Subwoofer purchase-from entry to mastery of science

Subwoofer purchase-from entry to mastery of science

There will be a lot of problems in decorating the home theater. As an important component, the subwoofer is also a headache for the owners.

Q：What is a subwoofer?

A: The subwoofer in English is Subwoofer. According to the literal interpretation, it has the meaning of "sub" bass or "under the bass". It can also be said to be "auxiliary bass" or "lower" subwoofer. In Taiwan, Subwoofer has long been called "super" woofer. This is another interpretation of "Sub", which is actually reasonable. After the popularity of home cinemas, the subwoofers in the audio industry were soon slammed into "subwoofers." Honestly, it's really not easy to understand the meaning of "overweight" just by looking at it literally! For audio fans, let's call Subwoofer a subwoofer!

Q： What is the purpose of the subwoofer?

A: As a part of the surround speaker system in a home theater, the subwoofer is responsible for regenerating the low frequency effect in movie sound effects. Used as a two-channel (also known as a 3D satellite speaker system), a subwoofer is used to make up for the low-frequency volume that the two-channel speaker lacks, or the low-frequency and extremely low-frequency areas that cannot be achieved.

Q What types of subwoofers can be divided into?

A: If you distinguish between built-in amplifiers, subwoofers can be divided into active subwoofers with built-in amplifiers and passive subwoofers without built-in amplifiers. If it is distinguished by the design of the cabinet, the subwoofer is the same as the general speaker, and can be roughly divided into a sealed design (Sealed System), a bass reflection design (Ported System), a band-pass design (Bandpass System), and a transmission line design ( Transmission Line System). If it is distinguished by the direction of sound, there are several kinds of sounds: frontal, rearward, floor, upper, and left and right. If distinguished by a single action method, there are a push-pull type consisting of two monomers, an in-phase sounding type consisting of two or three monomers, and an in-phase sounding type consisting of one monomer and one passive radiator. The most advanced subwoofer is in addition to the built-in amplifier, and a built-in bass equalization circuit, which can moderate the indoor low-frequency standing wave to a certain degree of attenuation, so that the low-frequency band below 100Hz or 80Hz can Compare straight frequency response curves. This subwoofer is the most ideal and the most expensive.

Q：What is a closed subwoofer?

A: The cabinet itself is sealed, there are no channels or gaps that are in contact with the outside, the bass unit can be placed facing forward, rear, side or above and below. This kind of box is the simplest in manufacturing and easy to meet the quality requirements. Its advantages are fast transient response, can withstand large power, and the box volume can be made smaller. Closed subwoofer is one of the subwoofers commonly seen on the market.

Q：What is a bass reflex subwoofer? A: The cabinet itself has one or more bass reflection holes, so that the back wave of the bass monomer in the cabinet can be transmitted out of the cabinet through the bass reflection hole and mixed with the positive wave of the bass monomer, thereby increasing the sense of low frequency. . Its advantages are relatively low distortion and relatively high power. The lowest cut-off frequency of the bass reflection type can be lower than that of the closed box (if the same bass unit is used). However, its transient response is not as good as a closed box. The bass-reflex subwoofer is also one of the subwoofers commonly seen on the market.

Q：What is a band-type subwoofer?

A: The design of the box with general formula usually includes two independent boxes, one is hidden, and the other is connected to the outside with a bass reflection hole. The cabinet room hidden inside is closed, and the bass unit is installed on this cabinet. The unit diaphragm faces the cabinet room outside, and the bass unit is not visible from the appearance of the cabinet. When the bass unit moves, the air that drives the outer box passes through the bass reflection hole to form an acoustic filter (Acoustic Filter), which generates a fourth-order (attenuates 24dB every octave, also designed as a sixth-order) filtering effect . Its advantage is that the power capacity may be the best of all the different cabinet types, and the transient response is probably only inferior to that of the closed cabinet. Band-type subwoofers are relatively rare on the market due to the high production cost of the cabinet.

Q：What is a transmission line subwoofer?

A: Like the transmission line horn, there must be a closed pipe with a large and small inside. The large end is connected to the back of the woofer and receives the back wave of the unit. The small end is the exit. The length of the pipe must be at least a quarter of the wavelength of the lowest cut-off frequency, and appropriate sound-absorbing materials must be placed on the inner wall of the pipe. The function of the pipe is to use the back wave of the bass unit to extend the low-frequency cut-off frequency downward to achieve a lower frequency response. The advantage is that the low frequencies can be extended downwards even lower. The manufacturing cost of the transmission subwoofer is the highest among all subwoofers, so it is more rare on the market.

Q：What is a passive subwoofer?

A: In the early days, there was no built-in amplifier in the subwoofer to drive the speaker alone. There was only a passive crossover network responsible for frequency division in the cabinet. This kind of subwoofer is called passive subwoofer. The so-called "passive" term originates from the fact that the subwoofer has no kinetic energy and must be driven by an external amplifier. The passive subwoofer's music signal is obtained from the rear speaker output terminal, and it also has a set of signal output terminals to pass the divided music signal to the two-channel speaker. Since the two-channel speaker and the subwoofer are driven by the same rear stage, the variables of the rear stage increase, and the driving force is also dispersed. At present, such subwoofers are quite rare.

