A microphone, commonly referred to as a microphone or microphone, is a device that converts sound signals (mechanical energy) into electrical signals. The circuit symbol of the microphone is shown in the following figure. Originally, microphones were represented by "S", "M", or "MIC" in circuits, but now they are often represented by "B" or "BM". 1. Microphones can be classified into dynamic coil type, crystal type, aluminum strip type, capacitive type, etc. according to the composition of microphones. Microphones can be divided into wired type and wireless type according to different use modes. At present, there are two commonly used types of microphones: dynamic coil and capacitive. 2. The principle of a microphone (1) Moving coil microphone The physical appearance of a common moving coil microphone is shown in Figure (a), and its interior is mainly composed of magnets, coils, vibration membranes, step-up transformers, soft iron, etc., as shown in Figure (b). Magnets and soft iron form a magnetic circuit, and the magnetic field is concentrated in the gaps formed by the core column and the outer soft iron. There is a vibrating film installed in front of the soft iron, which is equipped with a coil on top of it, which is precisely sheathed on the core column and located in a strong magnetic field. When the vibrating film vibrates before and after being subjected to sound wave pressure, the coil cuts magnetic lines of force and generates induced electromotive force, thereby converting the sound wave signal into an electrical signal. Due to the small number of turns, low impedance, and low output voltage of the microphone coil (usually referred to as the voice coil), it cannot meet the input signal requirements of the connected amplifier. Therefore, moving coil microphones are equipped with a step-up transformer, with a vibrating film coil (voice coil) on the primary side and an output line on the secondary side, which greatly improves the signal output from the microphone. According to the different turns ratio of the primary and secondary windings of the step-up transformer, the moving coil microphone has two output impedances: low impedance and high impedance. Among them, the low impedance is 200-600 Ω, and the high impedance is several tens of kiloohms. The frequency response of a dynamic microphone is generally 50-10 000Hz, with an output level of 50-70dB and no directionality. The frequency response of the combined dynamic microphone can reach 35-15 000Hz and has different directional characteristics for selection when using. (2) Capacitive microphones have excellent frequency response characteristics throughout the entire audio range, high sensitivity, and low distortion. However, their volume is larger than that of dynamic microphones, and they are often used in amplifying and recording work that requires high sound quality. The physical appearance of an ordinary capacitive microphone is shown in the following figure (a), and its interior is mainly composed of vibrating film, electrode plate, resistor, etc., as shown in Figure (b)