Speaker structure, working principle and performance index

The loudspeaker is a kind of energy conversion device that converts electrical signals into acoustic signals. The performance of the loudspeaker has a great impact on the sound quality. The speaker is a very weak component in the audio equipment, and it is an important component for the sound effect. Common speakers include electromagnetic speakers, dynamic speakers, electrostatic speakers, etc. So, what is the working principle of different types of speakers? Next, I will introduce the structure, working principle and performance index of the speakers one by one.

Structure of the speaker

The speaker is generally composed of a dust cap, a sound cone, a voice coil, a vibration plate, a basin frame, a binding post, upper and lower magnetic pole pieces, and magnetic steel.

1. Sound Basin

Use the vibration of the sound basin to push the air to vibrate to achieve the sound of the sound. Therefore, the material of the sound cone determines the personality of the speaker.

2, basin stand

The types and characteristics of the basin frame are as follows: iron sheet: lower price; die-casting: not easy to deform; synthetic material: light weight and not easy to deform.

3.Voice coil stand

The voice coil stand is mostly aluminum. Because the voice coil rack needs to consider heat dissipation, the aluminum skin has good heat dissipation, light weight and no deformation. It is also useful in paper, but it is now obsolete. There is also a KISV epoxy board, which has better performance.

4.Magnet

Ferrite: The most commonly used traditional, large size and low price.

NdFe: It is 7 times more magnetic than ferrite, but it is unstable and easily demagnetized, so it cannot replace ferrite.

Strontium magnet: It is characterized by high efficiency, but its volume is not large, so it is only used on tweeters.

5.Branches

The support plate is also called spring plate and elastic wave, which is the support of the speaker vibration. There are two main materials for centering support plate: cotton fabric and polyimide fiber.

6, folding ring

The folding ring is the connecting part of the sound basin and the basin frame, which is used to support the sound system of the sound basin and provide compliant restoring force and damping effect.

7.Dust cap

The main function is to prevent dust and debris from entering the magnetic gap. The material used is paper, cloth, aluminum, plastic or carbon fiber fabric, and the commonly used shape is a hemisphere.

How speakers work

1, moving coil type

The basic principle comes from Fleming's left-hand law. Put a current line and magnetic line perpendicularly between the north and south poles of the magnet. The wire will be moved by the interaction between the magnetic line and the current. Then a diaphragm is attached here. On the root track, the diaphragm will move forward and backward as the current changes. At present, more than 90% of cone cones are moving coil designs.

2.Electromagnetic

Magnetic speaker, also known as "reed speaker". In the structure of magnetic speaker, there is a moving core electromagnet between the two poles of the permanent magnet. The attraction of the phase-level attractive force is kept stationary in the center; when a current flows in the coil, the movable core is magnetized and becomes a bar 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 to promote air thermal vibration.

3.Inductive

Similar to the electromagnetic principle, but the armature doubles, and the two voice coils on the magnet are not symmetrical. When the signal current passes, the two armature will push and move with each other for different magnetic fluxes. Unlike electromagnetics, inductors can regenerate lower frequencies, but the efficiency is very low.

4, electrostatic

The basic principle is Coulomb's law. Usually, a plastic diaphragm and an inductive material such as aluminum are subjected to vacuum vaporization. The two diaphragms are placed face to face. When one of them adds a positive current and high voltage, the other will induce a small current. By pushing each other through the attraction and repulsion of each other, the air can make a sound.

The electrostatic monomer is light in weight and has small vibration dispersion, so it is easy to get clear and transparent midrange and treble, which affects the bass power, and its efficiency is not high, and it is easy to collect dust using a DC power supply.

5.Planar

The earliest design developed by Japan's SONY, the voice coil design is still the theme of the moving coil type, but the cone cone diaphragm is changed to a honeycomb plane diaphragm, because fewer people have a hollow effect, the characteristics are better, but the efficiency is also low. .

