catalogue
Classification of microphones
Introduction of microphone types
The history of microphones
Main technical indicators of microphone
Matters needing attention in the use of microphone
Maintenance of microphone
Classification of microphones
There are many kinds of microphones:
First, according to the principle of energy conversion, it can be divided into inductive, capacitive, electromagnetic, piezoelectric and semiconductor microphones;
Second, according to the directionality of received sound waves, it can be divided into two types: non-directional and directional. Directional microphone includes cardioid directivity, strong directivity and bi-directivity.
Three, according to the purpose can be divided into stereo, close call, wireless microphone, etc.
Introduction of microphone types
First, the dynamic microphone
This is one of the most commonly used microphones. It is mainly composed of diaphragm, voice coil, permanent magnet and booster transformer. Its working principle is that when a person speaks into a microphone, the diaphragm vibrates back and forth with the sound, thus driving the voice coil to cut the magnetic field lines in the magnetic field. According to the principle of electromagnetic induction, the two ends of the coil will produce induced audio electromotive force, thus completing the acoustic-electrical conversion. In order to improve the output induced electromotive force and impedance of the microphone, a step-up transformer is needed.
The invention has the advantages of simple and stable structure, convenient use and low inherent noise. Early dynamic microphone had low sensitivity and narrow frequency range. With the maturity of manufacturing technology, many professional dynamic microphones have appeared in recent years, which have good characteristics and technical indicators and are widely used in language broadcasting and sound reinforcement systems.
When picking up outdoor or human voice, the airflow when the wind is talking to people will impact the diaphragm of the acoustic-electrical converter, which will make the microphone make a lot of noise and even make the diaphragm unable to move freely. At this time, the wind is needed. That's what the hoodie does It consists of a metal cover of the shell and a sponge inside. The metal cover can resist the impact of external force and protect the microphone. The sponge will weaken and prevent the air flow from entering. In this way, the airflow movement when people speak and the airflow movement of the wind will not affect the pickup effect. Because the sound is not the directional movement of airflow, but the mechanical fluctuation, it is little influenced by the range hood. If it is in a strong wind, the interference of airflow may be great. At this time, it is necessary to add a layer of windproof device (windproof basket) to the shell of the microphone.
In addition, acoustic resistance, nylon mesh and resonant cavity are all acoustic treatment measures set by the microphone to improve the sound quality.
Second, the aluminum microphone
This is also a kind of microphone made by electromagnetic induction principle. Because of its excellent sound quality, it was widely used in professional fields before the 1960s. But it has one of the biggest disadvantages: it is very delicate and easy to be damaged. When using this kind of microphone, we should pay great attention to windproof measures, otherwise the blasting sound such as force and beating may damage the microphone, and of course it cannot be used for outdoor pickup work.
The reason for this feature is structural. Thin aluminum strips are used to replace the coils of moving-coil microphones. When this aluminum strip vibrates in a magnetic field, it can also induce current. This kind of aluminum strip is about several centimeters long, 2-4 millimeters wide and only a few microns thick. It is light in weight and can sense and transmit the vibration characteristics of sound waves, so it has good sound quality and is easy to be damaged.
Aluminum ribbon microphone has no diaphragm, and its aluminum ribbon is both a diaphragm and a coil. Its acoustic wave driving mode is also different from the moving coil driving mode, and it is a double-sided driving mode. When sound waves reach different distances on both sides of the aluminum strip, due to different phases, it can cause sound pressure difference, and the aluminum strip will vibrate due to this sound pressure difference.
Third, the condenser microphone.
Capacitor microphone is a kind of microphone with good performance at present, and its working core is capacitance. There are three main types: audio type, radio frequency type and electret type.
A condenser microphone works by changing the capacitance. It is mainly composed of diaphragm, rigid polar plate, power supply and load resistor. Its working principle is that when the diaphragm is under the pressure of sound waves and vibrates with different pressures and frequencies, the capacitance between diaphragm plates changes. At the same time, the charge on the polar plate changes accordingly, so that the current in the circuit changes accordingly, and there is a corresponding voltage output on the load resistance, thus completing the acoustic-electrical conversion.
