What is a Subwoofer?

                                      What is a subwoofer? 

An indispensable part of the subwoofer home audio system, so how much do you know about it? Presumably many people have only heard of him and don't have a preliminary understanding, so today I will introduce the difference and knowledge of subwoofers to help you understand.

The purpose of subwoofer:

As a part of the home theater system, it is responsible for regenerating the low-frequency sound effects in the movie sound effects to make up for the insufficient low-frequency feeling of the main speakers, or the low-frequency and extremely low-frequency areas that cannot be reached.

The distinction of subwoofer:

 If it is distinguished by whether it has a built-in amplifier, it can be divided into two types of active subwoofer and passive subwoofer. If the cabinet design is divided into bass reflex design, band-pass design, transmission line design and so on. If it is distinguished by the direction of sounding, it can be divided into sounding toward the front, sounding toward the back, sounding toward the ground, sounding upward, and sounding toward the left and right sides. If distinguished by a single action method, there is a push-pull type composed of two speakers. In-phase sounding type composed of two or three, in-phase sounding type composed of a single passive radiator. Among them, the most ideal subwoofer is in addition to the built-in amplifier, but also designed a set of bass equalization circuit, which can moderately attenuate the indoor medium and low frequency standing waves to a certain extent, so that the low frequency range below 100HZ or 80HZ can be compared Straight frequency response curve. Of course, the price of guns of this design is also quite expensive.

Closed subwoofer:

The cabinet itself is airtight, and there are no channels or gaps that communicate with the outside world. The woofer can be placed facing up, down, front, back, left, and right. This kind of cabinet is the simplest to manufacture, and can easily meet the quality requirements. It has a quick response time and can withstand higher power. At the same time, the cabinet volume can be made relatively small.

 Bass reflex subwoofer:

The cabinet itself has one or more bass reflex holes, so that the front wave of the woofer in the cabinet can be transmitted out of the cabinet through the bass reflex holes and mixed with the sine wave of the woofer, thereby increasing the volume of the low frequency. The distortion is relatively low, and the withstand power amplifier is relatively large. Compared with a closed cabinet, the lowest cut-off frequency of the bass reflex can be lower. However, the instant response is not as good as a closed box.

Band-pass subwoofer:

The band-pass cabinet design usually includes two independent cabinets, one is hidden inside, and the other is connected to the outside with a bass reflex hole. The chamber hidden inside is airtight. The woofer is installed in this closed room, with the diaphragm facing the other box room outside. The woofer cannot be seen from the appearance of the cabinet. When the bass unit starts to move, it drives the air in the outer box room through the bass reflex hole to form an acoustic wave filter, which produces a four-order filtering effect on a certain frequency band. The withstand power is the best among different cabinet designs. The instant response is only inferior to the closed design. This kind of design cost is relatively rare.

Transmission line type subwoofer:

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 smaller 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 extend the low-frequency cut-off frequency downward by the back wave of the bass unit to achieve a lower frequency response. The low frequency can be extended lower. The manufacturing cost of the transmission type subwoofer is also the most expensive, so the gun of this design is even rarer.

Passive subwoofer is a passive subwoofer:

In the early days, there was no dedicated drive amplifier in the subwoofer, only the passive crossover network responsible for crossover. This type of cannon is called a passive subwoofer and some people call it a passive subwoofer. It has no driving capability 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 pass the divided music signal to the main box. Since the main box and the gun are driven by the same power amplifier, the driving force will be dispersed when the load of the power amplifier increases. This type of cannon is very rare now, and it is thankless.

Active subwoofer is active subwoofer:

With the development 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 power amplifier. In addition, generally active subwoofers will also be equipped with high-level input and output terminals, which are connected by speaker cables.

It is impossible to connect the cannon with a speaker cable in a home theater, because the output of the AV power amplifier or the subwoofer before decoding is output through the low-level RCA terminal or CLR terminal, not from the speaker cable terminal. In the case of a two-channel gun, the music signal can be transmitted to the gun through the second output of the front stage or the back stage.

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

Most of the built-in active subwoofer amplifiers use the Class D amplification method. Because this method is very efficient, does not generate heat, the output power can easily reach hundreds of watts or even thousands of watts, and the cost is low. Furthermore, the bandwidth of the woofer is approximately between 20HZ-150Hz, and the distortion of the D-type amplifier is relatively high, which is not conducive to the shortcomings of the mid-frequency and high-frequency bands, which is not a big problem in the use of subwoofers.

There are also a few high-end subwoofers that insist on using the AB type method. The heat sink area on the back of this type of gun is bound to be quite large. The heat sink required for the output power of hundreds of watts is as large as the heat sink of a power amplifier of hundreds of watts.

The necessary input and output terminals of the subwoofer:

For active subwoofers, it must include low-level output and input. Some are also equipped with Link Out to connect the second cannon. High-level speaker cable input and output. Then there must be crossover point adjustment, volume adjustment, phase adjustment or phase switching, etc. A few guns do not yet have a high-pass output. The so-called high-pass output is to filter out the frequency band below the crossover point, and let the frequency band above the crossover point pass. The low-pass output is inverted, allowing 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, while the phase switching is only two phase shifts of 0 degrees or 180 degrees. Why phase adjustment is needed? 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 the same or close, the total low frequency sense is the sum of the two.

Conversely, if the two phases are opposite or close to opposite, the total low-frequency sense will be subtracted. In theory, when the gun 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 cannot place the gun on the same horizontal line as the main box and the center, but more on the corners or sides. At this time, the phase of the sound emitted by the gun may be different from other channels. It can be adjusted with phase adjustment.

 0 degree and 180 degree cannons are best placed in the corner or between the main speakers, as close as possible to the main speakers or the center channel. Guns with 0-270 degree stepless or segmented phase adjustment are more flexible. The adjustment selectivity is high. How to make sure that the adjusted phase is correct? You only need to use your ears to select the frequency band with the most abundant low-frequency volume. The pink noise test disc is a very good test software. Test each frequency of 20Hz-150Hz, or choose a single frequency to listen. By careful comparison, you can hear the richest frequency bands.

Written by : Davecl Audio

An audio equipments manufacturer

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