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Microphone, whose scientific name is microphone, is translated from English microphone, also known as microphone. A microphone is an energy converter that converts sound signals into electrical signals. It can be classified into dynamic coil type, capacitive type, electret and recently emerging silicon micro microphone, in addition to microphone and laser microphone. Most microphones are electret condenser microphones, which work by using a polymer vibrating membrane with permanent charge isolation
Microphone can be divided into electric microphone and condenser microphone according to its energy conversion principle. The electric type can be subdivided into dynamic microphone and aluminum band microphone.
Common types of commercial microphones are condenser microphones, crystal microphones, carbon microphones, and dynamic microphones. There are two kinds of energy sources commonly used in condenser microphones: DC bias power supply and electret film. These two kinds of condenser microphones and crystal microphones convert sound energy into electrical energy to generate a changing electric field. The carbon microphone uses a DC voltage source to change its resistance through sound vibration, thus converting the sound signal into an electrical signal. Capacitive, crystal and carbon microphones all produce a voltage signal proportional to the displacement of the sensitive film, while dynamic microphones produce a voltage signal proportional to the vibration rate of the sensitive film. Dynamic microphone uses permanent magnet as the energy source, and converts sound energy into electrical energy based on inductance effect
Most microphones are electret condenser microphones (ECM), which have been used for decades. The working principle of ECM is to use a polymer vibrating membrane with permanent charge isolation. Compared with the polymer vibration film of ECM, the performance of MEMS microphone is very stable at different temperatures, and will not be affected by temperature, vibration, humidity and time. Due to its high heat resistance, MEMS microphones can withstand the high temperature reflow soldering at 260 ℃ without any change in performance. Since the sensitivity changes little before and after assembly, this can even save the cost of audio debugging in the manufacturing process. At present, integrated circuit technology is more and more widely used in the manufacture of sensors and sensor interface integrated circuits. This micro manufacturing process has the advantages of precision, flexible design, miniaturization of size, integration with signal processing circuits, low cost and mass production. The early micro microphone was based on piezoresistive effect. According to research reports, it was made with (1 × 1)cm2、2 μ Microphone with m-thick polysilicon film as the sensing film. However, if there is no stress in the sensitive film, the first resonant frequency of such large and thin polysilicon film will be lower than 300Hz. The first-order resonant frequency in such a low frequency range will cause the frequency response of the microphone in the auditory frequency range to be extremely uneven (the change of sensitivity is greater than 40dB), which is unacceptable for microphone applications. When there is tensile stress in the sensing film, its resonant frequency will increase at the expense of sensitivity. Of course, a higher first-order resonant frequency can be obtained by adjusting the size of the sensitive film, but this will still reduce the sensitivity. It can be seen that the piezoresistive scheme is not suitable for the manufacture of micro microphones