microphone

Microphone, the scientific name is microphone, also called microphone, microphone. The microphone is an energy conversion device that converts sound signals into electrical signals. There are moving coil type, condenser type, electret and recently emerging silicon micro microphones, in addition to liquid microphones and laser microphones. Most microphones are electret condenser microphones, their working principle is to use a polymer material diaphragm with permanent charge isolation

Classification
Microphones can be divided into two types, electric microphones and condenser microphones, according to their energy conversion principles. Among them, the electric type can be subdivided into dynamic microphones and ribbon microphones.
Common types of commercial microphones include condenser microphones, crystal microphones, carbon microphones, and dynamic microphones. Commonly used condenser microphones use two kinds of energy sources: DC bias power and electret film. Both 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, thereby converting acoustic signals into electrical signals. Condenser, crystal, and carbon microphones all generate a voltage signal proportional to the displacement of the sensitive membrane, while dynamic microphones generate a voltage signal proportional to the vibration rate of the sensitive membrane. Dynamic microphones use permanent magnets as energy sources, and convert sound energy into electrical energy based on the inductive effect.

Features
Most microphones are electret condenser microphones (ECM), a technology that has been around for decades. The working principle of ECM is to use a polymer material vibrating membrane with permanent charge isolation. Compared with ECM's polymeric diaphragm, the performance of MEMS microphones is very stable at different temperatures and will not be affected by temperature, vibration, humidity and time. Due to its strong heat resistance, MEMS microphones can withstand high temperature reflow soldering at 260°C without any change in performance. Since the sensitivity change is small before and after assembly, this can even save audio debugging costs during 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 accuracy, flexible design, miniaturization, integration with signal processing circuits, low cost, and mass production. Early miniature microphones were based on the piezoresistive effect. Some studies reported that a microphone with a (1×1) cm2, 2μm thick polysilicon film as the sensitive film was produced. However, in the absence of stress in the sensitive film, the first-order resonance frequency of such a large and thin polysilicon film will be lower than 300 Hz. The first-order resonance 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 sensitivity change is greater than 40dB), which is unacceptable for microphone applications. When there is tensile stress in the sensitive film, its resonance frequency will increase, but at the expense of sensitivity. Of course, the size of the sensitive film can be adjusted to obtain a higher first-order resonance frequency, but this will still reduce the sensitivity. It can be seen that the piezoresistive solution is not suitable for the manufacture of miniature microphones.
A feasible solution is to use a capacitive solution to manufacture miniature microphones. The advantage of this method is that all materials used in the integrated circuit manufacturing process can be used for sensor manufacturing. However, it is quite difficult to manufacture a micro microphone with a single-chip process, because the air medium between the two capacitor plates can only have a small interval. Moreover, due to size limitations, the bias voltage is difficult to meet in some applications. Based on the above problems, the research on condenser microphones has not been interrupted.

working principle
At the beginning of the 20th century, microphones developed from acoustic-electric conversion through resistance to inductive and capacitive conversion. A large number of new microphone technologies were gradually developed, including microphones such as aluminum dynamic coils, as well as the widely used condenser microphones and electret microphones. . The working principle of the coil microphone is that the human voice vibrates the diaphragm through the air, and then the electromagnetic coil winding on the diaphragm and the magnet surrounding the moving coil microphone form a magnetic field cutting, forming a weak fluctuating current. The current is delivered to the loudspeaker, and the fluctuating current is turned into sound in the reverse process.

Ribbon microphone
For aluminum ribbon microphones, the aluminum ribbon used is both a microphone diaphragm and a conductor that moves in a magnetic field. The aluminum strip is usually made of aluminum silk, with a thickness of 0 to 1 mm, a width of 2 mm to 4 mm, and a mass of only 0.2 mg, in order to achieve a better transient response. In order to obtain an ideal resonance frequency between 2kHz and 4kHz, the aluminum strip is made into a corrugated shape to maintain a precise tension value. The aluminum strip as a conductor and microphone diaphragm is suspended in the magnetic field between the two magnetic pole faces, vibrating with the frequency of the incident sound wave, and at the same time, a certain voltage output is generated at both ends of the aluminum strip.

Capacitive
Condenser microphones have two metal plates, one of which is coated with an electret film (mostly polyperfluoroethylene propylene) and grounded, the other plate is connected to the gate of the field effect transistor, the gate and the source A diode is connected between the poles. When the electret diaphragm itself is charged, the electric quantity of the surface charge is Q, and the electric capacity between the plates is C, the ground voltage U=Q/C will be generated on the pole head, when it is subjected to vibration or air flow friction , Due to vibration changes the distance between the two plates, that is, the capacitance C changes, but the power Q does not change, it will cause a voltage change. The magnitude of the voltage change reflects the strength of the external sound pressure, and the voltage change frequency reflects The frequency of the outside sound, this is the working principle of electret microphone.
The diaphragms of condenser microphones are mostly polyperfluoroethylene propylene, which has good humidity performance, produces more surface charges and is less affected by humidity. Since this microphone is also a capacitive structure, the signal has a large internal resistance. In order to draw out and amplify the voltage signal generated by the sound, the output terminal must also use a field effect transistor.

