Speakers and Speakers Magnet

Everyone knows that magnets are needed in electroacoustic equipment such as speakers, stereos, and headphones. So what role does magnets play in electroacoustic devices? How does the performance of the magnet affect the sound output quality? Which magnet should be used in different quality speakers?

Today, yueci is here to explore speakers and speaker magnets with everyone.

What is a speaker?

The core component responsible for producing sound in audio equipment is the speaker, commonly known as the speaker. This key component is indispensable whether it is a speaker or a headset. The loudspeaker is a kind of transducer device that transforms the electric signal into the acoustic signal. The performance of the loudspeaker has a great influence on the sound quality. To understand speaker magnetism, you must first start with the sound principle of the speaker.

The sound principle of the speaker

The side view of the speaker below can help us understand the basic structure of the speaker. The speaker is generally composed of T iron, magnet, voice coil and diaphragm.

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Classification of speakers

  • According to the driving mode: electric type (moving coil type), electromagnetic type (reed type), electrostatic type (capacitive type), piezoelectric type (crystal type), etc.;
  • According to the playback frequency band: bass, midrange, treble, full-range speakers, etc.;
  • According to the shape of the diaphragm: cone, dome, belt, speaker, etc.;
  • According to the magnetic circuit structure: internal magnetic, external magnetic, dual magnetic, five magnetic speakers, etc.;
  • According to product usage: miniature, multimedia, car speakers, etc.

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The structure of the speaker

There are two main types: cone type and dome type. The basic components of moving coil speakers with different functions are the same:
It is composed of fixed frame, magnet, permanent magnet, voice coil, vibration system, accessories, etc.

Moving coil structure and principle

The interaction between the audio current flowing through the voice coil in the magnetic field and the permanent magnetic field will produce a force that vibrates the cone axis.
The force can use the formula: F=BLI. Electricity-force-mechanical vibration-sound
Among them:

  • B: Magnetic induction intensity in the magnetic air gap. (Unit: T)
  • L: Effective length of voice coil wire. (Unit: m)
  • I: The current flowing through the voice coil. (Unit: A)

Under this force, the paper cone moves as a whole.

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Technical parameters: rated impedance

The rated impedance is a pure resistance impedance specified by the manufacturer, which is used to replace the loudspeaker when determining the effective electric power of the signal source. It refers to the minimum value following the first maximum value on the impedance curve, including 4 Ω, 6 Ω, 8 Ω, 16 Ω, 32 Ω, 128 Ω, etc., which is the reference for calculating the electric power, and is generally marked on the loudspeaker body.

The choice of impedance depends on the output capacity of the front stage power amplifier current. Generally speaking, the impedance of large caliber speaker is small.

What is speaker magnet?

Speaker magnet refers to the magnet used in the speaker, which is the magnet used in the speaker, referred to as speaker magnet. When the changing current flows into the speaker, the magnet becomes an electromagnet. The direction of the current is constantly changing, and the electromagnet is constantly moving back and forth due to the “forced movement of the energized wire in the magnetic field”, which drives the paper cone to vibrate back and forth.

How does the speaker make a sound?

We all know that a magnetic field will be generated in the energized wire. The strength of the current affects the strength of the magnetic field (the direction of the magnetic field follows the right-hand rule). When the AC audio current passes through the coil of the speaker (ie the voice coil), the voice coil is A corresponding magnetic field is generated. This magnetic field generates an interaction force with the magnetic field generated by the magnet on the speaker. This force causes the voice coil to vibrate in the speaker’s magnetic field with the strength of the audio current. The diaphragm of the speaker and the voice coil are connected together. When the voice coil and the speaker diaphragm vibrate together, they push the surrounding air to vibrate, and the speaker produces sound. As shown in the figure below, this is the principle of the speaker.

