A microphone is a transducer that converts an acoustic signal into an electrical signal. Commonly known as the simple, microphone. The quality of the microphone will directly affect the quality of the sound.
(1) There are many varieties of microphones. According to the principle of transduction, they can be divided into electric, capacitive, electromagnetic, piezoelectric, and semiconductor microphones. According to the directionality of the received sound waves, they can be divided into non-directional and directional. Sexual two kinds of directional microphones include heart-shaped directivity, strong pointing, double-directionality, etc.; can be divided into flat sound, close-talking, wireless microphone, etc. according to the use.
1. Dynamic microphone This is the most commonly used microphone. Its structure is shown in Figure 2-2-1, which is mainly composed of vibrating diaphragm, voice coil, Yongyi magnet and step-up transformer. Its working principle is that when a person speaks into the microphone, the diaphragm trembles with the sound before and after, so that the voice coil moves the magnetic field of the cutting magnetic field in the magnetic field. According to the principle of electromagnetic induction, an induced audio electromotive force is generated at both ends of the coil, thereby completing the acoustic-electrical conversion. In order to improve the output induced electromotive force and impedance of the microphone, a step-up transformer is also required. Dynamic microphones are simple, stable, easy to use, and inherently low noise, and are commonly used in speech broadcast and sound reinforcement systems. However, the defect is low flexibility and narrow frequency range. In recent years, there have been special dynamic microphones, and their characteristics and technical indicators are better.
2. Capacitor microphone The condenser microphone works by changing the capacitance. Its structure is shown in Figure 2-2-2, which is mainly composed of a vibrating diaphragm, a rigid plate, a power supply and a load resistor. Its working principle is that when the diaphragm is subjected to the pressure of sound waves and vibrates with the magnitude and frequency of the pressure, the capacitance between the diaphragm plates changes. At the same time, the charge on the plate changes accordingly, so that the current in the circuit also changes accordingly, and the load resistor has a corresponding voltage output, thereby completing the acoustic-electric conversion. The condenser microphone has a wide frequency range, high flexibility, small distortion, and good sound quality, but the structure is complicated and the cost is high, and it is mostly used for high-quality broadcasting, recording, and sound amplification.
3, electret condenser microphone This microphone works the same as the condenser microphone, the difference is that it uses a PTFE data as a diaphragm. Since this kind of data is treated by special electric power, the appearance is permanently charged with a polarized charge, which replaces the plate of the condenser microphone, so it is called an electret condenser microphone. Its characteristics are small size, superior performance, and convenient use, and it is commonly used in cassette recorders as an internal microphone.
4, wireless microphone wireless microphone is a small sound reinforcement system in practice. It consists of a miniature transmitter. The transmitter is composed of a micro-electret condenser microphone, a frequency modulation circuit and a power supply. The wireless microphone adopts a frequency modulation method to modulate the signal, and the modulated signal is transmitted through the short open line of the microphone and the range of the transmission frequency. The national rule is between 100MHz and 120MHz, and every 2MHz is a channel to prevent mutual interference. The wireless microphone and the receiver should be corresponding one by one, and the supporting application should not be used to show the fault. The receiver is a dedicated FM receiver, but an ordinary FM radio only needs to adjust its tuning frequency to the frequency emitted by the wireless microphone, and can also hear the sound from the wireless microphone. The wireless microphone is small in size, convenient to use, good in sound quality, wireless between the microphone and the amplifier, and easy to move, and the transmission power is small, so it has been widely used in classrooms, stages, and television production.
(II) Performance Index of the Microphone The performance index of the microphone is an objective parameter for evaluating the quality of the microphone, and is also the basis for selecting the microphone. The performance of the microphone is mainly as follows:
1. Flexibility Flexibility refers to the size of the output of an electrical signal by a microphone under the influence of a certain intensity of sound. The high degree of flexibility means that the sound-electric conversion efficiency of the microphone is high, and the response to the small sound signal is flexible. It is technically used to indicate how flexible the microphone can output under the sound pressure of 0.1pa [μBar]. If the flexibility of a microphone is 1mV/μBar, it means that the signal voltage of the microphone is 1mV under the sound pressure of 1μBar. The habit is also often 2, the frequency characteristic microphone is different in the flexibility of sound waves under different frequencies. Normally, the flexibility is high in medium audio (such as 1 kHz), and the flexibility is low in low audio (such as tens of Hz) or high audio (ten terahertz). Based on the flexibility of the mid-audio, the frequency range in which the flexibility is reduced to a certain rule value is called the frequency characteristic of the microphone. The wide range of frequency characteristics indicates that the microphone has a higher flexibility for sounds in a wider frequency band, and the sound amplification effect is good. The ideal microphone frequency characteristic should be 20Hz~20kHz.