Q：What is an active subwoofer?

A: After entering the era of home cinemas, due to the development of the surround multi-channel speaker system, the subwoofer must have a built-in amplifier for the driver, so the active subwoofer immediately flooded the market. Due to the built-in amplifier, low-level music signals can be obtained from the subwoofer output of the AV surround amplifier (connected via the RCA terminal or XLR terminal). In addition, general active subwoofers will also be provided with high-level input and output terminals, which are connected by speaker cables. In the use of home cinemas, it is impossible to connect the subwoofer with a speaker cable, because whether it is an AV surround amplifier or AV processing pre-stage, the subwoofer signal output is output through the low-level RCA terminal or XLR terminal. It is not output from the speaker wire terminal. In the use of two-channel speakers plus a subwoofer, users can use the second set of output terminals of the previous stage to transmit music signals to the subwoofer, or the output cable of the subsequent stage to connect the subwoofer. .

Q：What kind of amplification circuit does the built-in amplifier of the subwoofer use?

A: For most active subwoofers, the built-in amplifiers use Class D amplifier circuits. Nowadays, they are also called digital amplifiers in more fashionable names. Why use a Class D amplifier? Because this kind of amplifying line is very efficient, does not generate heat, the output power can easily reach hundreds of watts or even kilowatts, and the cost is low. Furthermore, the bandwidth of the subwoofer is about 20Hz-150Hz. The distortion of the Class D amplifier is high, which is not conducive to the disadvantages of the mid-range and high-frequency bands. Of course, there are a few higher-level subwoofers that insist on using class AB amplifiers. If you really work with Class AB amplification, the area of the heat sink on the back of the subwoofer is bound to be quite large, because the area of the heat sink required for the output power of several hundred watts is as large as a hundred watt amplifier.

Q：What input / output terminals are necessary for a subwoofer?

A: For active subwoofers, the necessary input and output terminals include low-level output terminals, input terminals (some also have Link Out for connecting a second subwoofer), and high-level speakers. Line input and output. Then there must be crossover adjustment, volume adjustment, phase adjustment, or phase switching. A few subwoofers also have a high-pass output and a low-pass output. The so-called high pass is to filter out the frequency band below the crossover point and let the frequency band above the crossover point pass. The so-called low pass is to filter out the frequency band above the crossover point and let the frequency band below the crossover point pass.

Q：Volume adjustment or crossover adjustment is easy to understand. What is phase adjustment or phase switching?

A: The so-called phase adjustment is continuous or segmented adjustment from 0-270 degrees, and phase switching is only two kinds of phase switching of 0 degrees or 180 degrees. Why do we need phase adjustment? Simply put, when the low frequency emitted by the subwoofer is mixed with the low frequency emitted by other speakers, if the phase of the two is the same or close to each other, the total low frequency sense is the sum of the two. Conversely, if the two phases are opposite or close to each other, the total low-frequency amount will be subtracted. In theory, when the subwoofer is placed on the same horizontal line as the front, middle, or two-channel left and right speakers, the low-frequency phase should be the same as or close to that of the front, middle, or left and right channels. The problem is that due to various reasons, many people cannot place the subwoofer on the same horizontal line between the left and right speakers, but on a corner or side. At this time, the phase of the sound emitted by the subwoofer is different from the other The speakers will be different, so it is best to have a phase adjustment device. Generally, there are two types of phase adjustment on the subwoofer. The simpler is to switch between 0 and 180 degrees. This subwoofer is best placed in the corner or between the front channel or left and right speakers, even if it cannot , As close as possible to the front channel or left and right speakers. Another more accurate adjustment is 0-270 degrees stepless or segmented adjustment. This phase adjustment has a higher selectivity, and the position of the subwoofer can be more flexible. How do you make sure the phase is correct? It's very simple, just listen with your ears and choose the one with the most abundant low-frequency volume. Of course, you'd better have a pink noise or multiple frequency test piece, broadcast the 20Hz-150Hz frequency on the test piece, or choose a single frequency (such as 60Hz) to listen, so you can definitely hear the most abundant low-frequency volume. .

Q：When we choose the placement of the subwoofer, what should we pay attention to?

A: The placement of the subwoofer is the same as the placement of the left and right channel speakers, because the subwoofer is also affected by the reflection from the adjacent wall or floor, and it is enhanced at some frequencies (normal phase). Effect, it also produces an offsetting (inverted) effect at certain frequencies. In addition, the low-frequency or low-frequency standing waves that are unavoidable in the listening space will also affect the performance of the subwoofer. So, when you choose the placement of the subwoofer, the first thing to consider is: Do I need to increase the sense of volume of the subwoofer through the low-frequency enhancement of the corner. The second thing to consider is whether the standing waves (caused by the left and right channel speakers) that originally exist in the listening space will form an enhanced (normal phase) or offset (inverted) effect with the sound waves of the subwoofer. To be clearer, that is, when we place the subwoofer, we must not only consider the problem of the opposite phase of the sound waves between the subwoofer and the left and right channel speakers, but also consider the subwoofer itself and the phase. Adjacent floors and walls cause the opposite phase problems. As mentioned above, the problem of normal phase and reverse phase caused by the subwoofer and the left and right channel speakers can be solved by the phase adjustment device attached to the subwoofer. As for the subwoofer itself and the wall caused by the normal phase, how to deal with the problem? Of course, it can only be handled by the positioning of the subwoofer. Seeing this, I think readers have begun to realize that the seemingly simple placement of subwoofers involves such complicated considerations.

