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Get out of the misunderstanding of speakers

2021-04-10 52
In the audio system, the structure of the speaker is the simplest, but it plays the most important role. The speaker is the final link in the sound system and the key part of converting electrical signals into sound signals. Due to the lack of structural components of the speaker, any point of deficiency is caused and the entire replay effect is affected. The evaluation of speakers is based on subjective listening-the quality of the final music replay as the standard. Therefore, the following first problem arises: This is the unification of the technical test of the speaker and the subjective sound quality evaluation.
1. About the speaker test and listening
Friends with certain HiFi experience will have such a consensus: This is the gap between the technical indicators of the speaker and the subjective audition.
Why is this happening? This should start from the beginning, from the main technical indicators of the current speaker test.
Among the technical indicators of the speakers, the most important ones are as follows: These are the frequency range, power handling, sensitivity and phase characteristics, and transient characteristics.
Frequency range, also known as frequency response, refers to the replay range from bass to treble in the speaker. In some cases, the frequency response range marked by a pair of small civilian speakers and a pair of large monitor speakers may be the same; but the feelings of subjective listening are quite different. Why is this? Is the test inaccurate or someone deliberately falsified? Under normal circumstances, the above speculation is wrong, but there are other reasons.

Reason 1: The distance between the current test standard and the actual application
The current frequency response test standard still uses the 1W·m standard many years ago. When the sound test standard was formulated many years ago, the frequency response curve of a speaker at 1 watt and 1 meter, in many cases, represents a full power test, which is close to the situation in actual use. Speakers of that era were basically low-power, high-sensitivity speakers. For the input test power of 1W, it basically represents the actual working state of the low-power speaker. In recent years, with the development of speaker manufacturing technology, most speakers have become high-power and low-sensitivity. This makes the current frequency response test, and the gap between it and the actual use is getting bigger and bigger. At present, most speakers, mainly referring to household speakers, have a power rating of more than 50W, and some even reach 200W. The sensitivity can only reach 82-86dB.
In this case, if the frequency response test of 1 watt·m is still used, there will be a situation that the test does not match the use. The input 1W test signal is only equivalent to 1/50~1/200 of the power of most speakers, and the actual use power of these speakers is generally 15~50W, and even full power usage at the peak, which leads to a test itself Deviate from the actual problem.
An accurate and feasible measurement method should try to simulate the actual working state of the equipment under test, when the test results are the most effective.
Therefore, in the frequency response test of the speaker, if a half-power frequency response test can be added, then this half-power frequency response curve will have the most realistic impact on the actual work of the speaker.
For example, a small speaker with a nominal name of 200W, when you use a 100W half-power test signal to test it, you will see severe degradation in low frequency and a substantial increase in distortion.
Why is this? This is because the woofer unit is restricted by the equivalent volume of the speaker and influenced by factors such as inverters, as well as the increase in the resonance of the cabinet itself, which will inevitably produce the above results.
Testing the speaker with a low-power signal of 1W is only equivalent to singing a low-pitched opera by someone who has not received professional training. It is not the actual working state, but the real actual working state is to sing loudly on the stage, so the small voice is good and the performance on the stage is completely They are different concepts, but two different things.
Therefore, the frequency response test of increasing the half power of modern speakers not only has a very realistic guiding significance, but also has the effect of anti-counterfeiting-it can completely expose the original features of those small speakers with a nominal 2000W.

