〈1〉 Magnetic speaker: also known as "reed speaker", its structure is shown in Figure 4. There is a movable core electromagnet between the two poles of the permanent magnet. When there is no current in the coil of the electromagnet, the movable core Attracted by the attraction of the two magnetic pole phases of the permanent magnet, it remains stationary in the center; when a current flows in the coil, the movable core is magnetized and becomes a strip-shaped magnet. As the direction of the current changes, the polarity of the strip magnet changes accordingly, so that the movable iron core rotates around the fulcrum, and the vibration of the movable iron core is transmitted from the cantilever to the diaphragm (paper cone) to drive the air thermal vibration.
〈2〉 Electrostatic speaker: It is a speaker that works by using the electrostatic force applied to the capacitor plate. In terms of its structure, the positive and negative electrodes face each other into a capacitor, so it is also called a capacitive speaker. As shown in the figure, there are two thick and hard materials as the fixed plates, which can transmit sound, and the middle plate uses a thin and light material as the diaphragm (such as an aluminum film). The diaphragm is fixed and tightened around it to keep a considerable distance from the fixed pole, and even on a large diaphragm, it will not touch the fixed pole.
The original DC voltage between the two electrodes (called the bias voltage). If the audio voltage output by the amplifier is added between the two electrodes and overlaps with the original output voltage, an alternating pulsating voltage is formed. This pulsating voltage is generated by the change in the attractive force of the gap between the two poles, and the diaphragm vibrates and sounds.
The advantage of the electrostatic speaker is that the entire diaphragm vibrates in phase, the diaphragm is light, the distortion is small, and it can reproduce extremely crisp sounds, with good resolution, clear details, and realistic sound. Its disadvantages are low efficiency, high-voltage DC power, easy vacuuming, and increased distortion of the diaphragm, which is not suitable for listening to rock and heavy metal music, and the price is relatively expensive.
(3) Piezoelectric speakers: Speakers that work by using the inverse piezoelectric effect of piezoelectric materials are called piezoelectric speakers. Dielectrics (such as quartz, crystals of potassium sodium tartrate, etc.) undergo polarization under pressure to cause a potential difference between the two ends of the surface, which is called the "piezoelectric effect". Its inverse effect, that is, the dielectric deformed in an electric field, undergoes elastic deformation, which is called "inverse piezoelectric effect" or "electrostriction".
Piezo speakers do not require a magnetic circuit compared to electric speakers, and do not require bias voltage compared to electrostatic speakers. They are simple in structure, cheap in price, and disadvantages are large distortion and unstable operation.
(4) Flame speaker: When the flame of air and gas passes through the electrode, the electrode is applied with a DC voltage and a high-frequency signal, and the flame is modulated by the audio signal to make a sound. The flame is almost massless and the sound dynamics are good. But it has fatal drawbacks: insecurity and inconvenience.
(5) airflow modulation speakers: also known as airflow speakers. It is a loudspeaker that uses compressed air as its energy source and uses audio current to modulate the airflow. It consists of air chamber, modulation valve, horn and magnetic circuit. Compressed air flows from the air chamber through the valve 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 efficiency of the system. It is mainly used as a sound source for high-intensity noise environment tests or long-distance broadcasting.
<Seven> Magnetically Distorted Speaker. This is a special strong magnet that can vibrate under the action of a magnetic field.