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 poles of the permanent magnet. When there is no current in the coil of the electromagnet, the movable iron core receives The attraction of the two-pole phase attraction of the permanent magnet remains stationary in the center; when current flows in the coil, the movable core is magnetized and becomes a bar-shaped magnet. As the direction of the current changes, the polarity of the bar magnet also changes accordingly, so that the movable core rotates around the fulcrum, and the vibration of the movable core is transmitted from the cantilever to the diaphragm (paper cone) to push the air thermal vibration.

2. Electrostatic loudspeaker: It is a loudspeaker that works by using the electrostatic force applied to the polar plate of the capacitor. In terms of its structure, it is also called a capacitive loudspeaker because the positive and negative poles are opposite to form a capacitor. As shown in the figure, there are two thick and hard materials as fixed plates, which can transmit sound, and the middle plate uses a thin and light material as a diaphragm (such as an aluminum film). Fix and tighten the periphery of the diaphragm to maintain a considerable distance from the fixed pole. Even on the large diaphragm, it will not touch the fixed pole.

The original DC voltage between two electrodes (called bias). If the audio voltage output by the amplifier is added between the two electrodes, it overlaps with the original output voltage to form an alternating pulsating voltage. 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 advantages of electrostatic speakers are that the entire diaphragm vibrates in phase, the diaphragm is light, and the distortion is small. It can reproduce extremely crisp sound, with good resolution, clear details, and realistic sound. Its shortcomings are low efficiency, high-voltage DC power supply is required, easy to vacuum, the increase of the diaphragm distortion will also increase, not suitable for listening to rock, heavy metal music, the price is relatively expensive.

3. Piezoelectric speakers: Speakers that use the inverse piezoelectric effect of piezoelectric materials are called piezoelectric speakers. The dielectric (such as quartz, potassium sodium tartrate and other crystals) is polarized under the pressure to cause a potential difference between the surfaces of the two ends, which is called the "piezoelectric effect." Its inverse effect, that is, the dielectric placed in the electric field will elastically deform, is called "reverse piezoelectric effect" or "electrostriction".

Compared with electrodynamic speakers, piezoelectric speakers do not require a magnetic circuit, and do not require bias voltage compared with electrostatic speakers. The structure is simple and the price is low. The disadvantage is that the distortion is large and the operation is unstable.

4. Ionic speaker: Under normal conditions, the molecular weight of air is neutral and uncharged. But after high voltage discharge, it becomes charged particles, this phenomenon is called dissociation. The vibration of the ionized air with audio voltage generates sound waves. This is the principle of ion speakers.

In order to ionize, it is necessary to add a high-frequency voltage of 20MHz, and the audio signal piezoelectric is superimposed on it. It can be seen that the ion speaker is composed of a high-frequency oscillation part, an audio signal modulation part, a discharge cavity and a horn.