6.Ribbon

Without the traditional voice coil design, the diaphragm is made of very thin metal, and the current flows directly into the conductor to make it vibrate. Because its diaphragm is a voice coil, it is very light in weight, has excellent performance response and high frequency response. However, the efficiency and low impedance of ribbon speakers have always been a big challenge for amplifiers. Another method is to have a voice coil, but the voice coil is printed directly on the plastic sheet, which can solve some problems of low impedance.

7, horn type

The diaphragm pushes the air located at the bottom of the horn to work. Because the sound is not diffused during transmission, it is very efficient. However, because the shape and length of the horn will affect the sound, it is not easy to replay low frequencies. Now it is mostly used in giant PA systems or On the tweeter.

8.Piezoelectric

A speaker that uses the inverse piezoelectric effect of a piezoelectric material is called a piezoelectric speaker. The phenomenon that a dielectric material undergoes polarization under pressure causes a potential difference between the two surfaces 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".

9.Ion speakers

Ion loudspeakers use high voltage discharge to make air into charged protons. After the application of AC voltage, these free charged molecules will sound due to vibration. At present, it can only be used in high frequency monomers. Ion speakers differ from other speakers in that they have no diaphragm, so the transient characteristics and high-frequency characteristics are good, but the structure is too complicated.

10.Airflow modulation speaker

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.

11.Ultrasonic

It does not use any traditional form of speaker unit, but uses an ultrasonic generator to generate two specially processed ultrasonic beams. When these two beams act on the eardrum of the human ear at the same time, they can produce hearing by interaction.

Speaker performance indicators

1.Frequency response

This indicator reflects the main frequency range in which the speaker operates. When a constant voltage signal source is applied to the speaker and the frequency of the signal source is changed from low frequency to high frequency, the sound pressure generated by the speaker will change with the change of frequency. The resulting sound pressure-frequency curve, the wider this range, the better the sound reproduction characteristics

2.Rated impedance

It refers to the impedance value measured at the input of the speaker at a specific operating frequency. Usually it is indicated on the product trademark nameplate, given by the manufacturer, the rated impedance is usually the impedance mode value at which the maximum power can be expected in the rated frequency range. The rated impedance is generally 4 ohms, 8 ohms, 16 ohms, 32 ohms, etc. 3 ohms and 6 ohms are also used abroad.

3.Power

The power of the speaker is one of the important indicators when choosing to use the speaker. It is the input power when the speaker can work continuously for a long time without generating abnormal sound. In general testing, pink noise signal is used, and the test is performed in the rated frequency range through a specific filter.

The maximum noise power is different from the rated power, which indicates the speaker's ability to withstand large input power for a short time, and its test time is only a few seconds or minutes. Generally the maximum noise power is 2-4 times the rated power.

4.Sensitivity

Characteristic sensitivity refers to the sound pressure level measured at 1m in the axial direction when the speaker adds a pink noise signal voltage equivalent to 1W power on the rated impedance. Each speaker unit must be basically the same in the frequency band responsible for playback, so that the whole speaker has a balance of high, middle and bass during playback. Especially for stereo speakers, the units used for the left and right channels must be strictly screened and matched. It is required that the difference between the output sound pressure levels of the units used in the left and right channels should be within plus or minus 1 dB, otherwise the sound image localization will be affected.

5.Directivity

Directivity is used to describe the ability of a speaker to radiate sound waves in various directions in space. It is generally expressed by the curve of sound pressure level as a function of radiation angle. The directivity of a loudspeaker is related to frequency, and generally there is no obvious directivity at low frequencies. At high frequencies, due to the short wavelength of sound waves, the directivity will become sharp, so some speakers arrange several high-frequency units in different directions to improve the directivity. Directivity is also related to the caliber of the speaker. Generally, when the caliber is large, the directivity is also sharp; when the caliber is small, the directivity is wide.

6.Distortion

Distortions in speaker systems include chirp distortion, intermodulation distortion, and transient intermodulation distortion. The distortion characteristics of a speaker are more likely to cause the characteristics to deteriorate than a single speaker. Usually near the crossover point, distortion is greatly increased due to improper design or debugging. Harmonic distortion is mainly generated at low frequencies, especially near resonance frequencies. The minimum required harmonic distortion for high-fidelity speakers is not more than 2%.