Detail
Directivity
The directivity is also called the polar pattern of the microphone, which refers to the ability of the microphone to pick up sounds from different directions. Generally divided into omnidirectional, cardioid, super-cardioid, and figure-8.
Omnidirectional (Omnidirectional) is also called non-directional, it has the same sensitivity to sounds in all directions. Cardioid is a directional microphone with the strongest front-end sensitivity and the weakest back-end sensitivity. Supercardioid has a narrower pickup area than cardioid microphones, but the rear end also picks up sound. Figure 8 picks up the sound from the front and rear, but not from the side (90 degree angle).
Technical index
Sensitivity
Refers to the ratio of the microphone's open circuit voltage to the sound pressure acting on its diaphragm. In fact, the microphone will inevitably cause sound field scattering in the sound field, so there are two definitions of sensitivity. One is the sound pressure actually acting on the diaphragm, called sound pressure sensitivity, and the other refers to the sound pressure of the sound field where the microphone is not placed in the sound field, called sound field sensitivity. The sound field sensitivity is divided into free field sensitivity and diffusion Field sensitivity. Usually, the microphone for recording gives the sound pressure sensitivity, and the microphone for measurement gives the sound pressure or sound field sensitivity depending on the type of application.
The unit of sensitivity is Volt/Pa (Volt/Pascal, V/Pa), which is usually expressed by the sensitivity level, and the reference sensitivity is 1V/Pa.
Frequency response
It means that when the microphone receives sounds of different frequencies, the output signal will be amplified or attenuated as the frequency changes. The most ideal frequency response curve is a horizontal line, representing that the output signal can directly present the characteristics of the original sound, but this ideal situation is not easy to achieve. Generally speaking, the frequency response curve of a condenser microphone will be flatter than that of a moving coil. The frequency response curve of common microphones is mostly high and low frequency attenuation, while the middle and low frequencies are slightly amplified.
In the frequency response graph, the horizontal axis is frequency, the unit is Hertz, and most cases are represented by logarithms; the vertical axis is the sensitivity, the unit is decibels.
impedance
The 3-pin XLR connector can generate balanced output signals, which can effectively eliminate external noise interference. The three pins will be marked with 1, 2, and 3 numbers; in the US regulations, 1 represents the ground wire, 2 represents the normal phase (hot) signal, and 3 represents the reverse phase (cold) signal; in the European regulation, 1 represents the ground wire , 2 stands for cold signal, 3 stands for hot signal.
Signal to noise ratio
It is measured by the logarithm of the ratio of the microphone output signal voltage to the microphone's inherent noise voltage. The S/N value of general high-quality condenser microphones is 55~57dB.
Dynamic Range
Small dynamic range will cause sound distortion and deterioration of sound quality, so a sufficiently large dynamic range is required.
Equivalent noise level
The output voltage generated by the sound pressure of the sound wave on the microphone is equal to the output voltage generated by the inherent noise of the microphone itself, and the sound pressure of the sound wave is equal to the equivalent noise level of the microphone.
Total harmonic distortion (THD)
Harmonic distortion refers to the more harmonic components of the output signal than the input signal. Harmonic distortion is caused by the system not being completely linear. The sum of all additional harmonic levels is called total harmonic distortion. Generally speaking, the total harmonic distortion at a frequency of 500 Hz is the smallest, so many products use the distortion of this frequency as its indicator. The total harmonic distortion is less than F at 1%, which cannot be distinguished by the ear. If it exceeds 10%, the distortion component can be clearly heard. The smaller the value, the purer the sound, indicating the higher the quality of the product. The total harmonic distortion of general products is less than 1% (measured at a frequency of 500 Hz).

Connector
There are two types of 1/4-inch (6.3mm) connectors and 3.5mm connectors: mono and stereo. The simple way to distinguish them is to see that there are several black insulating rings on the connectors. The two insulating rings represent For stereo, an insulating ring represents mono.
Grounded
Right channel in stereo; inverted signal in balanced mono; or as mono power input
Left channel in stereo; positive phase signal in balanced mono; signal output terminal in unbalanced mono
Insulating ring
Sound collection angle
Just as the focal length of the lens has different changes, the angle at which the microphone collects sound is also different. The cardioid microphone can collect sound from multiple angles. The angle at which the supercardioid microphone collects sound is relatively small. The angle at which the gun-shaped microphone collects sound is narrower than the former two. Unlike the lens, the critical point of the microphone type is not precise. For single-person shooting, that is, shooting without the camera crew, the best microphone choice is a small gun-style microphone.
A voltage signal whose rate is proportional. The dynamic microphone uses permanent magnets as the energy source, and converts sound energy into electrical energy based on the inductive effect
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