Capacitive microphone has the advantages of wide frequency band, high sensitivity, small distortion and good sound quality, but its structure is complex and its cost is high. It is mostly used for high-quality broadcasting, recording and sound reinforcement. Four. Electret condenser microphone
The working principle of this microphone is the same as that of the condenser microphone, except that it uses a PTFE material as the diaphragm. This material is named electret condenser microphone because it is permanently charged on its surface after special electrical treatment, thus replacing the electrode plate of condenser microphone. It is characterized by small size, superior performance and convenient use, and is widely used as a built-in microphone in cassette recorders.
Five, wireless microphone
Wireless microphone is actually a small sound reinforcement system. It consists of a miniature transmitter. The transmitter consists of a miniature electret condenser microphone, a frequency modulation circuit and a power supply. The wireless microphone uses frequency modulation to modulate the signal, and the modulated signal is transmitted through the short opening of the microphone. According to national regulations, the transmission frequency ranges from 100 MHz to 120 MHz, and every 2MHz is a channel to avoid mutual interference.
Wireless microphones and receivers should be used in one-to-one correspondence, and there can be no mistakes. The receiver is a special FM receiver, but the general FM radio can also hear the sound emitted by the wireless microphone, as long as the tuning frequency is adjusted to the frequency emitted by the wireless microphone.
Wireless microphone is small in size, easy to use, good in sound quality, wireless between microphone and power amplifier, free to move and low in transmitting power, so it has been widely used in classrooms, stages and TV shooting.
The history of microphones
The history of microphone can be traced back to the end of 19, and scientists such as Alexander Graham Bell devoted themselves to finding a better pickup method to improve the latest invention at that time-telephone. During this period, they invented the liquid microphone and the carbon particle microphone. These microphones are not ideal, but they can barely work.
In the 20th century, the microphone changed from resistance type to inductance and capacitance type, and gradually developed a large number of new microphone technologies, including aluminum tape, moving coil and other microphones, as well as condenser microphones and electret microphones which are widely used at present.
Main technical indicators of microphone
First, sensitivity.
The sensitivity of microphone refers to the acoustic-electrical conversion ability of microphone. The specific value is that when the sound pressure of 10 Pascal acts on the microphone diaphragm, the microphone can be converted into a voltage of 1V, so the microphone sensitivity is 0dB. This is a very large number, which is beyond the reach of a microphone. The sensitivity of ordinary microphones is about-70dB, that of higher microphones is about-60dB, and that of professional high-sensitivity microphones can reach about-40dB.
A high-sensitivity microphone can pick up louder sound under the same conditions, reduce the burden of the post-amplifier, and easily obtain a high signal-to-noise ratio. Of course, excessive signal output should also consider the bearing capacity of the later equipment.
Second, directionality.
The directivity of microphone is one of the most important characteristics of microphone. Refers to the difference in sensitivity of microphones to sounds in different directions. This abstract meaning can usually be expressed intuitively in polar coordinates.
Polar coordinates use the angle and distance from the center to determine any point in the coordinates. When expressing the directivity of the microphone, it is equivalent to putting the microphone on the O point, the angle of 0 degrees is the positive direction of the microphone (the technical term is the main axis direction), and the distance from the O point is the sensitivity. In the use of polar coordinates, the length of 0 degree angle in this direction is designated as 1, so it is easy to see the difference of sensitivity in other directions. If the sensitivity of the 20-degree angle is 80% of the 0-degree angle, draw a point with a length of 0.8 at the 20-degree angle in the coordinates; If the sensitivity of a 90-degree angle is 50% of that of a 0-degree angle, draw a point with a length of 0.5 at the 90-degree angle in the coordinates. Similarly, the value of sensitivity within a 360-degree angle can be drawn, which is the polar diagram of microphone directivity.
Similarly, if the directional pattern is a perfect circle, it means that the microphone has the same sound sensitivity in all directions.
Third, proximity effect.
The short-range effect of microphone is a characteristic of differential pressure transducer microphone. This kind of microphone will obviously improve its low-frequency sensitivity when it picks up at close range, and the closer the distance is, the greater the low-frequency output will be. The lower the frequency, the stronger the short-range effect.
