How do speakers make sound?

Working principle of loudspeaker 1. The word speaker, commonly known as speaker; The electroacoustic lyrics published in 1993 point out that the loudspeaker is an electroacoustic transducer, which can convert electrical signals into acoustic signals and radiate them into the air. According to relevant data, the loudspeaker was first invented in 1877, and the German (E.W.Scimens) obtained the patent of the loudspeaker prototype. He first proposed an electrical structure consisting of a circular coil placed in a radial magnetic field. 1924, Americans C.W.Rice and E.W.Kollogg invented the electric loudspeaker. Second, the speaker is easy to ring but difficult to refine. Hundreds of millions of speakers are produced all over the world every year. They are widely used in communication, broadcasting, education, daily life and other fields. Like cloth, millet and millet, it has become something that people can't live without. For our technical personnel engaged in the design and manufacture of speakers, we need to have a thorough, systematic and comprehensive understanding of the theory, practice and technology of speakers. Some people say that the speaker is simple, but it's a trick. Anyone can make a speaker. This cannot be said to be completely unreasonable. Acoustics is a small subject, and the speaker is a small device. However, the production threshold of dozens to dozens of components is really not high, but the other side of the problem is that speakers are not easy to do. Speaker is an electro-acoustic device, which is one of the contents of electro-acoustic research. Electroacoustics is an interdisciplinary subject including electronics, acoustics, electromagnetism and magnetism. Although there are only dozens of components in the loudspeaker, its complexity is far beyond our imagination. This is because: (1) There are many levels of energy conversion and many feedbacks. Usually, the device energy conversion is only once. For example, an electric motor converts electrical energy into mechanical energy. The generator converts mechanical energy into electrical energy. Electric lights convert electric energy into light energy. The battery converts chemical energy into electrical energy. What happens here is only the conversion from one energy to another. The loudspeaker is different. It converts electrical energy into mechanical energy, and then converts mechanical energy into electrical energy, which is not common in various transducers. Its multi-level and multi-feedback naturally bring the complexity and diversity of the system. There are electrical part, acoustic part, energy part and mechanical part (mechanical vibration part) in a loudspeaker system. (2) The working state of the loudspeaker is not only static, but also vibrating, especially in three-dimensional space. This three-dimensional vibration system has many boundary conditions, and its vibration analysis is extremely complicated, so general mathematical tools are not enough. The cone differential equation derived by Dutch scholar Frankort is a simultaneous first-order differential equation with 14 variables, and the vibration of loudspeaker is also related to frequency and time. Actually, it's in multidimensional space. (3) The loudspeaker vibration system is a lumped parameter system only in the low frequency region. When the frequency increases, the vibration system is no longer a rigid body. When analyzing the loudspeaker, the equivalent circuit method is often used, and the loudspeaker is regarded as an equivalent circuit composed of concentrated parameters. Because we are familiar with circuit theory, it will be handy to analyze the speaker with circuit theory. When analyzing the vibration of the loudspeaker, it is assumed that the loudspeaker is a rigid body, which is convenient for analysis. But the above assumptions only apply to the bass band. When the frequency increases, the speaker is no longer a lumped parameter element, and the speaker diaphragm is no longer a rigid body, and the diaphragm will vibrate in a split mode. Therefore, in the high frequency band, the analysis derived from the rigid body vibration hypothesis is invalid, and the formula derived from the equivalent circuit is also invalid. The distributed parameter system is also characterized in that these scattered elements are not independent of each other. Specifically, the vibration of each point on the diaphragm is different, the amplitude and phase of each point are different, and each point affects each other. It can also be compared with familiar electronic technology. Because familiar electrical components (resistors, inductors, capacitors, transistors, integrated circuits, etc. ) and familiar circuit principle, an amplifier can be assembled according to the circuit diagram, and the difference between using these components is limited whether it is an experienced engineer or a novice middle school student. But for speakers and speakers, it is not so simple. If the same unit is assembled into a speaker, there may be a considerable gap if the experience is different. (4) The evaluation of loudspeakers depends not only on numerous objective test indicators, but also on the current objective test indicators, which can not completely summarize the quality of loudspeakers. There are as many objective test indicators of loudspeakers as 10, and there is an increasing trend. Most measurements need to be made in an anechoic room. Although there is computer-aided measurement now, it still cannot replace the measurement in anechoic room. The speaker's subjective evaluation is essential and very discrete. It often varies from person to person, from time to time, from place to place, from song to song, and is consciously or unconsciously influenced by various psychological hints. The result of evaluation not only depends on the listener's accomplishment, quality and psychological state, but also the sound itself is fleeting, which is more difficult than other items that need subjective evaluation, such as wine evaluation and tea evaluation, involving psychoacoustics, physiological acoustics, environmental acoustics, musical acoustics and mathematical statistics methods. (5) Speaker manufacturing technology involves many technical fields such as papermaking, chemical industry, adhesive, metal processing, magnet manufacturing, etc., which embodies its comprehensiveness and diversity. Among them, the change of loudspeaker diaphragm material is particularly important. Changing the diaphragm material only when the geometry is unchanged will not only change the objective test index, but also change the subjective sound quality. Because the above five aspects have brought many puzzling topics to electroacoustic workers, and also added fascinating colors to speaker technology. Speaker technology is one of the few technologies that can combine art with technology, interest with science. It is also the product of the combination of ancient acoustics and modern electronics; It is a technology with broad development space and close contact with hundreds of millions of people. It is a noble and beneficial contribution to develop loudspeaker technology. 