3. The output impedance of the output impedance microphone is the impedance between the two output lines of the microphone at 1KHz (ie 1 kHz). There are low resistance (such as 50Ω, 150Ω, 200Ω, 250Ω, 600Ω, etc.) and high resistance (such as 10KΩ, 20KΩ, 50KΩ).
4. Directional directionality indicates the characteristic that the flexibility of the microphone changes with the incident high direction of the sound wave. For example, bidirectionality means that only sound waves in one direction are flexible, while sound waves in other directions have no output at all. Non-directionality means that the sound waves of the same sound pressure in all directions can have approximately the same output. The directionality of the microphone is shown in Figure 2-2-3.
(3) The use of microphones The choice of microphones should be based on the location of the application and the request for sound quality, to separate the characteristics of the various microphones, and consider the choice. For example, high-quality recording and broadcasting, the main request sound quality is good, should use condenser microphone, aluminum belt microphone or advanced moving coil microphone; for ordinary amplification, you can use ordinary moving coil type; when the speaker position moves from time to time Or speaking with a large interval between the amplifier, such as karaoke singing, should use a dual-directional, less flexible microphone to reduce noise interference. In the application should pay attention to:
1. Impedance matching When using the transmitter, the output impedance of the microphone is the same as the input impedance of the amplifier. If the mismatch ratio is above 3:1, the transmission effect will be affected. For example, when a 50Ω microphone is connected to an input impedance of 150Ω, the output can be increased by nearly 7Db, but the sound of the upper and lower frequencies will be significantly lost.
2. The output voltage of the microphone is very low. In order to avoid loss and interference, the wiring must be as short as possible. The high-quality microphone should choose the double-core stranded metal isolation line. The ordinary microphone can use the single-core metal isolation line. The length of the high-impedance microphone transmission line should not exceed 5 meters, otherwise the high-pitched sound will be significantly lost. The connection of the low-resistance microphone can be extended to 30~50m.
3. Working interval and near-talk effect Generally, the working interval between the microphone and the mouth is 30cm~40cm. If the interval is too far, the reverberation will increase and the noise will increase relatively. If the working interval is too close, the signal will be distorted due to excessive signal. The low frequency sound is too heavy and affects the clarity of speech. This is because the directional microphone has a "near-speaking effect", that is, when the interval is broadcasted, the low-frequency sound will be significantly improved. However, sometimes the singer intends to apply "nearly speaking?"; making the singing effect more beautiful and beautiful.
4, the angle between the sound source and the microphone Each microphone has its effective angle, the ordinary sound source should be aligned with the microphone center line, the greater the off angle between the two, the higher the treble loss. Sometimes when using a microphone, it has a "long" sound, and when the microphone is deflected by some angle, it can be lightened.
5. Microphone position and height When the sound is amplified, the microphone should not be placed close to the speaker or aligned with the speaker, otherwise it will cause whistling. The height of the microphone should be determined according to the height of the sound source. If a person speaks or sings by a few people, the height of the microphone should be partially different from the singer's mouth; when there are a large number of people, the microphone should be placed at a uniform height and the singer should be properly distributed. And the accompaniment and the position of the various instruments in the team, do not make the sound too loud, light and light, and make all the sounds within the effective angle of the microphone.
If there is a lead singer or lead, a dedicated microphone should be placed if necessary. When several microphones are required for simultaneous use, the method of connecting and connecting can be adopted, but it is necessary to pay attention to the phase problems of several microphones. When the phases are different, they can be connected in parallel with each other, otherwise they will interfere with each other, so that the output is reduced and distorted.
Microphones of different models and different impedances should not be used in parallel. Because the high-impedance microphone is "short-circuited", the output voltage is reduced to a very low level. Normally, the microphones are used directly in parallel, which is not as effective as a single microphone. If you use a few microphones for a person to use at the same time, instead of separating the centers for different uses, then the microphone should choose the same model. Otherwise, the tone of the speech will change due to the walker or angle of the speaker. The microphone should be protected from knocking or falling during use. It is not advisable to experiment with the microphone by blowing or tapping, otherwise the microphone will be easily damaged. When the microphone is used outdoors, the windshield should be used to prevent the "beep" sound from entering the wind. The windshield also prevents dust from contaminating the microphone. Pay attention when using wireless microphones:
(1) Select the position where the receiver is placed, so that it should avoid the "dead point".
(2) When accepting, adjust the angle of the receiving antenna, adjust the frequency, and adjust the volume to make it in the best state.
(3) The antenna of the wireless microphone should sag naturally and be exposed outside the clothing.
(4) Avoid reverse polarity of the battery. At the end of use, remove the battery in time. Some microphones (such as electret condenser microphones, wireless microphones) are battery powered. If the voltage drops, the flexibility will be reduced and the distortion will increase. Therefore, when the sound is worse, check the battery voltage. Turn off the power switch when the microphone is not in use. Remove the battery when not in use for a long time.
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