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The influence of magnet performance on the sound output quality of speakers

In the case of the same magnet volume and the same voice coil, the performance of the magnet has a direct effect on the sound quality of the speaker:

  • The greater the magnetic flux density (magnetic induction) B of the magnet, the stronger the thrust acting on the sound film.
  • The greater the magnetic flux density (magnetic induction) B, the greater the relative power, and the higher the SPL sound pressure level (sensitivity). Headphone sensitivity refers to the sound pressure level that the headphones can emit when a 1mw, 1khz sine wave is input to the headphones. The unit of sound pressure is dB (decibel). The greater the sound pressure, the greater the volume, so the higher the sensitivity, the lower the impedance, and the easier it is for headphones to produce sound.
  • The greater the magnetic flux density (magnetic induction) B, the lower the total quality factor Q of the speaker. Q value (qualityfactor) refers to a set of parameters of the damping coefficient of the speaker. Qms is the damping of the mechanical system, which reflects the energy absorption and consumption of the various parts of the speaker. Qes is the damping of the power system, which is mainly reflected in the consumption of electric energy by the DC resistance of the voice coil; Qts is the total damping, which is related to the above two as Qts=Qms*Qes/(Qms+Qes).
  • The greater the magnetic flux density (magnetic induction) B, the better the transient. Transient can be understood as “fast response” to the signal, and the Qms is relatively high. Headphones with good transient response should respond as soon as the signal comes, and stop abruptly as soon as the signal stops. For example, the transition from the lead to the ensemble is the most obvious in drums and symphonies with larger scenes.

How to choose a speaker magnet?

The speaker magnets on the market mainly include AlNiCo, ferrite and neodymium iron boron:
Alnico is the earliest magnet used in speakers, such as speaker speakers (known as tweeters) in the 50s and 60s. Generally made of internal magnetic speaker (external magnetic type is also available). The disadvantage is that the power is small, the frequency range is narrow, hard and brittle, and processing is very inconvenient. In addition, cobalt is a scarce resource, and the price of AlNiCo is relatively high. From the perspective of cost performance, speaker magnets choose AlNiCo.
Ferrites are generally made of external magnetic speakers. Ferrites have relatively low magnetic properties and require a certain volume to meet the driving force of the speakers. Therefore, they are generally used in larger audio speakers. The advantage of ferrite is that it is cheap and cost-effective; the disadvantage is that it is larger in size, smaller in power, and narrow in frequency range.
The magnetic properties of NdFeB are far superior to AlNiCo and ferrite, and it is currently the most used magnet on speakers, especially high-end speakers. Its advantages are its small size, high power, and wide frequency range under the same magnetic flux. At present, HiFi headphones basically use such magnets. The disadvantage is that because it contains rare earth elements, the material price is relatively high.

Several factors to consider when choosing a speaker magnet

First of all, it is necessary to clarify the ambient temperature where the speaker is working, and determine which magnet should be selected according to the temperature. Different magnets have different temperature resistance characteristics, and the maximum operating temperature that can be supported is also different. When the working environment temperature of the magnet exceeds the maximum working temperature, the magnetic performance attenuation and demagnetization may occur, which will directly affect the sound effect of the speaker.

Performance comparison of permanent magnet materials

Theoretical maximum working temperature (℃) Oxidation resistance and corrosion resistance Price
Alnico 600 Good Medium
Permanent ferrite 300 Good Cheap
Sintered NdFeB 200 Bad Medium

Secondly, it is necessary to comprehensively consider the magnetic flux requirements and the volume of the magnet to select the speaker magnet. Someone asked if the louder the speaker magnet, the better the sound? In fact, the speaker is not the bigger the magnet, the better. From the influence of the magnet performance on the sound output quality of the speaker, we can find that the magnetic flux of the magnet has a great influence on the sound quality of the speaker. In the case of the same volume, the magnet performance: neodymium iron boron> alnico> ferrite; at the same magnetic flux Under the requirements, the required volume of NdFeB magnet is the smallest, and the ferrite is the largest. The same magnetic material (same material and same performance), the larger the diameter, the greater the magnetic induction, the greater the power of the speaker, the higher the sensitivity of the speaker, and the better the transient response. Therefore, it is necessary to consider the limitation of the volume of the speaker on the volume of the magnet and the requirements of the magnetic flux performance of the magnet to determine which magnetic material to choose.
Supplement: Someone asked how many cores and magnets the speaker mean?
When we talk about the magnetism of the speaker, we are talking about the diameter of the magnet in the speaker. For example, 100 magnet means that the diameter of the magnet is 100mm. The number of cores in the speaker refers to the diameter of the speaker’s voice coil. For example, 100 cores means the diameter of the voice coil is 100mm.
Source: China Permanent Magnet Manufacturer – www.ymagnet.com

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