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Thirteen questions about speaker knowledge

2021-08-07 473
1. What is a coaxial speaker?
In general speakers, the tweeter and woofer are arranged flat on the panel of the speaker, so their sound centers cannot overlap into one point. In this way, the distance between the treble and the bass reaches the listener is different, and this difference will cause The phase deviation thus affects the correct restoration of the sound image. The coaxial speaker uses a coaxial unit, which is actually a combination of a tweeter and a woofer. The treble is cleverly placed in the center of the bass diaphragm, so it can ensure that the acoustic center of the high and low frequencies are the same point. , Thereby solving the problem of phase deviation. The two most famous commercial coaxial speakers are British products, one is Tannoy (Tianlang) using "tulip" coaxial unit, and the other is KEF using Uni-Q coaxial unit.

2. What is a dumbbell arrangement?
That is, the tweeter is clamped tightly between two identical mid/woofers, which are a bit like two big dumbbells in the middle. The dumbbell arrangement can obtain a sound effect similar to that of a point source, which is good for stereo sound image positioning, so this design is more popular recently.

3. What is a two-wire crossover?
A conventional speaker has only one set of input binding posts. The full-band signal from the power amplifier is sent to the speaker with a set of speaker cables, and the high and low frequencies are separated by a crossover inside the speaker. Bi-wiring uses two sets of speaker cables to connect the amplifier and speakers, so that the high and low frequencies are separated from each other, and everyone does not involve each other. Two-wire crossover needs to separate the input terminals of the treble channel and the bass channel of the crossover, so the speaker must provide two sets of binding posts. Of course, the speaker that can split the sound of two lines can also adopt the conventional single-line connection, as long as the two sets of terminals are connected together into one set with the metal short-circuit strips that come with the box.
Similar to two-wire crossover, if three sets of speaker cables are used to transmit treble, midrange and bass respectively, this connection method is called Tri-wiring. However, three-wire partials are not as common as two-wire partials.

4. Is the two-wire crossover necessarily better than the regular connection?
The main reason for the two-wire split is that some speaker cables are suitable for transmitting low frequencies, and some are suitable for transmitting high frequencies. If the transmission is separated, the corresponding wires can be selected according to different needs to achieve the most ideal effect. However, this point of view is only a family statement, and some people think that two-wire crossovers do more harm than good. For example, the famous speaker manufacturers Dynaudio and Thiel insist on not using two-wire crossovers. They believe that the transmission characteristics of different cables are inconsistent, which will destroy the high and The consistency of the bass phase, if the same line is used, then why bother?

5. Why is the bass of usually larger speakers better?
The lower limit of the low frequency of the speaker is closely related to two factors, one is the resonant frequency of the speaker unit, and the other is the volume of the cabinet. Without boxing, the low-frequency resonant frequency of the woofer is generally considered to be the lower limit of the effective frequency response of the unit. The larger the aperture of the unit, the lower the resonant frequency, so the use of a large speaker will help restore lower low frequencies. In addition, a larger diaphragm area can push more air under the premise of the same amplitude, and it is easy to obtain more low-frequency volume. After the horn unit is boxed, its resonance frequency will rise due to the stiffness of the air in the box. The larger the box volume, the smaller the effect of air on the unit, and the smaller the rise in resonance frequency, which is beneficial to obtain a lower Comprehensive low frequency response. Large speakers on the one hand facilitate the use of large-caliber woofers, on the other hand they have a larger cabinet volume, so low frequencies are usually better.