The short-range effect destroys the good frequency response of the microphone, that is, the low-frequency part of the original sound field will increase abnormally after passing through this microphone. In this way, for heart-shaped and figure-eight microphones, the pickup distance should not be too close. Especially for the pickup of bass instruments, too strong low-frequency sound will cause serious interference and destroy the pickup balance of the whole band.
The solution is to have a low-frequency attenuation switch on the microphone. When this switch is turned on, the microphone uses the method of electrical signal processing to attenuate the bass component in the output signal. This switch is generally divided into three levels: off, music and voice. The latter two are sometimes abbreviated as M and V. The former means music and does not attenuate low-frequency signals, while the latter attenuates low-frequency signals.
Why does the microphone have music files and keep the bass effect? This is because the proximity effect also has its advantages.
According to psychological research, the high and low frequency bands of sound will make people feel "kind and sweet"; If the high and low frequencies are properly attenuated, the sound will give people a sense of distance, loudness and penetration. Therefore, some pop singers like to pick up the microphone close to their mouths, and also achieve the emotional atmosphere required by the song content.
Fourth, the signal-to-noise ratio.
The signal-to-noise ratio of microphone refers to the ratio of signal component to noise component when the microphone outputs.
This is an important technical index of microphone. The higher the SNR, the better the quality of the microphone. Because when the pickup object is a very weak sound, it is necessary to increase the playback in order to hear clearly during recording and playback. At this time, a microphone with high SNR can bring less noise, by going up one flight of stairs.
High-sensitivity microphone can reduce the noise of the amplified equipment, but high-sensitivity microphone can't reduce the noise of the output signal.
To sum up, this is the relationship: high signal-to-noise ratio can reduce the noise output of microphone, while high sensitivity can reduce the noise generated by subsequent equipment due to amplification.
Verb (abbreviation of verb) records and plays amplitude-frequency response.
The sensitivity of the microphone is different under the action of sound waves with different frequencies. Generally, the sensitivity is higher at intermediate frequencies (such as 1 kHz), but it is lower at low frequencies (such as tens of Hz) or high frequencies (more than ten kHz). Based on the intermediate frequency sensitivity, the frequency range where the sensitivity drops to a specified value is called the frequency characteristic of the microphone. The expression method is to draw a frequency response curve. Observe the smoothness of the curve and the frequency range within plus or minus 3 decibels. For example, the frequency response of a microphone is 55- 18KHz, which means that the output signal of this microphone changes within 3 dB within the range of 55- 18KHz.
Six, the output impedance
As mentioned in the antenna system, microphones or any other equipment have input and output impedance problems. The output impedance of microphone can be divided into three categories: high impedance (10-20kΩ); Medium resistance (600 Ω); In the case of low resistance (200 Ω), the output impedance of the microphone will affect the impedance matching mode of its connection with subsequent equipment. Moreover, for microphones, high-impedance microphones are more susceptible to noise, and professional microphones often output signals in a low-impedance way.
Seven, the maximum sound pressure
Excessive sound pressure will lead to poor pickup quality and may damage the microphone, so the microphone has a technical index of "maximum sound pressure". Generally, this value can reach more than 120dB, which can meet the requirements of general pickup work. However, for high sound pressure pickups (such as jet engines, steam hammers, etc. ), still have to consider. For extremely close pickup, although the sound pressure of the sound source is not very large, it may become very large because of the close distance. This indicator should also be considered at this time.
Matters needing attention in the use of microphone
When choosing a microphone, we should consider the choice comprehensively according to the use situation and the requirements for sound quality, combined with the characteristics of various microphones. For example, high-quality recording and broadcasting mainly requires good sound quality, and should choose condenser microphone, aluminum tape microphone or advanced dynamic microphone; For general amplification, you can choose ordinary moving coil type; When the speaker moves or moves away from the speaker from time to time, such as karaoke, one-way microphone with low sensitivity should be selected to reduce noise interference. It should be noted that:
1, impedance matching
When the transmitter is used, the output impedance of the microphone is the same as the input impedance of the amplifier, which is the best matching. If the mismatch ratio is above 3: 1, the transmission effect will be affected. For example, if a 50 Ω microphone is connected to an amplifier with an input impedance of150 Ω, the output can be improved by nearly 7Db, but the high and low frequency sounds will be obviously lost.
