First, the magnetic speaker (reed speaker)
Magnetic speakers are also known as "reed speakers." In the magnetic speaker structure, an electromagnet having 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 level attraction of the two magnetic pole phases of the permanent magnet, and remains stationary at the center; When a current flows in the coil, the movable iron core is magnetized to become a strip magnet.
As the direction of the current changes, the polarity of the strip magnet changes accordingly, causing the movable iron core to rotate around the fulcrum. The vibration of the movable iron core is transmitted from the cantilever to the diaphragm (paper cone) to promote thermal vibration of the air.
Second, the ion speaker
In the normal state, the molecules of air are neutral and uncharged. However, after high-voltage discharge, it becomes charged particles, and this phenomenon is called dissociation. By using the audio voltage to vibrate the free air, sound waves are generated, which is the principle of the ion speaker.
For ionization, a high frequency voltage of 20 MHz is applied, and the audio signal voltage is superimposed thereon. The ion speaker is composed of a high frequency oscillating portion, an audio signal modulating portion, a discharge chamber and a horn. The discharge chamber adopts a quartz rod with a diameter of 8 mm at the center to open a quartz tube, and one electrode is inserted therein, and the other electrode is cylindrically sleeved on the outside of the quartz tube. Since the form of silent discharge is used, only the center needle electrode has Loss, the center electrode can be replaced periodically.
The difference between the ion speaker and other speakers is that there is no diaphragm, so the transient characteristics and high frequency characteristics are very good, but the structure is very complicated.
Third, the ultrasonic speaker
The so-called ultrasonic speaker refers to the ultrasonic sound-reducing technology that has been successfully developed in the past few years and is entering the practical stage.
The principle of this ultrasonic sound reproduction technology: instead of using any conventional speaker unit, it uses an ultrasonic generator to generate two specially processed ultrasonic beams, when the two beams simultaneously act on the eardrum of the human ear. It is possible to produce hearing due to interaction.
We know that when only one beam is applied to the tympanic membrane, no sound is heard. Since the ultrasonic speed has a strong and controllable directivity, the intersection of the two beams can form a small sound-reducing area. When the human ear is in this area, the sound can be heard, and once the human ear You can't hear it when you leave the area.
With this feature, some of today's top luxury cars are equipped with multiple sets of such ultrasonic sound reproduction systems, forming a listening area in each seat, so that each passenger can choose their favorite content, but not each other. Any interference is generated without affecting the conversation between passengers.
Fourth, electrostatic speaker (capacitor speaker)
The electrostatic speaker is a speaker that operates by the electrostatic force applied to the capacitor plate. As seen from the structure, since the positive and negative electrodes face each other in a capacitor shape, they are also called capacitive speakers.
The electrostatic speaker has two thick and hard materials as the fixed plate, which can transmit sound through the plate, and the middle plate uses a thin and light material as the diaphragm (such as aluminum film). The diaphragm is fixed and tensioned to maintain a considerable distance from the fixed pole. Even on the large diaphragm, it does not touch the fixed pole.
The original DC voltage (called bias voltage) is between the two electrodes of the electrostatic speaker. If the audio voltage outputted by the amplifier is applied between the two electrodes, it overlaps with the original output voltage to form an alternating pulsating voltage. This pulsating voltage is generated due to the change in the attraction force of the two-pole gap, and the diaphragm vibrates to sound.
The advantage of the electrostatic speaker is that the entire diaphragm vibrates in phase, the diaphragm is light, the distortion is small, and the extremely crisp sound can be reproduced, and the resolution is good, the details are clear, and the sound is realistic. Its shortcoming is low efficiency, high-voltage DC power supply, easy to vacuum, and increased distortion of the diaphragm. It is not suitable for listening to rock and heavy metal music, and the price is relatively expensive.
Five, airflow modulation speaker (airflow speaker)
The airflow modulation speaker is a speaker that uses compressed air as an energy source to modulate the airflow with an audio current. It consists of a gas chamber, a modulating valve, a horn and a magnetic circuit. The compressed air flow passes through the valve through the air chamber and is modulated by the applied audio signal, so that the fluctuation of the air flow changes according to the applied 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 testing or long-distance broadcasting.
Sixth, piezoelectric speaker
A speaker that operates using the inverse piezoelectric effect of a piezoelectric material is called a piezoelectric speaker. Dielectrics (such as quartz, sodium potassium tartrate and other crystals) are polarized under pressure, causing a potential difference between the two ends of the surface, which we call the "piezoelectric effect." Its inverse effect, that is, the dielectric placed in the electric field undergoes elastic deformation, called "inverse piezoelectric effect" or "electrostriction".
Compared with an electric speaker, a piezoelectric speaker does not require a magnetic circuit, and does not require a bias voltage compared with an electrostatic speaker. The structure is simple and inexpensive, and the disadvantage is that the distortion is large and the operation is unstable.
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