of their attachment and location relative to the signal source:
It is extremely difficult to give any specific recommendations on the choice of microphones without taking into account specific conditions, since a microphone of a certain design solution and purpose (for example, a broadband condenser microphone for sound recording in studios) may be poorly compatible or even completely unacceptable for other conditions and purposes (for example , in conference systems or as a manual for soloists). It is possible to indicate only general rules that should be followed when choosing a microphone for one purpose or another. Broadcasting studios, as well as sound recording studios (television, film, recording) of music and artistic speech should be equipped with broadband microphones with the highest electro-acoustic parameters. Therefore, in studio conditions, as a rule, condenser microphones are used, which have a wide frequency and dynamic range, often with a switchable XH (double-membrane, the device of which is discussed above). In addition to the listed advantages, studio CMs have 5-10 times greater sensitivity than dynamic ones, and practically do not have audible transient distortion, since the resonance of the CM moving system lies near the upper limit of the nominal frequency range and has a very low quality factor. Therefore, in recording studios and in music sound reinforcement systems, small cardioid KMs, such as KM84, KM184 (Neumann), C460B (AKG), from domestic ones - MKE-13M ("Microphone-M") are increasingly being used as universal instrumental microphones. The disadvantages of CMs include the need for a constant voltage source, which is usually a mains power supply, as well as the fact that CMs do not tolerate humidity well, as well as a sharp change in temperature. The latter is due to the fact that the input impedance of the built-in KM amplifier has a value of 0,5 ... 2 GΩ, therefore, under conditions of high humidity and dew, this resistance decreases with a change in air temperature, which leads to a “blockage” of low frequencies and an increase in noise. Therefore, CM is rarely used outdoors and in portable installations. In studio conditions, the use of CM does not cause any difficulties. Microphones with unidirectional directivity are used at a wide angle of location of performers and when recording with several microphones for a clear separation of individual groups of musical instruments, as well as in cases where it is necessary to reduce the influence of extraneous noise or reduce the reverberation component in the recorded signal. A microphone with a two-way directivity is used when recording a duet, dialogue, singer and accompanist, when recording small musical compositions (string quartet), and also when it is necessary to detune from directional sources of noise or strong reflections from the ceiling and floor. In this case, the microphone is oriented with a zone of minimum sensitivity to noise sources or reflective surfaces. The figure-eight microphone is also used in cases where they want to specifically highlight the low frequencies of the voice of a soloist or a separate musical instrument, placing the microphone in this case in close proximity to the performer. Here the so-called "near zone effect" is used, associated with the manifestation of the sphericity of a sound wave at a close distance from the sound source, when the first and second acoustic inputs of the microphone are affected by sound pressures that are different not only in phase, but also in amplitude. This effect is most noticeable with "eight" microphones and is completely absent in omnidirectional ones. Omnidirectional microphones are used to transmit the general acoustic environment of the room when recording with several microphones, as well as when recording speech, singing, music in heavily muffled rooms, when recording various meetings and round table conversations. Recently, "boundary layer" microphones have been increasingly used for such recordings, in which the membrane of a very small size is located parallel to the plane of the table at a very small distance from its surface, and the microphone itself is designed as a small flat object, which, when placed on a table or on the floor is practically a continuation of its surface. Due to this, reflections from the table surface do not fall on the membrane of such a microphone, and the characteristic curve of such a microphone is determined by the direction and dimensions of the surface on which the microphone lies, and is close to a hemisphere in the sound range. As an example of such "boundary layer" microphones, one can cite C562BL (AKG), and from domestic models - MK403 ("Nevaton"). Omnidirectional CMs are also used as lavaliers built into furniture or tape recorders for acoustic measurements. Microphones in studios, except for the special cases mentioned above, are usually mounted on floor or boom stands. Since the microphone is not moved or touched during recording, and the stands create good shock absorption from the floor, as a rule, there are no special requirements for studio microphones in terms of vibration susceptibility. Many of the principles of sound recording, which require accurate microphone placement, taking into account the environment of the performer, in television are determined mainly by visual requirements. So, the microphone entering the frame should be small, with a surface that excludes glare, guaranteed to accurately reproduce the color of television. Outside the frame, microphones are used on mobile stands. Since the movement of the microphone occurs frequently during transmission, special measures are taken to protect it from air currents, vibrations (external shock absorbers, wind protection). Relatively large distances from sound sources and a high noise level require the use of directional and often highly directional microphones. For video cameras, as a rule, light, relatively small microphones are used with a slightly sharpened characteristic compared to a cardioid one, structurally compatible with the camera, often with the use of special measures in the microphone design to reduce vibration interference that occurs when the camera is moved during video recording. As an example, the MKE-24 and MKE-25 microphones ("Microphone-M"). Another group of professional microphones is for sound amplification systems for music and artistic speech in concert halls and theaters and broadcasting from these facilities. The main feature of the operation of microphones in sound amplification systems (C3U) is the possibility of their self-excitation as a result of the occurrence of parasitic acoustic feedback at certain frequencies, due to the sound signal from the loudspeaker (direct) or reflected from the walls of the ceiling, other surfaces on the microphone. This phenomenon usually limits the amount of sound pressure in the sounding of halls. Improving the stability of C3U is achieved both by special electronic signal processing and by a few simple considerations outlined below. 