6. What are the main performance indicators of the speakers?
There are many technical indicators to objectively measure the performance of speakers. We often see in product catalogs or speaker manuals: frequency response, impedance, sensitivity, maximum carrying power, and maximum output sound pressure level. The frequency response represents the relationship between the output sound pressure level of the speaker and the frequency. If drawn as a graph, it is a function curve with frequency as the abscissa and output sound pressure (or sound pressure in decibels) as the ordinate. The overall trend of this curve in the mid-frequency band is horizontal. Of course, there may be many small fluctuations in the middle because the system is not perfect. At the low-frequency and high-frequency ends, the curve shows a downward trend, and the output of the speaker will decrease. Usually, the two points where the output of the low-frequency and high-frequency ends drop by 3dB relative to the middle horizontal section are the low-frequency cut-off point and the high-frequency cut-off point. The frequency band between these two points is the frequency response range of the speaker. Obviously, the wider the frequency response range, the better, so that the wider sound range of the music signal can be restored. For current speakers, the high-frequency end is not a problem. It has already reached the upper limit of 20kHz for audio. Some products are far beyond. The difficulty lies in the low-frequency end. Generally, the bookshelf box reaches about 50-60Hz, and the floor box reaches about 30-40Hz. Very good. In addition, the flatter the curve in the frequency response range and the smaller the fluctuation, the better, which means that the speaker can reproduce all frequency signals in the frequency band equally without distortion of balance.
In layman's terms, impedance is the resistance to the input current signal, in ohms (Ω). The most common impedance values of speakers are 8Ω, 4Ω, and 6Ω. Of course, there are other values such as 3Ω, 5Ω, and 10Ω, but they are not common. Need a special note: the impedance of the speaker is only a nominal value, and the actual impedance of the speaker varies with frequency. For example, a speaker with a nominal 8Ω has an impedance of 8Ω only at certain frequencies, and may be 10Ω at other frequencies. , 20Ω, other frequencies may be as low as 6Ω or 4Ω. The characteristics of impedance changing with frequency can be seen clearly on the impedance curve of the speaker. This change increases the difficulty of driving the amplifier. Sensitivity is a measure of the electrical-to-acoustic conversion efficiency of a speaker. The unit is dB/W/m. It means the sound pressure level that can be obtained at a distance of 1m from the axis of the speaker when the power of 1W is input. For example, the sensitivity is 90dB/W/m. The speaker indicates that the input power is 1W, and the sound pressure level of 90dB can be obtained at a distance of 1m directly in front of the speaker. High-sensitivity speakers save amplifier power, which should be considered an advantage. However, sometimes sensitivity and other performance indicators are not easy to balance. Under the trade-off, it is often better to sacrifice a little sensitivity in exchange for better other performance. This is because the current high-power amplifiers are very common and the price is not too high. The sensitivity is not too high. It's a big problem. The maximum load-bearing power is the safety index of the speaker, indicating the input power that the speaker can withstand for a long time. Input below this value is obviously safe. If the limit is exceeded for a long time, the voice coil will easily be overheated and burned. The maximum load-bearing power index provides a reference for us to use speakers safely, but we should also pay attention to the premise of "long time". It is permissible to exceed the maximum load-bearing power for a short time. For example, there are many short-term peaks in the music signal, and its power intensity It exceeds the average power several times or even dozens of times, but the duration is very short, that is, it is fleeting. To play such a signal, as long as the average power does not exceed the maximum load-bearing value of the speaker, there is no problem at all. Maximum output sound pressure level It means the loudspeaker's maximum output capacity when the distortion does not exceed a certain standard. The popular saying is how loud the loudspeaker can be. Generally, the maximum output sound pressure level of household speakers is about 100dB~110dB, and a few high output speakers can reach about 120dB. Obviously, the higher the maximum output sound pressure level, the better. If this indicator is too low, dynamic compression is likely to occur.

7. What are the criteria for evaluating the quality of speakers?
A truly excellent speaker should have both excellent objective performance indicators and good subjective listening evaluation. Excellent performance indicators include wide and flat frequency response, little distortion, fast transient response, high sound pressure output capability, high power carrying capacity, suitable impedance characteristics and reasonable sensitivity. What is a good subjective listening evaluation is an "art", and everyone's standards are different. Theoretically speaking, since the speaker is a link of the sound reproduction system (and the final link that has the most obvious impact on the quality of the sound reproduction), it should be restored absolutely faithfully. Distorted or embellished and beautified, if it meets or approaches such a standard, it is a good speaker. This is the so-called "only true school" view. However, some people think that since the speakers are used to reproduce music, the sound quality of the speakers is the standard for testing the quality of the speakers. This is the so-called "aestheticism" point of view. "Aestheticism" allows speakers to make reasonable modifications to the music signal, and does not care about whether the technical indicators are perfect, as long as the sound is "good". The "Aestheticism" view is more suitable for those of us who use music as entertainment. However, there is no uniform standard for what is "good", and it is easy to fall into misunderstandings in blind pursuit of goodness regardless of performance or personality. Therefore, objectively speaking, even a good speaker recognized by the "Aestheticism" should be built on the premise of ensuring the basic performance indicators.

