How does electricity become sound

In middle school, we learned Faraday electromagnetic induction: the change of magnetic flux will produce electric current. For example, when a part of the conductor of a closed circuit cuts the magnetic line of induction in a magnetic field, it will generate current, which is simply magnetized electricity.

Vice versa, electric current also generates a magnetic field. The working principle of the earphone is that the change of the current produces a changing magnetic field, and the vibration of the diaphragm is generated by the movement of the magnet, thereby making a sound.


In the headset, there are two magnets. One is the strong magnetic magnet in the picture below, which is a permanent magnet, which is fixed on the frame of the earphone and cannot be moved. The other is the voice coil connected to the diaphragm, which is an electromagnet. It produces magnetism and is movable.


The sound is produced by the vibration of the air driven by the diaphragm. When the audio current flows through the earphone cable and flows through the red voice coil in the earphone in the figure below, a changing magnetic field will be generated, and this magnetic field will interact with the permanent magnetic material fixed in the earphone, and the force of the voice coil will be different. The movement of the amplitude, which drives the diaphragm to vibrate, produces a changing sound. This is how the speaker works!


The process by which different earphones turn current into sound

According to different structures and working principles, there are mainly moving coil type, moving iron type, electromagnetic type and electrostatic type.

1. Electromagnetic

Electromagnetic earphones are an older one, as shown in Figure 1.


Its structure is that two coils are respectively sleeved on the N and S poles of the U-shaped magnet, and then the iron vibrating film is placed on the top (to facilitate vibration, there is a gap between the film and the magnetic pole) and attracted by the magnetic pole. When an audio current passes through the coil, an alternating magnetic field that changes synchronously with the audio signal will be generated. This alternating magnetic field causes the magnetic pole to change the attractive force of the diaphragm. The constantly undulating diaphragm will push the surrounding air to produce sound waves. This completes the entire process of turning current into sound. Electromagnetic headphones are characterized by high impedance (generally 2000Ω) and high sensitivity. Earlier electronic tubes and mineral radios used this kind of earphones.

2. Dynamic headphones


Its basic structure is to place the coil (commonly known as the voice coil) in the gap of the toroidal magnetic field. When an audio current passes through, the alternating magnetic field generated by the voice coil will interact with the toroidal magnetic field, thereby driving the diaphragm to emit sound. The moving coil type is a headset with mature technology. It is characterized by a low impedance (usually a few Ω to tens of Ω) and a wide sound range and superior low-frequency characteristics. According to the different crafts and materials, the price ranges from a few yuan to tens of thousands of yuan. It is also the headset with market share.

In addition, there is a kind of "isomagnetic" earphone that directly "prints" the voice coil on the diaphragm, but the structure is somewhat similar to the "electrostatic type" described later, except that the solid electrode is replaced with a special structure. magnet. It also has a good sound effect.

3. Moving iron earphone

Its basic structure is shown in Figure 3.


The structure of the moving iron headset is somewhat similar to the "reed" speaker in an old radio (of course, its craftsmanship and sound effects are not comparable to the "reed" type). It transfers the alternating magnetic field generated by the coil to the gap between the magnetic poles through a thin U-shaped iron, interacts with the magnetic pole, and then drives the "vibrating membrane" through the conductive rod to emit sound. Its characteristic is that the volume can be made small. Earphones made with this structure can even be inserted into the ear canal, which is difficult for other earphones to achieve, so it is often used in hearing aids. But its disadvantage is that the sound range is relatively narrow and the low frequency characteristics are slightly worse. However, due to its small size, multiple units of different ranges can be placed in a headset to make up for this shortcoming, but this will bring some very complicated acoustic problems.

Because the moving iron earphones are small and exquisite, they have high requirements on the production process, so the price is expensive, generally in the hundreds to thousands of dollars.

4. Electrostatic headphones

This is a less popular headset. The general price is also a few thousand yuan, even sold for hundreds of thousands. Its basic structure is shown in Figure 4.


The working principle is shown on the right side of Figure 4. Its structure is very fine, mainly composed of a few micrometers thick plastic sheet with metal coating as the vibrating membrane, suspended between two polar plates. When working, the vibrating membrane is energized with hundreds of volts of direct current to form an electrostatic field. The amplified and boosted audio signal leads to two fixed electrodes, and a constantly changing electric field is generated between the electrodes. The interaction of these two electric fields drives the diaphragm to vibrate and produce sound.

Because the diaphragm is thin and light, and it vibrates as a whole, it is sensitive to the details of music, has a good transient response, a very wide range and a flat curve, which can make a variety of sounds reproduced well. But the inconvenience of this kind of headphones is that they need to be equipped with a dedicated headphone amplifier with high voltage, which cannot be directly driven by ordinary audio equipment.