Q：How should the subwoofer be placed in order to solve the problem of normal phase and reverse phase of the sound waves formed between it and the adjacent wall?

A: The best way to solve the problem of normal phase and reverse phase of the sound waves is to keep the sound waves reflected from the adjacent wall and the sound waves reproduced by the bass unit to maintain the same phase, that is, they are mutually positive. To be more precise, if you want to keep the four sound waves of the woofer, floor, side wall, and rear wall in the normal phase, the safest way is to keep the reflected sound wave and the woofer sound wave within 90 degrees. If the 90-degree phase is converted into a wavelength, it is within a quarter of a wavelength. However, since the sound wave will reach the wall first and then reflect back after it is emitted from the monomer, its travel route is already twice, so we divide the quarter wavelength by two, which is one eighth in actual calculation. The wavelength is fine. Perhaps this is what many people do n’t understand, let me give an example to explain: If the sub-woofer's crossover point is 100Hz, then we take the highest 100Hz wavelength for calculation (other lower frequencies are already covered). A 100Hz wavelength is 340 meters (approximate speed of sound waves per second) divided by 100Hz, which is equal to .3.4 meters. One-eighth wavelength of 3.4 meters is 42.5 cm. Therefore, we should place the subwoofer within 42.5 cm from the side wall, back wall and floor. In this way, the sound waves reflected from the back wall, side wall and floor of the subwoofer can all be reflected. It has a positive relationship with the main sound wave from the bass unit. When all the reflected waves are positively related to the main wave, it is the time when the low-frequency volume is most abundant. This is why placing the subwoofer in the corner can get the richest sense of low-frequency volume. Seeing this, you may think of a question: Since the subwoofer can be placed in the corner to obtain the richest sense of low-frequency, isn't it possible to put the left and right channel speakers in the corner to get the most abundant low-frequency feel? That's right! You can also get the richest sense of low-frequency volume, but do n’t forget that the left and right channel speakers are not only woofers, but also midrange and tweeters. The frequency range of these units is much higher than that of woofers. If you calculate according to the above formula, you will find that the distance required for each monomer is different, so it is easy to produce complex sound waves mutually strengthen or cancel each other out, which is harmful. Therefore, in addition to the subwoofer, the general speaker is not suitable for placing in the corner. Another advantage of placing the subwoofer in the corner is that you can turn the volume knob down because the low-frequency volume is very sufficient. When the volume knob is turned down, it means that the amplifier does not have to output so much power, and the bass unit does not have to move so violently, both of which help reduce distortion.

Q：In addition to the above-mentioned ways to choose the placement of the subwoofer, what other methods can help users find the most appropriate placement?

A: There is another method that is commonly used by ordinary people. It is to try to change the position of the subwoofer while playing familiar music software. This method is also effective, but you must be sure of the music software you are playing to determine how low-frequency effects should behave correctly. In addition, you must have a correct understanding of the low-frequency performance of the instrument, knowing how much low-frequency volume and what kind of low-frequency texture are correct. If you have the above capabilities, using listening methods, try the position of the subwoofer repeatedly. This is an effective method.

Q：Can the subwoofer be placed in front of the listening position and used as a coffee table?

A: Generally, it is not suitable to be placed in front of the listening position, because this position is not easy to achieve the same phase with the left and right channel speakers. In addition, because it is too close to the listener, the low frequencies emitted by the subwoofer cannot be easily mixed with the left and right channel speakers. If the crossover point of the subwoofer is adjusted to a low value (60Hz), you may not feel the low frequency coming from the subwoofer when listening to some music. In short, this is not a good subwoofer setup.

Q：Can the subwoofer be placed behind the listening position?

A: If it is directly behind the listening position, the same problem of phase and low frequency cannot be mixed. It is easy to let people hear that the low frequency band is emitted from the subwoofer at the back. If there is a distance from the listening position, then the phase issue must still be considered.

Q：Can the subwoofer be placed on the side of the listening position?

A: If you are as close as possible to the sides of the left and right channel speakers, the problem is not very large. After phase adjustment, you can still get a perfect low-frequency mixing feeling. You cannot hear the low-frequency sound coming from the subwoofer alone. If the subwoofer is placed on the side immediately adjacent to the listening position, it is not the ideal position. Unless it is two subwoofers, and the crossover point is low enough, plus proper phase adjustment.

Q：Some people say that the subwoofer can be positioned freely because there is no directivity at low frequencies?

A: To discuss the problem of no directivity at low frequencies, first of all