Can an 8.4Ω speaker be connected to an 8Ω amplifier?
This is a very common problem and a typical problem with conceptual errors. The statement "8 ohm power amplifier" itself is not correct. The questioner may see some power amplifiers marked with words like "100W/8Ω" and think that the output impedance of this power amplifier is 8Ω. This is actually a misunderstanding, correct. The explanation is: with 8Ω load as the test condition, the output power of this power amplifier is 100W.
Power amplifiers are constant voltage output power amplifiers, whether transistor machines or electronic tube machines, and their output impedance is very small. Transistor machines are generally below 0.1Ω, and tube machines are higher, but they are generally below 1Ω instead of 8Ω. Transistor amplifiers have a strong load capacity. In principle, speakers with any impedance can be connected. Of course, it should be noted that the impedance cannot be too low to make the amplifier overwhelmed or even overloaded. For example, connect a pair of 2Ω speakers (if any). Most power amplifiers of medium and low power will be overwhelming. For tube amplifiers, there is a problem of "optimal load", that is, the performance of the circuit is the best when the load impedance is a certain value. The optimal load impedance is usually a few thousand ohms to several tens of thousand ohms, while the impedance of a speaker is only a few. Ohm, the difference is too large, so an output transformer is used for impedance transformation. The output transformer of the tube machine is generally equipped with different taps. No matter the impedance of the speaker, as long as the set of taps with the same (or close) value on the output transformer is selected, it can be "mapped" to the best load required by the power amplifier. In summary, when the power amplifier is matched with speakers, there is no need to worry about the impedance of the speakers at all. The transistor machine can be connected to speakers of any impedance, and the tube machine can adapt to speakers of various impedances by selecting the tap of the output transformer.

Thirteen questions about speaker knowledge

9. Why do some speakers consume so much power and what causes it?
Two reasons: First, the sensitivity of the speakers may be relatively low. For speakers with a sensitivity difference of only 3dB, to obtain the same volume level (or sound pressure level), the input power difference will be doubled. For example, if a box of 90dB/W/m wants to obtain a sound pressure level of 100dB at a distance of 1m , As long as the input power of 10W is enough, and for the 87dB/W/m speaker, 20W power is needed. If the sensitivity difference of the speakers is 10dB, then the input power under the same output sound pressure condition will reach a difference of 10 times. For example, if the front 87dB/W/m speaker is replaced with an 80dB/W/m sensitivity speaker, the sound pressure of 100dB is still obtained at 1m away, and the required input power is as high as 100W, which is higher than the 90dB/W/m box. 10 times.
Second, maybe the sensitivity is not low, but the impedance characteristic is abnormal. For example, for some speakers, the sensitivity is above 87-90dB/W/m, which is not low, but looking at their impedance curve, the impedance at certain frequencies may be as low as 2Ω or even 1Ω. Such a low impedance is for ordinary amplifiers. It's close to short circuit, how to push it? It is certain that serious overload distortion will occur at these frequencies. To tame such speakers, only Krell, Mark Levinson, and other super powerful amplifiers whose power can maintain a linear increase when the load impedance is reduced to 1Ω, can be used. If you encounter boxes with low sensitivity and weird impedance characteristics at the same time, the requirements on the amplifier will be even more demanding.

