Do you really know subwoofers?

                                                    Do you really know subwoofers?

As a part of the home theater system, it is responsible for the low-frequency sound effects in the movie sound effects, which are used to make up for the lack of low-frequency volume of the main speaker, or the low-frequency and extremely low-frequency areas that cannot be reached.

If it is distinguished by whether it has a built-in amplifier, it can be divided into two types: active subwoofer and passive subwoofer. According to the cabinet design, it can be roughly divided into bass reflex design, bandpass design, transmission line design and so on. If it is distinguished by the direction of the sound, it can be divided into sounding toward the front, sounding toward the back, sounding toward the ground, sounding toward the top, and sounding toward the left and right sides. If it is distinguished by a single action mode, there is a push-pull type composed of two speakers. The same-phase sounding type composed of two or three, the same-phase sounding type composed of a single passive radiator. Among them, the most ideal subwoofer is designed with a set of bass equalization circuit in addition to the built-in amplifier, which can moderately attenuate the indoor medium and low frequency standing waves to a certain extent, so that the low frequency band below 100HZ or 80HZ can reach a relatively low frequency. Flat frequency response curve. Of course, guns of this design are also quite expensive.

sealed subwoofer

The box body itself is airtight, and there are no channels or gaps communicating with the outside world. The woofer can be placed facing the front, rear, left, right, up and down. This kind of cabinet is the easiest to manufacture, and it is easy to meet the quality requirements. It responds quickly in an instant, can withstand large power, and the volume of the cabinet can also be made relatively small.

Bass Reflex Subwoofer

It is the most popular type of speaker on the market. The speaker is designed with an inverter tube, which is the so-called bass reflex design.

The box itself has one or more bass reflex holes, so that the front wave of the woofer in the box can pass out of the box through the bass reflex hole, and mix with the positive wave of the woofer, thereby increasing the volume of the low frequency. The distortion is relatively low, and the power amplifier is relatively large. Compared with closed cabinets, the lowest cut-off frequency of the bass reflex type can be lower. But the instant response is not as good as the closed box.

Bandpass Subwoofer

The bandpass cabinet design usually includes two independent cabinets, one hidden inside and one connected to the outside through the bass reflex hole. The chamber hidden inside is airtight. The woofer is installed in this airtight room, and the diaphragm faces another box room outside. The woofer cannot be seen from the outside of the cabinet. When the woofer starts to move, the air that drives the outer box chamber passes through the bass reflex hole to form a sound wave filter, which produces a four-order filter effect on a certain frequency band. The power to withstand is the highest among different box designs. The instant response is only inferior to the closed design. This design cost is relatively rare.

Transmission Line Subwoofer

It is named after the transmission line of classical electrical theory. Usually, the sound tubes of this kind of speakers are mostly stacked in a labyrinth, so they are also called labyrinth or labyrinth.

There must be a closed pipe from large to small inside. The large end is connected to the back of the woofer to receive the back wave of the unit, and the small end is the outlet. The length of the pipe must be at least a quarter of the wavelength of the lowest cutoff frequency, and the inner wall of the pipe needs to be equipped with appropriate sound-absorbing materials. The function of the pipe is to use the back wave of the woofer to extend the low-frequency cutoff frequency down to achieve a lower frequency response. Low frequencies can be extended even lower. The cabinet manufacturing cost of the transmission type subwoofer is also the most expensive, so the gun of this design is even rarer.

passive subwoofer

In the early days, there was no dedicated driver amplifier in the subwoofer, only a passive crossover network responsible for frequency division. This kind of gun is called a passive subwoofer, and some people call it a passive subwoofer. It has no driving ability and must be driven by an external power amplifier. The music signal is obtained from the output of the power amplifier, and it also has a set of signal output terminals to transmit the divided music signal to the main box. Since both the main box and the cannon are driven by the same power amplifier, the driving force will be dispersed as the load of the power amplifier increases. This kind of gun is very rare now, and it is thankless.

active subwoofer 

With the formulation of a home theater surround multi-channel system, the subwoofer must have a built-in amplifier. Therefore, low-level music signals can be obtained from the subwoofer output of the AV amplifier. In addition, the general active subwoofer will also have another high-level input and output terminal, that is, use the speaker cable to connect. It is impossible to connect the cannon with a speaker cable in a home theater, because whether it is an AV power amplifier or a pre-decoder subwoofer signal output is output through a low-level RCA terminal or CLR terminal, not from a speaker cable terminal. In the case of two-channel cannon, the music signal can be transmitted to the cannon through the second output of the front stage or the rear stage.

What kind of amplification method is used by the built-in amplifier of the subwoofer

The vast majority of active subwoofer built-in amplifiers use Class D amplification. Because this method 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 frequency bandwidth of the woofer is about 20HZ-150Hz, and the class D amplifier has high distortion, which is not conducive to the disadvantages of mid-frequency and high-frequency bands, but it is not a big problem in the use of subwoofers. There are also a small number of high-end subwoofers that insist on using the AB method. The area of the heat sink on the back of this type of gun is bound to be quite large.