Reason 2: The current test standards are too loose
There is another reason for the inconsistency of subjective listening and frequency response testing. This is that the current test indicators are too loose and not strict enough.
The current speaker frequency response test uses -3 dB as the standard. In other words, the frequency at which the sound from the high-frequency and low-frequency ends is attenuated by half is used as the final frequency of measurement.
With the continuous development and maturity of tweeter production technology today, it is no problem for a tweeter above mid-range to produce a 20 kHz tweeter. At present, the high frequency of the world's most advanced silk film soft dome tweeter has reached 40 kHz and the high frequency limit of the metal film tweeter has reached 80 kHz. In addition, the tweeter is less affected by the speaker cabinet, so the current new speakers above the mid-range will not cause problems in high frequencies. The big influence on high-frequency replay is only limited to the selection of the low-end crossover point of the tweeter, whether the connection with the mid-range unit is excellent, and whether there is sufficient power headroom.
For the woofer, the situation is much more complicated. Although the manufacturing technology of the woofer has also been greatly developed, especially the new diaphragm materials are constantly coming out, the technical indicators of the woofer are also good. But the gap between the test index of the woofer and the actual index after it is installed on the speaker is very obvious. Sometimes, an excellent woofer is installed on a faulty speaker cabinet, which will completely lose its original advantages.
For example, a poorly designed cabinet will produce severe mid- and low-frequency resonance. This resonance will not only cause the sound of the speaker to be replayed, but also cause the deterioration of the test curve.
When it comes to the pros and cons of the low-frequency test curve, it is mentioned that the current test standards are too loose and not strict enough.
The low frequency response of the two curves shown in Figure 2a and Figure 2b can both be finally measured as 30 Hz, but the performance of the replay bass is quite different. Why is there a significant difference in the curve but the same result after measurement and interpretation? This is because the criterion of -3dB is too loose. If the measurement standard is -2dB or -1dB, the measurement results of the two curves will be quite different.
It is generally believed that an audio frequency response with an attenuation within -3dB is an effective frequency response. Attenuation too much, its role in the replay is basically overwhelmed and ineffective. However, a lot of practice has proved that the human ear after rigorous training can achieve quite high resolution and accuracy. The human ear can clearly distinguish the difference in replay sound caused by the difference of a few tenths of a dB. Therefore, if the speaker frequency response test is performed, in addition to providing a standard result of -3dB, it can also provide a test result of -1dB, which will provide a more practical reference for the speaker.
The test indicators of large monitor speakers are more "quasi". In fact, the word "quasi" here does not mean correctness, but refers to the test indicators are closer to subjective listening. Since I mentioned that the current test method of 1 watt · 1 meter has shortcomings, within the scope of the current test standards, which speakers have the closest technical test and subjective listening results? The answer is as follows: this is a large-scale monitor Speakers and large civilian speakers of better quality.
Why is the result a large speaker? There are three reasons. One is the large volume of the cabinet of large speakers, so the low frequency response in actual work is limited by the volume of the cabinet. The second is that large-scale speakers have greater power and power margin, and the possibility of overload distortion during work is relatively small. Especially for large monitor speakers, the power margin left is very large, and there is no possibility of overload distortion at all. The third is that the woofer of large speakers has a relatively large caliber. When producing the same bass, the stroke of the unit is very small, and the distortion is relatively small.

Second, specific misunderstandings about speakers
1. The statement about the long stroke of the large voice coil is not accurate enough
It is often heard that someone mentions the long stroke of the large voice coil. The so-called large voice coil means that the diameter of the voice coil is relatively large, which is easy to understand. But long trips are another matter. Because the long stroke is only a relative concept, under the premise of true comparability, the larger the voice coil, the shorter the stroke can only be relatively short and it is impossible to extend it at all.
From Figure 3a and Figure 3b, we can see the reason for the short stroke of the large voice coil. There are two key factors that determine the length of the woofer's stroke. One is whether the folding ring (mostly made of rubber, foam and processed cloth) glued to the outer ring of the speaker cone is wide enough and has sufficient elastic range. In theory, the wider the folding ring, the stronger the elasticity, and the longer the stroke obtained. However, the folding ring cannot be too large, nor can it be too flexible. If the folding ring is too large, the effective driving area of the unit will be reduced. If a 6.5-inch woofer ring is too large, it can only function as a 5-inch woofer. Moreover, no matter how long the stroke of the small-diameter unit is, it is impossible to send out low-frequency and practical low-frequency.

Another factor that determines the travel of the woofer is the fixed core support sheet pasted on the bottom of the speaker cone. Because the speaker cone can be reliably moved back and forth only by fixing the two points of the folding ring and the fixed core support piece. The fixed core piece is made of cotton and linen fiber with glue and synthetic material. The elastic range of the fixed core support piece is limited. Therefore, it is not the rubber folding ring that restricts the travel of the woofer at present, but the fixed core support piece. Since the diameter of the fixed core support piece cannot be made too large, it will never reach or exceed the diameter of the rubber folding ring, so the larger the diameter of the voice coil, and the diameter of the fixed core support piece is limited, leaving the fixed core support piece The smaller the range of motion, in other words the smaller the speaker stroke. Only when the voice coil is small, the movable range of the fixed core support piece is relatively wide. Therefore, the large voice coil and long-stroke woofer is only a relatively speaking, and it is not accurate.

Get out of the misunderstanding of speakers

2. The long-stroke small-diameter woofer cannot replace the large-diameter woofer
It is often said that a small-diameter long-stroke woofer can emit enough bass as long as the stroke is long enough. This is a wrong view. In theory, as long as the same volume of air is driven in the same unit time, the same level of sound pressure can be generated. But as far as the woofer is concerned, this is unrealistic. Because of too large stroke and too strong air compression ratio, the replay sound will be seriously distorted. For a large-diameter woofer (above 200mm in diameter), when sufficient sound pressure is reached, due to the large driving area of the speaker cone and short stroke, the distortion during playback is small and the sound is better. Even in a relatively ideal state, the current speaker sound distortion can only achieve 1%. When working with a small-diameter, long-stroke woofer, the distortion will rise rapidly due to the excessive stroke, which is an illusion that the quantity is not guaranteed. Therefore, for most small, small-caliber speakers, even the low-power test low-frequency speakers are okay, but in actual use, compared with large large-caliber speakers, the gap is still quite large, which is an essential difference. So when conditions permit, it makes sense to choose large speakers.

