Introduction to speaker units and crossover technology

Introduction to speaker units and crossover technology

Although different speakers may differ in the design of internal sound-absorbing cotton, bass reflex tube, reinforcement plate/reinforced baffle and other components, from the perspective of basic components, at least the following three parts are necessary in any speaker: speaker unit (or loudspeaker unit), cabinet and crossover.

The speaker unit of the speaker
　　The speaker unit converts the electrical signal sent by the power amplifier into sound output, and plays the most critical role in the electro-acoustic conversion of the speaker. The performance index and sound quality of the speaker depend to a great extent on the performance of the speaker unit. Therefore, the prerequisite for making a good speaker is to select a speaker unit with excellent performance.

　　The performance requirements for speaker units can be summarized as large power-carrying capacity, low distortion, wide frequency response, good transient response, and high sensitivity. However, it is very difficult to balance distortion, transient, power and other performances in such a wide full-band range of 20Hz-20KHZ. Although the so-called all-round players can achieve a certain overall balance, they are often mediocre in individual details. Therefore, the simplest idea in speaker design is to let the speaker units perform their respective functions and use frequency-band playback.

　　For this reason, speaker manufacturers produce different types of units. Some are only responsible for playing bass, called woofers, those that play mid-range are called mid-range units, and tweeters are only responsible for playing treble. In this way, targeted designs can be adopted to make the performance of each unit better.

　　Therefore, although it is theoretically possible to design a speaker with a full-band speaker, due to the above considerations, the design method of using a combination of multiple units to cover the entire audio frequency band still accounts for the vast majority. The specific number of units used depends on the frequency division method of the audio range. If it is a simple division of two speakers, high and low (or mid-low), it is enough; if it is a three-way speaker with high, mid-low and three-way divisions, then at least three units must be used. Now the design method of two low-frequency units working in parallel is also very popular, so the total number of units may reach four; some large speakers have a finer frequency division. If the design of units working in parallel is adopted, the total number of speaker units will be more. In the information or manual of the speaker, there is usually a scheme such as "X channels and X units", which is a specific description of the number of frequency division channels and the total number of units used in the speaker. For example, "three channels and four units" means that this is a three-way speaker design, with a total of four speaker units, and the rest are similar.

Frequency divider and speaker unit:
　　Since almost all current speakers are designed to play back in multiple frequency bands, there must be a device that can divide the full-band music signal sent by the power amplifier into treble and bass output or treble, midrange and bass output as needed, so that it can be connected to the corresponding speaker unit. The frequency divider is such a device. If the full-band signal is directly sent to the high, midrange and bass units without distribution, the "excess signal" outside the unit frequency response range will have an adverse effect on the signal restoration within the normal frequency band, and may even damage the treble and midrange units.

　　From the perspective of circuit structure, the crossover is essentially an LC filter network composed of capacitors and inductors. The treble channel is a high-pass filter, which only allows high-frequency signals to pass through and blocks low-frequency signals; the bass channel is just the opposite, it only allows bass to pass through and blocks high-frequency signals; the mid-range channel is a band-pass filter, except for the frequency between the two crossover points, one low and one high, which can pass, and both high-frequency and low-frequency components will be blocked. In actual crossovers, sometimes in order to balance the sensitivity difference between the high and low frequency units, attenuation resistors are added; in addition, some crossovers also add an impedance compensation network composed of resistors and capacitors, the purpose of which is to make the impedance curve of the speaker psychologically flatter, so as to facilitate the amplifier drive. For a more detailed discussion of the speaker crossover method, please refer to the article "Electronic Crossover Technology of PC Multimedia Speakers".

Types of speaker units There
　　are many types of speaker units, and the classification methods are also different. If classified according to the principle of electro-acoustic conversion, there are cone units, flat units, dome units, ribbon units and other types. Among them, cone units and flat units are more suitable for tweeters, and some midrange units also use dome designs; from the perspective of the frequency bands covered, some midrange units also use dome designs; from the perspective of the frequency bands covered, speaker units can be divided into bass units, midrange units, tweeters and full-band units.