Necessary input and output terminals for subwoofer

For an active subwoofer, it must include a low-level output terminal and an input terminal. Some also have Link Out to connect the second cannon. High-level speaker line input and output. Then there must be crossover point adjustment, volume adjustment, phase adjustment or phase switching, etc. A few guns do not have a high-pass output. The so-called high-pass output is to filter out the frequency bands below the crossover point and let the frequency bands above the crossover point pass. The low-pass output is inverted to allow the frequency band below the crossover point to pass.

Phase Adjustment and Phase Switching

Phase adjustment is continuous or segmental adjustment from 0-270 degrees, and phase switching is only two phases of 0 degree or 180 degrees. Why do you 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 phases of the two are consistent or close, the total low frequency sense is the sum of the two.

Conversely, if the phases of the two are opposite or nearly opposite, the overall sense of low-frequency volume will be subtracted. Theoretically, when the cannon is on the same horizontal line as the main box and the center, its low-frequency phase should be the same or close to the main box and the center. In fact, many factors make it impossible to place the cannon on the same horizontal line as the main box and the center, but more often in the corner or on the side. At this time, the sound phase of the cannon may be different from that of other channels. It can be adjusted directly with phase adjustment. The cannon that switches between 0 degrees and 180 degrees is best placed in the corner or between the main speakers, as close to the main speaker or center channel as possible. The guns with 0-270 degree stepless or segmented phase adjustment are more flexible in placement. High adjustment selectivity. How to confirm that the adjusted phase is correct? You just need to listen to the frequency band with the richest sense of volume in the low frequency with your ears. The pink noise test disc is a very good test software. Test each frequency of 20Hz-150Hz, or choose a single frequency to listen. Careful comparison can hear the most abundant frequency band.

Subwoofer placement

The cannon is also the same as the main speaker to pay attention to the placement, because the subwoofer will also be affected by the reflection of the adjacent wall or ground, and will enhance (positive phase) at certain frequencies, and will also cancel at certain frequencies ( Reverse) effect. In addition, the inherently unavoidable mid-low frequency or low-frequency standing waves in the listening space will also affect the performance of the subwoofer. Therefore, when you choose the position of the cannon, the first thing to consider is: Do I need to use the low-frequency enhancement of the wall solution to increase the volume of the subwoofer. The second thing to consider is: whether the standing waves originally existing in the listening space (caused by the left and right channel units) will enhance (positive phase) or cancel (antiphase) the sound waves of the subwoofer. That is, when we place the cannon, we should not only consider the positive and negative phase of the sound waves between the cannon and the left and right channels, but also consider the positive and negative phase caused by the cannon itself and the adjacent ground and wall. question.

The problem of positive phase and reverse phase between the cannon and the left and right channels can be solved by adjusting the phase of the cannon symbol. As for the normal phase and reverse phase problems caused by the cannon itself and the wall and ground, in addition to changing the environment, it can only be solved by the placement of the cannon. Simple positioning actually involves many complicated considerations. The best way to solve this problem is to keep the sound waves reflected by the adjacent walls and the sound waves reproduced by the subwoofer in the same phase, that is, they are in phase with each other. To keep the four sound waves of the subwoofer, the ground, the side wall, and the rear wall in positive phase, the safest way is to keep the reflected sound wave and the subwoofer sound wave within 90 degrees of phase. If the 90-degree phase is converted into a wavelength, it is within a quarter of the wavelength.

However, since the sound wave will first reach the wall and then reflect back after being sent out from the unit, its travel route is already doubled, so we divide the quarter wavelength by two, that is, in the actual calculation, take one-eighth of the wavelength as Can. For example: the chosen crossover point of the gun is 100Hz, then we will take the highest 100Hz wavelength for calculation (other lower frequencies have been covered). The wavelength of 100Hz is 340 meters (approximate speed of sound waves per second) divided by 100Hz, which is equal to 3.4M. One-eighth wavelength of 3.4M is 42.5CM. Therefore, we should place the gun within 42.5CM from the side wall, back wall, and ground. In this way, the sound waves reflected from the back wall, side wall, and ground of the gun can be matched with the subwoofer. The emitted main sound wave maintains a positive phase relationship.

When all the reflected waves are in positive phase relationship with the main wave, it is the time when the low frequency is the most abundant. This is why placing the cannon in the corner can obtain the richest sense of low-frequency volume. Another advantage of placing the cannon in the corner is that you can turn down the volume because the low-frequency sense is very sufficient, which means that the amplifier does not have to output so much power, and the unit does not have to move so violently. Helps reduce distortion.

If you think the above method is too complicated, you can also use the simple method, playing the clip or music you are familiar with, while trying to change the position of the gun. To get it right, you also have to have a good sense of the instrument's low-frequency performance.