A retrospective
Distortion caused by cabinets and crossovers
For a speaker, the performance of the driver unit is important. But as a whole, the cabinet and the crossover are all part of the speaker, and these two parts can also produce distortion.
For frequency dividers, there are usually electronic components such as capacitors, inductors, and resistors in the frequency divider, and audio electrical signals will be distorted after passing through these components, so good capacitors and inductors are not cheap, and some audio companies even need to be customized, and the price is naturally higher. In addition, the improper setting of the crossover point can also cause distortion. For example, a crossover does not cut the sound frequency blanketly, but attenuates the frequency outside the crossover point. If the design of the crossover point is not reasonable, although it can be connected from the perspective of frequency response, but it just falls in the area where the distortion of the two drive units is large, then a large distortion will be formed near the crossover point under the action of the two units.
For the cabinet, the inside of the cabinet is equivalent to a room, there will also be standing waves, reflected sound problems, the resonance caused by the cabinet will produce distortion, in the sense of hearing will make the midrange, or the bass becomes unclean, the clarity is not enough, it is necessary to properly solve the problem through the ratio of the length to width and height of the cabinet, the cabinet material, the structure, and the internal accessories. The most common practice is to calculate the volume of the cabinet, grasp the proportion of the cabinet, avoid some standing waves, and put a certain amount of sound-absorbing cotton inside the cabinet to absorb the sound reflection inside the cabinet.
Bowers & Wilkins' Matrix structure is designed to reduce cabinet vibration and reduce coloration
The metal structure inside the Magico Q5 speaker
Sound-absorbing materials are typically used to absorb sound from inside the cabinet, mainly for bass drivers, or midrange drivers. Because the back of the cabinet is mostly open, it radiates sound to the inside of the cabinet. In contrast, treble drivers, and even some midrange drivers, have a sealed back, and usually have sound-absorbing cotton inside the closed cavity that absorbs the sound waves from the back of the treble driver. But there are also some audio companies that take a targeted approach to reduce distortion. For example, the "Nautilus" flagship loudspeaker launched by Bowers & Wilkins in the United Kingdom has a long duct on the back of the tweeter driver and midrange driver, which is designed to absorb the back wave of the tweeter to reduce distortion. The same is true of KEF's MAT technology introduced in recent years, but instead of a catheter, it is a labyrinth-like MAT component that is mounted behind the coaxial drive. The MAT unit has several pipes of different lengths, each corresponding to a specific frequency, to absorb the back wave of the tweeter driver. For the cabinet structure, the audio company will take some support structures inside the cabinet to reinforce the cabinet. For example, Bowers & Wilkins famously reinforces the cabinet, as well as the complex metal support structure inside the Magico loudspeaker to reinforce the cabinet and prevent resonance. Contemporary audio companies use finite element analysis (FEA) to analyze the vibration distribution of the cabinet, and to make targeted reinforcements for particularly vibrating parts.
The new KEF loudspeakers use MAT technology, which is the part behind the driver that absorbs the back waves of the high-frequency driver
MAT technology is mainly used for absorption at frequencies above 600 Hz
For bass-reflex speakers, the design of the inverter is important
Bass-reflex design is common in loudspeakers and can increase sensitivity and reduce low frequency extension. But don't think that bass reflex is as simple as digging a hole in the cabinet and fitting a tube. In fact, the design of the inverted tube is also very knowledgeable, otherwise the inverted tube is also a component that causes distortion. When the driver unit makes a sound, the diaphragm moves backward to compress the air in the box, and then discharges it from the inverted tube, when the diaphragm moves forward, it sucks in air from outside the box, so there will be air flow in and out of the pipe, and the air flow in the pipe has flow resistance, and the flow resistance is nonlinear, depending on the air velocity in the pipe. When the driver plays the sound, the amplitude of the diaphragm is constantly changing, the air velocity in the duct is also constantly changing, and the flow resistance is also constantly changing, which will cause the air flow in the duct to be disturbed and produce noise. Also, air has inertia. When the diaphragm moves backwards, the air inside the cabinet is compressed, and then pushed through the ducts to the outside. When the diaphragm moves forward, it takes a certain amount of time to inhale due to air but inertia. However, at this time, the diaphragm has moved forward, forming a certain lag with the entry of air, which affects the movement of the voice coil and will also cause distortion of the magnetic circuit.
From the inverted tube to the compression driver
Speaking of inverted tubes, I also think of compression drives. It is widely used in professional loudspeakers and is usually responsible for the playback of high and medium frequencies. In addition, a small number of home speakers also use compression drivers. The compression driver is used in conjunction with a horn waveguide, which is known as a "horn horn". The good thing about horns is that they are highly sensitive and emit powerful sound pressure with ease. They usually work with a large bore bass driver. For audiophiles, the horn speakers listen to symphonies, piano recordings, momentum and dynamics are excellent.
For the compression driver, because there is a compression cavity, there is a throat. When the diaphragm moves forward, it compresses the air inside the cavity, then outputs it from the throat, and when it moves backwards, it sucks in air from the outside, which is similar to the inverted pipe, which produces distortion. And there is also the issue of compression ratio. For example, the larger the diaphragm, the smaller the throat, the larger the compression ratio, the greater the distortion, and the smaller the compression ratio, the smaller the distortion pair. So it is not easy to design a "HIFI-class" compression drive.
Compression driver + horn is the most commonly used sound reinforcement method in professional audio, this method is highly efficient, can output strong sound pressure, and there are also people in audiophiles who like to compress the driver + horn sound
The frequency response is distorted
"Distortion of frequency response" I'm sure many people have seen this for the first time. In fact, the meaning of distortion refers to the deviation of the signal compared with the original signal (or standard) during transmission. When testing the speaker, the frequency response of the test audio signal ranges from 20Hz~20kHz, the frequency response curve is straight, and finally received by the test microphone after the speaker is played, and the frequency response curve of the speaker is finally obtained, which is very different from the original standard, and the frequency response of the speaker cannot be as straight as the input signal, so there is no speaker without frequency response distortion. But at the same time, you'll find that almost every speaker has a period of frequency response that is in the relatively smooth, highest average output sound pressure level, which is the frequency response of the speaker.
In addition, the frequency response curves here are usually the results of near-field tests or in professional anechoic chambers. In fact, when the speaker arrives at the consumer's home, the frequency response curve will be changed to become more severe due to the influence of room acoustics, and the frequency response will be more distorted. Audiophiles who are interested and can afford to have a separate listening room, then do a good job of acoustic treatment, and even use an audio processor to correct the frequency response curve.
I believe many students are familiar with the use of audio processors to correct the frequency response curve. It works by raising or lowering the level of a certain frequency to change the output sound pressure of the speaker, which is used to compensate for the frequency response curve of the listening area. If the level is increased to compensate, it means that the total harmonic distortion of the speaker will increase, and the greater the degree of compensation, the greater the distortion. Use the equalization function of the audio processor with caution.
Frequency response before correction
Corrected frequency response
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To be continued