Superheavy bass speakers, commonly known as subwoofers, play a very important role in creating a stunning momentum effect. Most brands of AV amplifier plus five speakers and subwoofers in the form of a package to promote home theater products, subwoofers are already an essential configuration, in fact, the design standard, exquisite production, excellent effect of subwoofers. The effect on audio playback in home theater systems is quite fascinating. Unfortunately, the price of the subwoofer effect on the market is difficult for the average person to accept. The price is affordable, but the effect is not acceptable, and things in the world are often not satisfactory. However, good at hands-on audio and video lovers but "do it yourself, plenty of clothes and food", based on this, this article intends to make a simple introduction to the design principle of subwoofer, for interested sound reference.
  Generally speaking, from the composition of the subwoofer, the bass is also divided into active and passive two categories, the so-called active subwoofer refers to the subwoofer containing power amplifier, of which the circuit part in addition to power amplification. Usually also has audio frequency filtering (filters out the audio frequency components above the bass), phase adjustment. Volume adjustment and other units; The passive subwoofer is no different from the general speaker, consisting of a unit and a passive power divider, in which the divider is a low-pass filter. Make the playback frequency range only for super-heavy bass audio. The following subwoofer - large unit speaker, power amplification are introduced.

1, subwoofer box design principle and classification
  On the design principle of subwoofers, it can be roughly divided into three categories, namely closed speakers, inverting speakers and bandpass filter speakers

1.1, closed speaker
  As the name suggests, this speaker box is completely closed, as is the general so-called closed box structure, see the figure1。

  The closed speaker is characterized by simple structure and good transient response. The sense of hearing is deep and clear. The disadvantage is that under the same volume, compared with other types of speakers, its low frequency dive cutoff frequency is higher than other speakers, therefore, if you want to obtain a lower low frequency dive frequency, you usually need a larger box volume and use a larger caliber speaker unit, and the efficiency of the speaker is lower than other types of speakers.
  In the case volume design, there is an engineering design data for reference. When the resonant frequency Fs of the horn unit is less than 50Hz, the box volume should be greater than 1.4 liters. When Fs is greater than 50Hz, the box volume should preferably be greater than 2 litres.
  Closed box in the production, adjustment usually also need to fill a large number of absorbent cotton in the box, the material is based on glass fiber, long fiber wool, can improve the flexibility of the speaker, can also achieve the equivalent increase in box volume effect, up to 40% in theory, practical can be calculated according to the equivalent increase in volume 15%-24%, equivalent to reducing the volume of the box. In addition, filling the sound absorption cotton, can also improve the efficiency of the speaker, the correct filling amount, the maximum can improve the efficiency of the speaker up to 15%, the amount of sound absorption cotton usually needs to be determined by repeated listening to the amount of filling, with the sound is not cloudy (less), dull (too much) as the principle, other types of speakers are the same.
  For closed box subwoofers, the unit requirements are stricter than other types of speakers, which hope that Fs is less than 40Hz, Qts should be 0.3-0.6, Fs/Qts≤50. In addition, the diameter of the unit should be greater than 20cm, and it belongs to the long stroke design.

1.2, reverse phase speaker
  It is the most popular type of speaker on the market, and the speaker is designed with a inverter tube, that is, the so-called bass reflection design, see Figure 2.

  Inverter speaker, when the unit works at the resonant frequency Fs above the cone displacement is relatively small, so the power bearing capacity is high, and the resonant distortion is small, but below the resonant frequency, the cone displacement is greatly increased, and the resonant distortion is increased, under the same volume and unit conditions, the inverter speaker can obtain a lower low frequency diving cutoff frequency than the closed box. In addition, in theory, the efficiency of the invert speaker can be greater than the closed box of about 3dB.
  Of course, the design, production and adjustment of inverting speakers, including inverting tubes, is more difficult than that of closed boxes. The interior of the inverting speaker also needs to be filled with an appropriate amount of sound-absorbing cotton, usually less than the closed box.
  In terms of unit selection, Fs should be low dry 45Hz, Qts should be less than 0.5, and Fs/Qts should be about 100, unit diameter should be greater than 17cm, in order to obtain a larger sound pressure power, as closed box, it is appropriate to choose a long stroke design unit.

1.3, bandpass filter speaker
  This kind of speaker is relatively rare, see Figure 3, Figure 4, as can be seen from the figure, it is developed on the basis of closed box and inverting speaker. Both closed box design traces, but also the characteristics of the inverting speaker, of which the speaker shown in figure 3 is also called a four-level band-pass speaker, the speaker shown in Figure 4 can be called a six-level band-pass speaker.