3. The tone of low-sensitivity speakers is different
There was a time when the saying was popular that low-sensitivity speakers sound good. In fact, this is only a one-sided and unscientific statement.
The main factors that determine the sound quality of a speaker are only frequency response, transient characteristics, damping characteristics, and withstand power. It has nothing to do with sensitivity. In other words, for speakers with the same power, the lower the sensitivity of the speakers, the greater the input power required, the closer to the overload, and the relative increase in distortion.
Twenty years ago, the technical quality of the speaker unit itself had not yet reached a relatively good level. When making speakers, only a lot of attenuation correction circuits can be added to the crossover. The final result is that the frequency response curve is straight, but the sensitivity is greatly reduced, only reaching about 82dB. In this case, it is usually necessary to use a high-power amplifier to drive low-sensitivity speakers. However, under high-power driving, low-sensitivity small speakers are prone to overload distortion, and even cannot play some large dynamics. musical work.
Due to the development of audio technology, there have been many models of monitor-level and Hi-Fi-level speakers with a sensitivity exceeding 100dB, and their highest sensitivity has been close to 110dB.

4. The sound-absorbing cotton in the speaker does not represent the quality level
"A speaker without sound-absorbing cotton is a low-end speaker", this statement is inaccurate.
There is no sound-absorbing material in low-end speakers, which is a reality.
Basically, there is no sound-absorbing material in the packaged machine and the cheap finished speakers. Therefore, some people have come to the conclusion that there is no sound-absorbing cotton in low-end speakers, and adding filling materials to low-end speakers can improve the playback effect.
In fact, the above conclusion has no causal relationship, and there is no inherent connection.
The sound-absorbing material only plays two roles in the sound box. One is to eliminate some resonance and coloring of the sound box; the other is to appropriately reduce the volume of the sound box. It doesn't matter which class the speakers belong to.
Some people think that adding fillers to speakers is a panacea, and this is a big mistake.
First, as long as the cabinet design of the speaker is reasonable, there is no obvious resonance, and the cabinet is large enough to make a high-quality speaker without filling materials. In the field of speaker production all over the world, there are many such successful examples. No filling material is added to the speaker box, which is good for the transient characteristics of the speaker.
A speaker that has been carefully designed and processed carefully has basically reached an ideal state when it leaves the factory. In this case, randomly changing the presence and amount of filling materials in the speaker will have a lot of impact on the playback of the speaker, and most of these effects are negative.
Too much filler will cause the sound of the replay to become fleshy, poor transient characteristics, and weak. Although the curve will be improved during the test, the sound performance will be degraded during subjective listening. One thing must be clear, this is that the speakers are for listening, not for watching.

5. About the speaker's crossover
In the speaker frequency divider, there are only three main types of components: these are inductors, capacitors and resistors.
The role of the resistor is an attenuator, which is used to balance the sound ratio of each frequency band. As long as the power is large enough for selection. For the treble attenuation resistors of small speakers, the effect of using metal film resistors will be better.
The role of the inductor is to filter out the treble and strobe the bass. In recent years, many kinds of inductance coils wound with special-shaped enameled wire have become popular. Among them are multi-stranded enameled wire, hexagonal enameled wire and ribbon enameled wire, etc. (Figure 4). Every kind of inductance coil of special-shaped wire is said to have a certain magical power. But is this really the case? Actually it is not.
There are only three requirements for inductance coils for speakers. One is that the inductance value is accurate; the other is that its own DC resistance is low; the other is that it is not easy to produce saturation distortion.
Regarding the accuracy of the value, as long as it is measured and calibrated with high-precision instruments one by one during the production process.
In order to reduce the self-resistance of the inductance coil, it is necessary to improve the conductivity of the enameled wire itself. The conductive ability of enameled wires and their cross-sectional area are inseparable and directly related to the cross-sectional shape. When the cross-section of the wire is round, square, or hexagonal (Figure 5a, b), the efficiency is the highest.

Specifically for winding inductors, the hexagonal cross-section enameled wire can effectively reduce the gap between the turns and improve the efficiency of the inductor. Especially for multi-layer coils with a large number of turns, the improvement effect will be very obvious. However, when hexagonal enameled wire is used, the production cost will also be greatly increased. Therefore, if it is not used in very high-end occasions, use round oxygen-free copper enameled wire with a purity of more than 4N.

