<1> Magnetic speaker: also known as "reed speaker", its 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 Attracted by the attraction of the two magnetic poles of the permanent magnet, it stays still in the center; when current flows in the coil, the movable iron core is magnetized and becomes a bar magnet. As the direction of the current changes, the polarity of the bar magnet changes accordingly, causing the movable iron core to rotate around the fulcrum, and the vibration of the movable iron core is transmitted from the cantilever to the diaphragm (paper cone) to promote air thermal vibration.
<2> Electrostatic speaker: It is a speaker that uses the electrostatic force applied to the plate of the capacitor to work. In terms of its structure, the positive and negative poles face each other into a capacitor shape, so it is also called a capacitor speaker. As shown in the figure, there are two thick and hard materials as fixed pole plates, which can transmit sound, and the middle pole plate uses thin and light material as the diaphragm (such as aluminum film). Fix and tighten around the diaphragm to keep a considerable distance from the fixed pole. Even on the large diaphragm, it will not collide with the fixed pole.
<3> Piezoelectric speakers: The speakers that use the inverse piezoelectric effect of piezoelectric materials are called piezoelectric speakers. The phenomenon in which dielectrics (such as quartz, potassium sodium tartrate and other crystals) are polarized under pressure to cause a potential difference between the two ends of the surface is called the "piezoelectric effect". Its reverse effect, that is, elastic deformation of the dielectric medium placed in the electric field, is called "reverse piezoelectric effect" or "electrostriction".
<4> Ion speaker: Under normal conditions, the molecular weight of air is neutral and uncharged. But after high-voltage discharge, they become charged particles. This phenomenon is called ionization. The ionized air is vibrated by audio voltage to generate sound waves, which is the principle of ion speakers.
<5> Flame speaker: When the flame of air and gas burning passes through the electrode, the electrode is applied with DC voltage and high-frequency signal, and the flame is modulated by the audio signal to make a sound. The flame has almost no quality and the sound dynamics are excellent. But it has fatal shortcomings: unsafe and inconvenient.
<6> Airflow modulation speaker: also known as airflow speaker. It is a speaker that uses compressed air as energy source and uses audio current to modulate airflow to produce sound. It is composed of air chamber, modulation 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. At the same time, the modulated air flow is coupled by the horn to improve the efficiency of the system. It is mainly used as a sound source for high-intensity noise environmental testing or long-distance broadcasting.
<7> Magnetically induced distortion speakers. This is a special strong magnet that can vibrate and produce sound under the action of a magnetic field.