1. The maximum approximation of the microphone to the source of the primary signal (singer, speaker, musical instrument), i.e. the use of lavalier (for speech) and handheld microphones. Note that lavalier microphones are usually omnidirectional, so bringing them closer to the speaker does not affect their frequency response. In handheld microphones, usually unidirectional, special measures are taken to cut low frequencies in order to compensate for their rise when working with a close signal source. 2. The maximum possible distance of the speaker and microphone from loudspeakers and reflective surfaces (microphone on stands at the level of the performer's mouth or musical instrument). 3. The correct choice of the XH of the microphone and the orientation of its working axis relative to both the source of interference (reflections) and relative to the working axis of the nearest loudspeakers and speakers. We note here that, according to the results of our studies, the most versatile, in terms of C3U stability, is a microphone with a supercardioid voltage characteristic, this is especially significant in the range from 200 to 3000 Hz. In C3U and television broadcasts, microphones should be preferred as small as possible so that they do not interfere with the audience watching what is happening on the stage or stage. For the same reasons, microphones with shiny and bright colors should not be used. In the theater environment, microphones are often placed along the ramp, where they are exposed to the strong electromagnetic fields generated by lighting installations. Here you should use microphones with reliable shielding, with a balanced output, and in dynamic ones, an antiphonal coil is required. In a concert hall, stage, podium, there is a danger of great interference due to shocks and vibrations, and therefore most of the stands have a vibration absorber, usually on the base, and the stands embedded in the stands often include a shock absorber. However, they do not completely eliminate the transmission of vibrations from table, floor or podium shaking. In addition, there is always the possibility that the speaker will touch the stand, not to mention the microphones for soloists, which are mainly operated by hand. These microphones provide special measures for vibration protection: the capsule is shock-absorbed or untied relative to the microphone body, electric filters are used that cut off low frequencies. Dozens of models of such microphones are produced by many European companies (AKG, Sennheiser, Beyerdynamic), American (Electro-Voice, Shure), from domestic - "Byton-2". It should be noted that dynamic microphones are fundamentally more sensitive to vibrations than condenser ones, and directional microphones are more sensitive than pressure receivers. In speech amplification systems (conference halls, meeting rooms, drama theaters, etc.), the main criterion is speech intelligibility, and not the correct transmission of timbre, therefore it is better to limit the frequency range of microphones to the range of 100 ... 10 Hz with a "blockage" of low frequencies , starting from 000...300 Hz up to 400...10 dB at 12 Hz. As an example of such microphones, one can cite models D100, D541В, D558, С590 (AKG), from domestic ones - MD-580, MD-91, MD-96 ("Microphone-M"). Further narrowing of the microphone frequency range is possible to 97...500 Hz with almost no loss of intelligibility, but this leads to a noticeable distortion of the speaker's voice timbre, which is also undesirable in high-quality C5000 speech. Therefore, microphones with a frequency range of 3 ... 500 Hz, and even narrower, are used only in communication devices where the transmission of the voice timbre is not essential, but it is necessary to correctly convey the meaning of actions, commands, etc. The narrowing of the frequency range in microphones for C3U speech to 100 ... 10 Hz is a certain compromise between intelligibility and transmission of speech timbre and is also advisable because the spectrum of aerodynamic (wind, from the speaker's breathing), vibration (friction and body shock) noise , as well as reverberant interference in poorly damped rooms, which are most meeting and conference rooms, has a pronounced low-frequency character. Therefore, from the point of view of the "useful signal/noise" ratio, it is not advisable to have microphones with a wide range of low frequencies. Moreover, the C000U uses unidirectional microphones, which, when placed near the speaker, cause a rise in low frequencies, which compensates for their drop in the microphone's frequency response, taken in a free field at a standard distance of 3 m. In the absence of such a drop, low frequencies are emphasized, which causes the effect “mumbling”, “barrel-shaped” sound of the microphone, speech intelligibility is reduced. To improve speech intelligibility and vocal transparency, microphones for C1U usually have a smooth rise in frequency response at frequencies of 3 ... 3 kHz up to 7 ... 3 dB. A separate group of microphones include lapel, or as they are also called lavalier, microphones used both on television and in C3U. Lavalier microphone - usually a pressure receiver, light and small in size, with a special attachment to clothing; these are, for example, microphones SK97-O (AKG), MKE10 (Sennheiser), KMKE400 (Nevaton). The use of such microphones has both advantages and disadvantages. The obvious advantages are the freedom of the speaker's hands and the proximity of the microphone to the source of the useful signal. Let's list a few disadvantages. This is the contact of the microphone with the chest, which affects the color of the sound at low frequencies; it depends on the type of clothing and characteristics of the speaker. In addition, there is often nowhere to mount the power supply on the speaker. Often the microphone is shielded by the chin, and the sound loses the effect of presence, sometimes nasal tones are emphasized, which leads to nasal sound and poor intelligibility. Microphone cable touching clothing causes rustling noises. In addition, there are psychological difficulties in the use of such microphones. Microphones for outdoor operation must be suitable for use in any weather: in rain, snow, wind, etc., therefore, dynamic microphones are usually used for these purposes, which, compared to condenser and electret ones, have significantly greater resistance to temperature and moisture that do not require constant power, more reliable. To reduce wind noise, such microphones usually have a streamlined shape, an external windshield, since the built-in windshield, usually used in handheld microphones and for C3U speech, is not enough for outdoor operation in windy conditions. When reporting from the street, it is more expedient to use omnidirectional microphones as hand-held microphones, since they are fundamentally less susceptible to wind, vibrations, and accidental shocks. At the same time, of course, special measures to reduce the influence of vibrations and wind should not be excluded in the designs of such microphones. As an example of reporting microphones - F-115 (Sony), and from domestic ones - MD-83 ("Microphone-M"). In C3U outdoors, for the same reasons as indoors, directional microphones should be used, but one should still try to avoid the possibility of precipitation on the microphone (installation of canopies, booths, etc.). Author: Sh.Vakhitov
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