1. Types of microphones: According to their different structures, microphones are generally divided into dynamic type, crystal type, carbon particle type, ribbon type and condenser type. Among them, the most commonly used are dynamic microphone and condenser microphone. The former is durable and cheap, while the latter is delicate and expensive, but with excellent characteristics.
The dynamic microphone uses the diaphragm to induce changes in air pressure caused by sound waves, and drives a coil placed in a magnetic field to cut magnetic lines of force to generate a weak current signal corresponding to the change in sound pressure intensity. Usually dynamic microphones have low noise, no need to feed power, easy to use, stable and reliable performance.
The core of a condenser microphone is a capacitive sensor. The two poles of the capacitor are separated by a narrow air gap, which forms the capacitor's dielectric. When a voltage is applied between the two poles of the capacitor, the acoustic vibration causes the capacitance to change, and the current in the circuit also changes. Amplify this signal and output it, and you can get an audio signal of very good quality. In addition, there is a static-type condenser microphone, which uses static-type materials to make the microphone diaphragm electrode. It can work without external polarization voltage and simplifies the structure. Therefore, this microphone is very small and cheap, and it also has the characteristics of a condenser microphone. , Is widely used in various audio equipment and pickup environments. Condenser microphones have high sensitivity, good frequency response, and good sound quality.
2. The main technical characteristics of the microphone
At a frequency of 1KHz, 0.1Pa specifies that when the sound pressure is input from the 0° main shaft on the front of the microphone, the microphone's output terminal will output an open-circuit output voltage, in units of 10mV/Pa. Sensitivity is related to output impedance. Sometimes expressed in decibels, and 10V/Pa is specified as 0dB. Because the microphone output is generally at the millivolt level, the decibel value of its sensitivity is always negative.
2. Frequency response characteristics: The sensitivity of the microphone on the 0° spindle changes with frequency. A suitable frequency response range is required, and the characteristic curve in this range should be as smooth as possible to improve sound quality and suppress acoustic feedback. The same sound pressure, but different frequencies of sound when applied to the microphone sensitivity is not the same, the frequency response characteristics are usually expressed in decibels of the difference in sensitivity within the passband range. The wider the passband range, the smaller the decibel difference, the better the frequency response characteristic of the microphone, that is, the smaller the frequency distortion of the microphone.
The sensitivity of the microphone to the sound from different directions will be different, which is called the directionality of the microphone. The directivity is related to the frequency, the higher the frequency, the stronger the directivity. In order to ensure sound quality, the microphone is required to have a relatively consistent directivity within the frequency response range. The directivity is expressed by the difference in the sensitivity between the 0° direction on the front side of the microphone and the 180° direction on the back side of the microphone, and the difference greater than 15dB is called a strong directional microphone. The directional polar coordinate response curve pattern of the main frequency is often given in the product manual. The general types are: unidirectional "cardioid"; bidirectional "8-shaped"; and non-directional "circular"; and single Directivity "super-cardioid". The directivity of microphone sensitivity is an important factor in choosing a microphone. Some microphones are unidirectional, some are omnidirectional, and some are in between, and their directionality is cardioid.
The omnidirectional microphone has the same performance in picking up sound from all directions. This type of microphone is more suitable when the speaker wants to walk back and forth, but it is not suitable under conditions of large environmental noise.
The sensitivity of a cardioid pointing microphone is heart-shaped in the horizontal direction, with the highest sensitivity on the front side and the smallest on the back side. This microphone has excellent performance in a variety of sound reinforcement systems.
The unidirectional microphone is also called the super cardioid directional microphone. Its directivity is sharper than that of the cardioid microphone. The front sensitivity is extremely high, and the sensitivity in other directions is attenuated sharply. It is especially suitable for high-noise environments.
4. Output impedance:
The impedance of the microphone itself seen from both ends of the lead wire of the microphone is called the output impedance. At present, common microphones are divided into high impedance and low impedance. The high impedance value is about 1000 to 20000 ohms, which can be directly connected to the amplifier; the low impedance type is 50 to 1000 ohms, and it can be connected to the amplifier only after the transformer is matched. The output voltage of the high impedance is slightly higher, but the bypass effect of the lead capacitor is larger, which reduces the high frequency and is also susceptible to the interference of the external electromagnetic field. Therefore, the microphone lead should not be too long, generally 10 to 20 meters. should. Low-impedance output does not have this defect, so the noise level is low, the microphone lead can be lengthened correspondingly, and the low-impedance microphone lead of some public address equipment can reach 100 meters. If the distance is longer, a pre-amplifier should be added.