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Introduction to microphone performance indicators

Author:admin   AddTime:2021-07-16

Frequency response: The frequency response of a microphone refers to the change in the forward sensitivity of the microphone with frequency. The characteristic of this frequency response is usually expressed by a frequency response curve, which is generally a free-field frequency response.
Different use occasions have different requirements on the frequency response of the microphone. Microphones with different frequency response curves should be selected according to the use occasion. Generally speaking, the wider the frequency response curve, the better, but it is not absolute. To proceed from the actual effect, choose a moderate microphone. If the frequency response is too wide, unnecessary frequencies will be included, and the noise will increase, especially the low frequency extension. In the live sound reinforcement, the probability of acoustic feedback will increase, because the microphone has poor directivity in the low frequency range. In addition, from an economic point of view, the ones with wider frequency response are usually more expensive. But no matter whether you choose a wide or narrow frequency response curve, the unevenness in the frequency range should be small, that is, the curve should be straight. If there is a peak point, the sensitivity at the peak point will be high, which is likely to cause acoustic feedback. The sound frequency range of human speech is narrower than that of music, and band performance is wider than solo, chorus, etc.
In order to obtain special sound effects or make up for the lack of direct sound frequency response, many microphones are specifically designed with uneven frequency characteristics. For example, special microphones for certain musical instruments can be low-frequency boosted or high-frequency boosted, neck-mounted and wearable microphones used for broadcasting are high-frequency boosted, and some microphones are designed with adjustable frequency.
The frequency characteristic of the sound system is usually described by the frequency response curve with the ordinate of the decibel scale representing the power and the abscissa of the logarithmic scale representing the frequency.
The human hearing range is from 20 Hz to 20000 Hz. In theory, a frequency response of 20 to 20000 Hz is sufficient. Sounds below 20Hz can not be heard but other human sensory organs can perceive the so-called bass power. Therefore, in order to perfectly play various musical instruments and language signals, the amplifier must achieve the goal of high fidelity. Replay all harmonics of the tone. Therefore, the frequency band of the amplifier should be extended, the lower limit should be extended below 20 Hz, and the upper limit should be increased to above 20000 Hz. The way of expressing the frequency response of the signal source (radio head, recording stand, CD player, etc.) is different. For example, the frequency response of the FM stereo broadcast specified by the European Broadcasting Union is ±2dB at 40~15000Hz, and the lowest frequency response index specified by the International Electrotechnical Commission for the recording station: ±2.5 ± 4.5dB (normal band) at 40~12500Hz, The actual achievable indicators are significantly higher than this value. The upper limit of the frequency response of the CD player is 20000Hz, and the low frequency end can be very low, only a few hertz, which is one of the reasons for the good sound quality of the CD player.
The harmonic components that make up the sound are very complicated, and it is not that the wider the frequency range, the better the sound.
Sensitivity: The sensitivity of a microphone is an indicator of the microphone's sound-to-electric conversion capability. It is a physical quantity that characterizes how much electrical output the microphone can produce under a certain sound pressure. Generally speaking, it is the complex ratio of the output voltage of the microphone to the sound pressure received by the microphone.
Sensitivity is based on different test conditions. There are no-load sensitivity, load sensitivity, sound field sensitivity, sound pressure sensitivity, etc., which can be measured in different units such as mV/ubar (millivolt/microbar), mV/Pa (millivolt/Pa), dB, etc. Said.
1. No-load sensitivity (open circuit sensitivity) There is no special note. The sensitivity referred to in the general technical conditions of the microphone is the no-load sensitivity.
No-load sensitivity is the ratio of the measured output voltage to the sound pressure acting on the microphone diaphragm when the output terminal of the microphone is in an open circuit state.
2. Load sensitivity
Load sensitivity is the ratio of the output voltage measured on the load to the sound pressure acting on the microphone diaphragm when the rated load is applied to the output end of the microphone.
The sensitivity of a microphone is related to its own output impedance. The increase in output impedance can increase the sensitivity. The sensitivity of the moving coil type is about 1.5-4 mV/Pa, while the sensitivity of the capacitive type is about 10 times higher than that of the moving coil type, about 20 mV/Pa.
Good sensitivity? Still small?
Generally speaking, the high sensitivity of the microphone is a good thing, it can provide a higher input level to the mixer, can improve the signal-to-noise ratio. However, the output voltage is too high, which is prone to over-excited distortion. The higher the better, the choice of sensitivity should be based on actual needs.
When recording acoustic musical instruments, you should choose a microphone with higher sensitivity. When recording drums and other percussion, choosing a microphone with high sensitivity is often easy to be distorted; when recording voice signals, choosing a microphone with relatively low sensitivity can often avoid other noises. Enter to make the sound cleaner. In live sound reinforcement, we need the signal to reach a certain sound level in the field, which means that there is enough sound transmission gain in the hall and venue, so we choose a microphone with high sensitivity, but at the same time it will increase the possibility of receiving it.
 Discussion on the sensitivity and stability of electret condenser microphone products

With the rapid development of China's electronics industry, many electro-acoustic equipment have higher and higher electrical performance requirements for electret condenser microphones (referred to as sound heads), and strict requirements on the stability of product sensitivity. To achieve this goal, we must clarify the following issues.



 First of all, we must talk about the electrical performance of the sound head. The sensitivity of the sound head is in dB. Its formula is Lm=20LgM/Mr (dB), (sensitivity -40dB conversion is 10MV/PA). From this formula, you can It can be seen that the dB value is unstable, that is, the instability of M. The unit of M is V/Pa or V/ubar.
Then the output voltage of the sound head changes at different times when the sound pressure is constant. The output voltage of the sound head is mainly determined by the distance between the inner diaphragm of the sound head and the back plate of the field effect tube. The amount of charge is determined by the three.
Usually the distance between the diaphragm and the back plate is a rubber pad, and the possibility of its change is very small.
The improper doping concentration of the FET, the channel widening and narrowing for a period of time is different from another period of time, will cause M instability, this phenomenon mostly occurs in the capsule manufacturer, if the high static voltage, or It is the high voltage that will cause the tube to be induced to cause a soft breakdown of the PN junction, so M is unstable. Therefore, the sound head should be anti-static and away from high voltage.
The change of the charge of the diaphragm in the capsule can be caused by the following reasons, which is caused by the welding of the whole machine on the capsule. Because the diaphragm is an insulator, it can store charge. When the temperature is led to the diaphragm during welding (temperature is the main factor affecting the resistivity of the insulator), once the resistivity of the diaphragm decreases, it means that the electrical charge of the diaphragm is reduced. Loss, the M of the sound head will drop, so the welding time must be ensured that the temperature of the diaphragm cannot rise. Once the temperature of the diaphragm rises, it will also cause the expansion of the diaphragm and paper. Because the diaphragm is electrostatically charged, it will If the time between the welding and the test of the whole machine is short, the M is increased because the distance between the two poles is getting closer. These phenomena mainly occur when the manufacturer produces the whole machine. That is, the shorter the welding, the better

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