3. There are many kinds of speaker classification methods. Today, three classification methods are introduced: (1) direct radiation speakers, speakers, headphones, Haier speakers are classified according to radiation mode; (2) Classification of hi-fi (household) speakers; Loudspeaker for sound reinforcement; Musical instrument speaker; Speakers for movies; Radio; Television; Loudspeaker for recorder alarm; Underwater loudspeaker; Speakers for ships and automobiles (III) Electric speakers are classified according to their working principles: electromagnetic speakers, electrostatic speakers, piezoelectric speakers, ion speakers, flame speakers, airflow modulation speakers, and magnetic distortion speakers. Faraday not only discovered the phenomenon of electromagnetic induction, but also summarized the same law of electromagnetic induction. 1) When the magnetic flux passing through the area around the conductor loop changes with time, an induced electromotive force is generated in the loop, thereby generating an induced current. This change of magnetic flux can be caused by the change of magnetic field, and it can also be caused by the movement of conductor loop in magnetic field or the movement of a part of conductor loop cutting magnetic lines. 2) The magnitude of induced electromotive force is related to the speed of flux change, or it is directly proportional to the speed of flux change with time. In a word, the essence of electromagnetic induction phenomenon is that the change of magnetic flux produces induced electromotive force. 3) The direction of induced electromotive force always tries to establish an additional magnetic flux through the induced current generated by it, thus hindering the change of magnetic flux that causes induced electromotive force. 1845, Faraday's experimental law was written in mathematical form by F. E.Neumamn and others. If the change rate of magnetic flux is in Weber/second and the unit of induced electromotive force is in volt, Faraday's experimental law can be expressed as ε =-D φ/dt by mathematical formula. This equation is called Faraday's law of electromagnetic induction. With regard to Faraday's law of electromagnetic induction, I would like to emphasize the following points: Since 1, induced electromotive force is established in the circuit by electromagnetic induction, which is more essential than induced current. Even if the resistance in the loop is infinite and the current is zero, the induced electromotive force still exists. Even if the loop is not closed, induced electromotive force can be generated in a section of conductor. 2) The cause of induced electromotive force in the loop is the change of magnetic flux passing through the plane surrounded by the loop, not the magnetic flux itself. Even if the magnetic flux passing through the loop plane is very large, as long as it does not change with time, the induced electromotive force will not be generated in the loop. 3) Regarding the physical meaning of the "-"sign in Faraday's law of electromagnetic induction, the negative sign here indicates that the direction of the induced electromotive force is always like this: the magnetic field generated by the induced current passes through the magnetic flux of the loop, which hinders the change of the magnetic flux that causes the induced current. Direction of electromotive force: It is an objective fact to specify the direction of electromotive force from the negative pole to the positive pole of power supply. But when the power supply is connected to the circuit, the electromotive force ε is recorded as "positive" or "negative", depending on the bypass direction of the selected circuit. If the detour direction is consistent with the direction of electromotive force ε, the electromotive force is recorded as "+ε", and if the detour direction is opposite to the direction of electromotive force ε, the electromotive force is recorded as "-ε". V. Working principle of electric loudspeaker Electric loudspeaker is also called dynamic loudspeaker (as shown in figure1); It is an electro-acoustic transducer applying electrodynamics principle. It is the most widely used speaker at present. There are three main reasons: (1) The electric speaker is simple in structure, easy to produce and does not need large space, which leads to its low price and can be widely popularized. (2) This loudspeaker can achieve excellent performance and uniform frequency response in the middle frequency band. (3) This kind of loudspeaker is constantly improving. The development history of loudspeaker for several decades is the history of continuous improvement of loudspeaker design, technology and materials, and the history of performance advancing with the times. The shapes of electric speakers are mostly conical and dome-shaped; The structure of the cone speaker is shown in the figure. The structure of conical loudspeaker can be divided into three parts:1>; The vibration system includes diaphragm, voice coil, centering bracket, dust cover, etc. 2> magnetic circuit system includes magnetic upper plate, magnetic column, magnetic lower plate, magnet, etc. The 3> auxiliary system includes a basin frame, a blank holder, a wiring frame and a phase stop. According to Faraday's law, when a current-carrying conductor passes through a magnetic field, it will be subjected to electromotive force, and the direction of electromotive force conforms to Fleming's left-hand rule (Figure 2.3). Force is perpendicular to the direction of current and magnetic field, and force is proportional to current, wire length and magnetic flux density. When the voice coil inputs alternating