10. Someone uses a tube amplifier with a power of only a few watts to push a pair of very large speakers. Does it make sense?
Some people think that the large-caliber speakers used in large speakers are very heavy, and the amplifiers with small power cannot be pushed. In fact, it is a kind of taken for granted. The speaker's demand for amplifier power is mainly related to the sensitivity of the speaker, and has nothing to do with the size of the unit. Many large speakers, especially boxes with large-caliber paper cone woofers, such as the products of JBL, Klipsch and other companies in the United States, have very high sensitivity, usually above 90dB, and some even reach 95dB. For such speakers, use A single-ended tube amplifier with an output power of a few watts can push them to bursts. Some enthusiasts may know that in Japan, using 8W single-ended 300B amplifiers to push high-sensitivity JBL speakers is a very popular way to play.

11. The speaker is 100W, can it be pushed with a 50W amplifier? If you use a 200W power amplifier to push, will it burn the speakers?
First of all, it must be clear that the 100W power indicated on the speaker manual or nameplate refers to the carrying power of the speaker, which means that as long as the power of more than 100W is not input for a long time, the speaker will not be damaged, rather than the need to use a 100W power amplifier. As for how many watts of power amplifier is needed to drive, it mainly depends on the sensitivity of the speakers and the required output sound pressure level, which has nothing to do with the carrying power of the speakers. Generally speaking, there are always a few speakers with particularly low sensitivity, and the sound pressure level required in an ordinary home environment is not very large. A 50W power amplifier can already satisfy many speakers. Of course, the impedance characteristics of some speakers are quite special, which is a severe test for the amplifier. At this time, a powerful power amplifier with high power and high current output is needed to tame them.
Let's look at the second question: Will the speaker burn with a 200W power amplifier? It depends on how you use this amplifier. Indeed, 200W has surpassed twice the maximum carrying power of the speaker. If you turn on the volume to the maximum and keep the amplifier at full power output, then the pair of speakers will definitely burn. But this situation almost never happens. No one will suddenly turn the volume of the power amplifier to the end to use it. In fact, when the volume is so large that it is close to overload distortion (breaking noise), no one will continue to increase the volume ( It is equal to increasing the power input to the speaker) to make the speaker completely break, but it will reduce some volume to make the speaker emit a normal sound. In this way, the average power input to the speaker is always controlled within a safe range that it can withstand. No matter how high the power of the amplifier is, it only outputs the part that the speaker can withstand. How can it burn the speaker? On the contrary, the use of high-power power amplifiers increases the power reserve, which can avoid the overload distortion of the momentary large dynamic peak music signal, which is also beneficial to ensuring the quality of the sound.

12. What does "20-200W" on the speaker nameplate mean, and what is its power?
This is not an indicator of the speaker's carrying power, but the recommended amplifier power range, that is, the manufacturer recommends using an amplifier with a power in the range of 20-200W to drive.

When the speaker is in use, is it better to take off the grille or put it on?

The face net looks like a thin layer of yarn, in fact

It has an impact on sound radiation. If you use an instrument to measure the frequency response of the mesh with and without mesh, you will find that there is a big difference between the two. Most of the speakers are tested and adjusted when they leave the factory without the face screen, so the face screen should also be removed when using it. Of course, a few speakers are said to have been tested and tuned under the condition of wearing the net, so they should not be removed when listening. It is said that this is the case with Avalon speakers in the United States.

13. What are the special requirements for speakers in the home theater system?
Home theater systems generally use surround sound playback systems, so there are many channels. The current popular Dolby Digital and DTS systems use five wideband speakers to form the front and rear channels, and there is also a subwoofer. The requirements for these speakers are no different from those for high-fidelity two-channel systems. They are still the common requirements for wide frequency response, low distortion, less sound coloration, and good transient response. In principle, you only need to listen to speakers that perform well in music. , It is no problem for home theater, but those speakers with weak dynamic output capability (such as LS3/5A) are not suitable, otherwise they will easily be overloaded and distorted when encountering large dynamic and hot scenes. Since the center speaker is generally used on a TV, it should have antimagnetic performance. In addition, the sound of the speakers of each channel should be coordinated, and it is best to use the same series of products from the same manufacturer.

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