Step 2 connect the wires
The output voltage of the microphone is very low. In order to avoid loss and interference, the connecting wire must be as short as possible. For high-quality microphones, double-core stranded metal isolation wires should be selected, and for general microphones, single-core metal isolation wires can be used. The transmission line length of high impedance microphone should not exceed 5 meters, otherwise the treble will be obviously lost. The connecting wire of low resistance microphone can be extended to 30~50m.
3. Working distance and proximity effect
Usually, the working distance between the microphone and the mouth is 30cm~40cm. If the distance is too far, the echo will increase and the noise will increase relatively. If the working distance is too close, it will be distorted because of the strong signal, and the low-frequency sound will be too heavy, which will affect the clarity of the language. This is because the directional microphone has a "close-talk effect", that is, the low-frequency sound will be obviously improved when it is played at close range. However, sometimes singers deliberately use "near effect"? Quot makes the singing effect more beautiful.
4. Angle between sound source and microphone
Each microphone has its effective angle. Generally speaking, the sound source should be aimed at the center line of the microphone. The greater the deviation angle, the greater the treble loss. Sometimes when using the microphone, there will be a "rumbling" sound. At this time, it can be alleviated by deflecting the microphone at some angle.
5. Microphone position and height
When amplifying, don't put the microphone near or aim at the speaker first, otherwise it will cause howling.
The height of the microphone should depend on the height of the sound source. If one person speaks or several people sing, the height of the microphone should be consistent with the singer's mouth. When there are many people, the microphone should be placed at an average height, and the positions of singers, accompanists and various musical instruments in the team should be reasonably allocated. Don't put the sound too loud and too light, and all the sounds should be within the effective angle of the microphone. If there is a lead singer or orchestra, a dedicated microphone should be placed if necessary.
When several microphones need to be used at the same time, parallel connection can be adopted, but the phase of several microphones must be paid attention to. Only when the phases are consistent can they be connected in parallel, otherwise they will interfere with each other, reduce the output and distort the output. Microphones of different models and impedances should not be used in parallel, because the high impedance microphone is short-circuited and the output voltage is very low. Usually microphones are used directly in parallel, and the effect is not as good as that of a single microphone.
If several microphones are used for one person to speak at the same time, it is better to choose the same model than to divide several places for different purposes. Otherwise, the tone of speech will change due to the change of the speaker's position or angle.
Attention should be paid when using wireless microphone:
(1) Select the location where the receiver is placed to avoid "dead spots".
(2) When receiving, 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 droop naturally and be exposed outside the clothes.
(4) Prevent the polarity of the battery from being reversed, and take out the battery in time after use.
Some microphones, such as electret condenser microphones and wireless microphones, are powered by batteries. If the voltage drops, the sensitivity will decrease and the distortion will increase. Therefore, when the sound becomes worse, check the battery voltage, turn off the power switch when the microphone is not used, and take out the battery when it is not used for a long time.
Maintenance of microphone
The most vulnerable parts of the microphone are the diaphragm and the coil.
In order to transmit the sound vibration well, the shape of the diaphragm has been carefully designed, and the surface has many ups and downs. Moreover, for good vibration performance, it is generally very thin, so as to be light in weight and small in inertia. Too strong external force can easily deform and damage the microphone, which will reduce the quality of the microphone and completely damage its pickup ability. For professional work, the quality of pickup is very high, so it is very important to protect the microphone.
The same is true for the coil of the dynamic microphone, because it needs to vibrate freely in the narrow gap of the magnet, so its equilibrium position will not be destroyed by external force. Its deformation and shedding are difficult to repair. Therefore, the first point of microphone maintenance is to avoid violent vibration and blowing and knocking during use.
In order to transmit sound information better, the microphone has a precise acoustic resonance cavity with some small holes communicating with the outside to meet some acoustic design requirements (for example, the heart-shaped microphone has sound channels to bypass the back of the diaphragm), so that these sound channels cannot be artificially blocked in use. Common mistakes in use are: incorrect cloth wrapping and hand holding methods.
The most important thing for the preservation of microphones is moisture and dust prevention. In particular, condenser microphones should be stored in a drying oven in humid weather, and the humidity in the oven should not be too high or too low.
Turn off the power switch when the microphone is not used, and take out the battery when it is not used for a long time.