　　At present, the most common bass units and midrange units are all electric speakers in terms of the conversion principle. They mostly use cone-shaped diaphragms because the design of this shape of diaphragms is mature and has good performance. There are many kinds of diaphragm materials, including traditional paper diaphragms, diaphragms made of polymer synthetic materials (such as polyolefins), and diaphragms made of metal materials such as aluminum and magnesium. The requirements for the diaphragm are good rigidity (not easy to produce split vibration), light weight (good transient response), and appropriate internal damping characteristics (suppress resonance), but these requirements are not easy to meet at the same time. The rigidity of the metal diaphragm is very good, but the damping is poor; the polyolefin diaphragm takes all aspects into consideration relatively well and has been widely used in recent years. In addition, some manufacturers use very complex processes to manufacture diaphragms, and the "sandwich" composite structure is one of them. Its upper and lower surfaces are sandwiched with a middle layer of a honeycomb structure. It has high rigidity as a whole, and at the same time it has the characteristics of light weight and good damping, and has a promising future for fever.

　　The most commonly used tweeter is the dome tweeter, which is also an electric unit in terms of working principle. The diaphragm of the dome tweeter can be made of metal materials (such as aluminum, titanium, beryllium, etc.), called hard dome, or made of soft fabrics (such as silk, chemical fiber), called soft dome. Usually, the high-frequency response of the hard dome is better, while the sound of the soft dome is softer. In recent years, ribbon tweeters and electrostatic tweeters have also been used to a certain extent. Their common advantages are that the diaphragm is particularly light, so the high-frequency response is excellent, and the sound is delicate and transparent. However, the production of these two tweeters is not as easy as the dome tweeter, and their application is not very popular. There is also a horn tweeter, which is composed of a dome-type driving part and a horn-shaped horn. It is characterized by strong sound directivity and high efficiency, so it is widely used in speakers in the field of professional sound amplification. There is also a coaxial unit, which is actually a combination of bass and tweeter units. For specific features, please refer to the relevant questions and answers.

Installation requirements for woofers and tweeters
　　Generally speaking, woofers must be installed in boxes, while tweeters can be installed or not. There are two reasons why woofers must be installed in boxes: one is to eliminate the "sound short circuit" phenomenon; the other is to suppress the low-frequency resonance peak of the speaker unit.

　　When the diaphragm of the bass unit moves forward and backward, in addition to the sound waves radiating forward, the sound radiation phases in the two directions are exactly opposite, that is, 180 degrees apart. Since the wavelength of low-frequency sound waves is very long, its diffraction ability is very strong, that is, the directionality of low-frequency sound waves is very weak. If the speaker unit is not installed in a box, the sound waves radiated backward will go around to the front and cancel out the radiation in front. The overall forward sound wave radiation energy will be greatly weakened. This phenomenon is called "sound short circuit". The "sound short circuit" phenomenon must be eliminated, otherwise the low frequency cannot be effectively radiated. If the speaker unit is installed in a box, the radiation behind the diaphragm will be blocked by the box, and there will be no "sound short circuit".

　　Each electric low-frequency unit has a low-frequency resonance point. The output at this resonance point reaches a peak value, but the distortion is also very high and the transient response is very poor. If this resonance peak is not suppressed, the quality of the reproduced sound will inevitably be seriously affected. If the unit is boxed, the stiffness of the air in the box will inhibit the movement of the diaphragm, thus achieving the purpose of lowering the resonance peak and improving performance. In addition, by rationally selecting the structure and parameters of the box, the purpose of widening the low-frequency response can be achieved. Well-designed bass reflex boxes, passive radiator speakers, and transmission line speakers can all achieve this effect.

　　For the tweeter, the situation is completely different, because the wavelength of the tweeter is short and the diffraction ability is weak, there is no "sound short circuit" phenomenon, and it does not need to suppress the low-frequency resonance peak like the woofer. Therefore, for the tweeter, the role of the speaker is just a support.