The fruit is already pretty good. When choosing the wire diameter of the enameled wire, there is never a saying that the thicker the wire diameter, the better. As long as the inductance meets the requirements, the DC resistance is about one-tenth of the DC resistance of the woofer's voice coil. The DC resistance of the inductor is too large, which directly affects the low-frequency damping characteristics of the speaker; if the DC resistance of the inductor is too small, it will increase the production cost needlessly.
As for the choice of the frequency divider capacitor, there will never be a single capacitor that will fundamentally change the reproduction of the speaker. For friends who make their own speakers or want to improve the finished speakers, they must first have a clearer goal. When making speakers, the grade of crossover capacitor should be selected according to the investment of the speaker unit.
For example, if you buy a tweeter with a price of tens of yuan per unit, it is not worthwhile to spend another 20 yuan to choose a capacitor for it. You might as well buy a tweeter for one hundred yuan, and choose a crossover capacitor for a few yuan to get the benefit. If you have bought a tweeter for 300 yuan per unit, and spend dozens of dollars to buy a crossover capacitor, this is a good match.
For audio capacitors, different brands and materials have different effects on playback. But these inherent and detailed differences can be better reflected in the tweeter above the mid-range. For the frequency division capacitors in the low frequency band, the main focus is on accurate capacitance, voltage resistance, and reliability. Compared with treble crossover capacitors, the requirements can be relatively lower, because high-end capacitors with large capacitance values are too expensive. Often in a state where the gains outweigh the losses after use.

6. The advantages and disadvantages of NdFeB
For speaker units, magnetic materials are their bones and the basis of their power. It is a good way to improve the sensitivity of the loudspeaker by using materials with high magnetic energy product to make the loudspeaker unit. But it is not the only way.
For the magnetic material of the speaker, especially the magnetic material of the high-power woofer, there is a very important criterion that the thermal stability must be good. The magnetic energy product of NdFeB magnetic material is very large. But it also has fatal injuries. One is that it is easily oxidized; the other is that it has poor thermal stability. The Curie temperature of neodymium iron boron magnetic material is very low. At 80°C, its performance will drop to 80% of the reference temperature (the reference temperature is 24°C). This explains such a problem: If it is a neodymium iron boron woofer without special heat dissipation treatment, when working at high power, due to the effect of temperature rise, it will lead to insufficient bass. This kind of timbre variation is obvious to most music lovers.
Therefore, most of the current foreign speaker manufacturers basically use neodymium iron boron as the magnetic material of the tweeter, and take more effective heat dissipation measures. Few NdFeB magnetic materials are applied to the manufacturing process of the bass unit.
After years of HiFi practice, most people can basically clearly distinguish the slight differences in the replay sound caused by different audio signal lines. However, due to the poor thermal stability of the NdFeB magnetic material, the frequency response variation of the woofer is higher than 10%, which will be no small regret.

7. It is not advisable to use the demagnetization method to improve the sound quality
A while ago, there was a popular saying that the demagnetization method improves the sound quality. The so-called demagnetization method refers to the absorption of some large iron nails on the magnet of the speaker unit of the finished speaker. The magnetism of the unit itself is dispersed to a certain extent, the sensitivity of the speaker unit is reduced, and the original Q value is changed. Using the demagnetization method to adjust the replay effect of the speaker will play a certain role. But the demagnetization method is only suitable for those speakers that originally listened to the sound dry and tight. It is impossible to apply to most occasions. For speakers that use demagnetization to improve the broadcast, the method of adjusting the sound-absorbing material in the speaker and adjusting the inverter tube of the speaker can also achieve the goal, and the sensitivity of the speaker will not be lost.

8. The frequency response curve of an ideal small speaker
Since the bass replay ability of small speakers is most restricted by the speaker cabinet, the gap between the frequency response test curve and subjective listening is also the largest, which also puts forward a compromise and new requirement for the frequency response test curve of small speakers. This requirement is: the low end of the frequency response is not too much to pursue a lower digitization effect, and the extension of the low frequency end should not be deliberately pursued below 35Hz or more, because for small bookshelves that use small-diameter woofers (under 6.5 inches) Type speakers, which have a test frequency response lower than 40Hz, are of little significance in practical applications. However, if the low-end frequency of the frequency response test is changed to 50Hz, and the curve becomes a curve with a small peak at the low end and a slight drop in the high-frequency end (Figure 6), it will be greatly improved without exceeding the test standard. Improve the sound effect during replay.
With a small speaker with the curve shown in Figure 6, when replaying music, you will feel the bass is fuller and stronger, and the replay sound is kind and sweet.
There are many misunderstandings about speakers, and it is impossible to explain them clearly in a text. With the development of science and technology, old misunderstandings have been overcome, and new misunderstandings will be encountered. Therefore, if you want to avoid detours, you must truly be in line with international standards and continue to learn.

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