Application of electroacoustic transducer - headphone

Transducer is a kind of energy conversion device. As its name implies, electro-acoustic transducer is a device of mutual conversion between electrical energy and mechanical energy.Vibration produces sound.Earphone is a kind of audio product that converts the electric signal from the audio source into the sound that can be heard by human ear through the principle of electro-acoustic conversion.The characteristics of transducers depend on the selection of materials and production process. The performance and service life of transducers of the same size and shape are extremely different, and so are earphones.In terms of the development history of earphones, the electric transducer technology was first applied in small headphone speakers by German scientist Eugen Beyer in 1924. After continuous development and maturity of the technology, the earphone of today was finally formed.

Unlike loudspeakers, headphones generate sound pressure in a small cavity, and loudspeakers radiate sound energy in free space.Basic work in ear headphones to hear bands 20 hz to 20 KHZ, namely in the acoustic spectrum, it is a limited area, but even such a narrow band, its design must be sound in the machine, electricity, the three systems many contradictions that exist in the factors of compromise, in the design of the headset with many somewhat subjective thinking and skills of the designer, thus make the different brands and types.

There are many categories of earphones. According to the wearing mode and size, earphones are divided into headwear type, ear-sticking type, ear-hanging type and in-ear type.According to the working characteristics, it can be divided into moving coil type, moving iron type, electrostatic type, plane diaphragm type, etc., and open type, semi-open type and closed type according to the degree of openness.In recent years, with the in-depth development of earphone technology, active noise-canceling earphones and wireless earphones have also appeared, greatly enhancing the adaptability of earphones in outdoor environment.

This study is mainly classified into working characteristics, including moving coil type, moving iron type, static type, magnetic type and piezoelectric type earphones.

At present, most of the headwear earphones adopt the principle of moving coil electro-acoustic conversion.Moving coil type, as the first electro-acoustic conversion mode applied in earphones, is also known as electro-acoustic transducer. It is made of the principle of displacement caused by conduction of conductance in a constant magnetic field.Figure 3 is a schematic diagram of a moving coil electroacoustic transducer.Structurally, the diaphragm is connected to a cylindrical coil in the permanent magnet field, which drives the diaphragm to produce sound driven by the current of the signal.Figure 4 is a typical structure of a moving coil earphone sound unit.The working efficiency of moving coil earphone is higher. Theoretically, the larger the size of the diaphragm, namely the drive unit, the better the performance.

The structure and working mode of the moving iron earphone are completely different from the conventional moving coil earphone.Moving iron electroacoustic transducer is mainly composed of coil fixed in magnetic circuit and ferromagnetic component which can vibrate.The alternating magnetic field is generated when the alternating current passes through the coil, which makes the force of the iron diaphragm or armature in the magnetic circuit change and makes the armature vibrate and sound.Why is the moving iron element called the balanced armature?The main reason is that the iron sheet is usually at the center of a magnetic field and is in equilibrium when no magnetic force is applied to it.According to this structure, only a slight electrical signal can cause the moving iron unit to emit sound.Therefore, the moving iron earphone has very excellent electro-acoustic conversion ability.

The small size of the moving-iron unit allows the moving-iron headset to be placed deeper into the ear canal to improve hearing.As the diameter of the moving iron unit increases, the weight will be greatly increased, which is the main reason why the moving iron technology has not been applied in headphone.

In addition to the advantages in size, moving iron earphones generally have a very high sensitivity, but need to pay attention to, this does not mean that moving iron earphones must be easy to drive, in the face of 3 units or more moving iron earphones, often need high power output audio source or with ear amplifier to drive.When using earplugs, however, attention must be paid to impedance matching to achieve optimal performance.The high sensitivity means that the moving iron earphone has better sound resolution than the moving coil earphone, especially in the medium and high frequency detail level performance is very good.

As for the electrostatic type, due to the high manufacturing technology and cost, the price of electrostatic headphones is usually more than ten thousand yuan.The front and rear plates are usually made of very rigid metal sheets to avoid resonance.In order to penetrate the sound, the plate is punched with evenly distributed circular holes, whose area generally accounts for about 30% of the total area of the plate.The working principle of electrostatic earphone is different from that of moving coil earphone. The main reason is that the diaphragm is polarized by high dc voltage and in the static field formed by the change of two fixed metal plates. Driven by electric field force, the diaphragm makes sound.Since static headphones need to convert audio signals into hundreds of volts, special amplifiers must be used to drive them.

The structure of static headphones has inherent advantages over moving coil headphones. The most important thing is that the diaphragm is very thin, which is only about 1/10 of the diaphragm of moving coil headphones.This lighter and thinner diaphragm allows for faster, better transient response and finer performance of sound.In addition, due to the structural limitations of the diaphragm of moving coil earphone, there is always the problem of segmentation vibration, which affects the uniformity of three frequencies.However, the diaphragm of electrostatic earphone is fixed between parallel fixed plates, and the electric field received is completely uniform, which can realize linear drive.

Iv. The flat diaphragm earphone is also called the isomagnetic earphone, which is different from the moving coil and electrostatic earphone in working principle. The flat diaphragm electroacoustic transducer USES the flat mode, and there are several groups of voice coil lines on the flat diaphragm, and there is a magnetic circuit corresponding to one.Because the diaphragm is thin and light, and can be fully driven, it has wide frequency band, good high frequency characteristics and transient characteristics, low distortion, but lower sensitivity than ordinary moving coil electro-acoustic transducer.The magnets are concentrated on one or both sides of the diaphragm, which vibrates in the magnetic field formed by it.Flat diaphragm earphone can withstand greater input power, lower distortion under loud pressure, but also lower working efficiency and sensitivity, often need high power and large current output of the ear to cooperate.In addition, the large size and weight are not conducive to wearing for a long time.

Piezoelectric earphones

Piezoelectric electro-acoustic transducer is made by using the piezoelectric effect of some natural crystals.The so-called piezoelectric effect is the electromotive force (emf) that occurs when a piezoelectric crystal deforms between two specific opposite surfaces.Because the piezoelectric effect is reversible, the chip vibrates when an alternating voltage is applied to it.The impedance of piezoelectric electro-acoustic transducers is generally between 3500 ohms and 9000 ohms, with a maximum power of about 0.1w.In order to improve the sensitivity and better coupling with the driving medium, some products use multilayer chip structure.

The advantages of piezoelectric electro-acoustic transducer are stable performance, high temperature resistance, high humidity, high voltage performance and simple structure.The study of new piezoelectric materials (such as polyvinylidene fluoride film) also improves its linearity and distortion.However, due to high impedance, the application of headphone has not become a mainstream.

Earphones generally use a single type of electro-acoustic transducer, but in order to broaden the frequency of sound playback and improve its performance, there are also individual earphones that use two kinds of electro-acoustic transducers.However, due to the complexity of the structure of this kind of earphone and the constant emergence of new diaphragm materials and technologies, the structure of this kind of double-electro-acoustic transducer has not been widely used in the current earphone manufacturing, and it will not be introduced too much here.