audio current, the voice coil is driven by alternating driving force to generate alternating motion, which drives the paper basin to vibrate and repeatedly pushes the air to generate sound. The force that causes the vibration of the diaphragm of the electric loudspeaker is the magnetic field force on the current-carrying conductor. This effect is called the force effect of the electric transducer, and its magnitude is specified by the following formula: F=B L i, where: b is the magnetic induction density (intensity) in the magnetic gap, and its unit is n/(a.m), also called Tesla (T)L is the length of the wire of the voice coil, and the unit: m I is the current flowing through the voice coil, and the unit: ampere F is the force of the magnetic field on the voice coil, and the unit: Newton. However, when the voice coil is energized, it will cut the magnetic field lines in the magnetic gap and produce induced electromotive force in the voice coil. This effect is called the electric effect of the electric transducer, and the magnitude of the induced electromotive force is е = в i. VI. Working principle of other speakers: Magnetic speaker: also known as "reed speaker", whose structure is shown in Figure 4. There is an electromagnet with a movable iron core between the two poles of the permanent magnet. When there is no current in the coil of the electromagnet, the movable iron core is attracted by the attraction of the two poles of the permanent magnet. When current flows in the coil, the movable iron core is magnetized and becomes a bar magnet. With the change of current direction, the polarity of the bar magnet also changes, which makes the moving iron core rotate around the fulcrum, and the vibration of the moving iron core is transmitted from the cantilever to the diaphragm (paper basin) to push the air thermal vibration. 〈 2 〉 Electrostatic speaker: It is a kind of speaker that works by electrostatic force applied to the capacitor plate. As far as its structure is concerned, it is also called a condenser speaker because the positive and negative electrodes are opposite. As shown in the picture, there are two thick and hard plates as fixed plates, one of which can transmit sound, and the middle plate is made of light and thin material as diaphragm (such as aluminum film). Fix and tighten the periphery of the diaphragm to keep a certain distance from the fixed pole, and even on the large diaphragm, it will not collide with the fixed pole. As shown in fig. 5, a DC voltage (called bias voltage) exists between the two electrodes. If the audio voltage output by the amplifier is added between the two electrodes, it will be superimposed with the original output voltage to form an alternating pulsating voltage, which is generated by the change of attractive force between the two electrodes, and the diaphragm vibrates to produce sound. The advantages of electrostatic speaker are that the whole diaphragm vibrates in phase, the diaphragm is light, the distortion is small, the sound can be played back very clearly, the resolution is good, the details are clear and the sound is realistic. Its disadvantages are low efficiency, DC high-voltage power supply, easy vacuum cleaning, increased deformation of diaphragm, unsuitable for listening to rock and heavy metal music, and more expensive. (3) Piezoelectric speaker: A speaker that works by using the inverse piezoelectric effect of piezoelectric materials is called a piezoelectric speaker (as shown in Figure 6). Dielectric (such as timely, potassium sodium tartrate, etc.). Polarization under pressure leads to a potential difference between the two ends, which is called "piezoelectric effect". Its inverse effect, that is, the dielectric placed in the electric field will deform elastically, is called "inverse piezoelectric effect" or "electrostriction". Compared with the electric speaker, the piezoelectric speaker does not need a magnetic circuit, and compared with the electrostatic speaker, it is simple in structure and low in price, but it has the disadvantages of large distortion and unstable operation. 〈 4 〉 Ion speaker: Generally speaking, the molecular weight of air is neutral and uncharged. But after high voltage discharge, it becomes charged particles, which is called ionization. When ionized air is vibrated by audio voltage, sound waves are generated, which is the principle of ion sound box (Figure 7a). For ionization, a high-frequency voltage of 20MHz is applied and a piezoelectric audio signal is superimposed on it. As can be seen from (fig. 7d), the ion speaker consists of a high-frequency oscillation part, an audio signal modulation part, a discharge cavity and a loudspeaker. In the discharge chamber, a quartz tube is formed by a response bar with a diameter of 8mm at the center opening, one electrode is inserted into the quartz tube, and the other electrode (as shown in Figure 7b) is sleeved outside the quartz tube in a cylindrical shape. Because of silent discharge, only the central needle electrode is worn, so the central electrode can be replaced regularly. Unlike other speakers, ion speakers have no diaphragm, so the transient characteristics and high frequency characteristics are good, but the structure is too complicated. 〈 5 〉 Flame speaker: As shown in Figure 8, when the flame of air and gas combustion passes through the electrode, DC voltage and high-frequency signal are applied to the electrode, and the flame is modulated by audio signal to generate sound. The flame has almost no mass and the sound is very dynamic. But it has fatal shortcomings: unsafe and inconvenient. < airflow modulation speaker: also known as airflow speaker (as shown in Figure 9). It is a loudspeaker that uses compressed air as energy source and uses audio current to modulate airflow to produce sound. It consists of air chamber, regulating valve, horn and magnetic circuit. The compressed air flow passes through the valve from the air chamber and is modulated by the external audio signal, so that the fluctuation of the air flow changes according to the external audio signal, and the modulated air flow is coupled through the horn to improve the system efficiency. It is mainly used as the sound source for high-intensity noise environment testing or long-distance broadcasting. ↓7↓ Magnetically twisted speakers. This is a special kind of strong magnet, which can vibrate and sound under the action of magnetic field.