A, four-stage tape speaker

  An open mouth is added in the closed chamber, part of which works in the closed box mode, the other part works in the inverted mode, therefore, this speaker has the advantages of closed box, but also has the characteristics of the inverted speaker, its efficiency is higher than the pure closed box, low frequency diving cutoff frequency and the inverted speaker is similar, you can use a smaller caliber unit to obtain a lower diving cutoff frequency. In addition, its bandpass frequency can be adjusted, so the divider can be simplified, because the speaker itself is equivalent to a natural divider.
  In the selection of units, it is similar to the closed box in principle, but because the efficiency is slightly higher than the closed box, and the displacement of the cone is relatively small, a smaller caliber and short stroke unit can be used.

B, six level tape speaker
  An open mouth is added in the closed chamber of the four-order bandpass speaker, that is, there are two open mouths, one of which works at a lower frequency, the other works at a higher frequency, and the two have a frequency response with a certain bandwidth. Compared with the above four-order bandpass speaker, the efficiency and bandwidth adjustability are more flexible. In addition, lower and deeper bass effects can be obtained using smaller caliber units, and at the same time, the cone displacement is smaller and the resonant distortion is lower.
      In the unit selection, it is basically similar to the inverting speaker, but the Qts should be mastered at about 0.4, and the unit caliber is basically not strict requirements, if you want to obtain high sound pressure power and low distortion output, the unit caliber is of course as large as possible.
     Because the airflow and sound pressure of the inverting hole of the tape speaker at work are usually relatively large, especially in the case of large dynamic and ultra-low frequency signals, therefore, whether it is a four-order tape speaker or a six-order tape speaker, the inverting tube should be as large as possible to avoid airflow sound at work.
     In the box design, the value of its volume is not based on theoretical calculation in practical application, especially the commodity box, mainly based on the coordination of beauty and size, electroacoustic indicators by inverting tubes, sound absorption cotton adjustment to achieve the best level can, of course, the closer its volume to the engineering calculation value, the more performance can reach the best level. In addition, in the box production, the role of the internal reinforcement can not be ignored, in the box joint and the middle of the big plate to add some reinforcement to reduce the resonance of the speaker, so the box is always good.

2. Composition of the circuit
  Subwoofer in the home theater system has been widely used, the reason is that the film audio decoding and restoration process to obtain a heavy bass signal, whether in the analog Dolby system or today's very popular digital surround system, since there is a heavy bass signal, it is necessary to need a special speaker to replay.
  Subwoofer circuit composition analysis, generally by the pre-stage amplification, low-pass filtering, phase adjustment, power amplification, protection and power supply components, in terms of its role, the pre-stage amplification is the AV amplifier output of the overweight bass signal further amplified enough to drive the power amplifier part of the full power output of the amplitude, Because each grade of AV power amplifier provides the overweight bass signal voltage is not the same, generally ranging from 0.3-1 volts, so the pre-stage amplification is still necessary, the pre-stage circuit also has an important role is to play the significance of isolation buffer, because the various grades of power amplifier output of the overweight bass signal is different, some manufacturers steal material in the design, resulting in its output internal resistance is very high. If the power amplification unit of the subwoofer is directly driven, the effect may be very bad; Low-pass filter is a subwoofer circuit part of a more important unit, its role is to mix in the power amplifier output of the super-heavy bass signal in the low-frequency signal to further filter out, the general design will be 80-180Hz (many high-end products will filter the low-end cut-off frequency design to continuously adjustable), if it belongs to a fixed frequency filter, The general value is about 110-150Hz, too low speaker is easy to produce mixed sound, too high, easy to mix with human ear recognizable music signals; There is no uniform regulation on whether the overweight bass signal used for the output of various grades of AV power amplifier is reversed or positive, therefore, phase adjustment is to use the subwoofer in positive or reverse phase according to the needs of the system connection when the subwoofer is placed, depending on the effect, and generally essential; The power amplifier unit is not worded, is the core of the active subwoofer, similarly, in order to protect the safe work of the subwoofer and in the event of an anomaly to protect the equipment from being damaged valuable parts or to expand the fault, the protection circuit is generally necessary; The power supply is the power of each circuit unit and is the basic component. It should be added that recently some subwoofers have also designed an automatic power control function, so that the subwoofer automatically turns off the main power supply of the subwoofer when there is no signal.
  This paper provides a relatively well-designed signal processing circuit of the superheavy bass pre-stage, in which the first stage is signal amplification, and the amplification times of this stage can be adjusted according to needs; the second stage is the lifting circuit of the superheavy bass below 50Hz, which is not available in the general subwoofer circuit; the third stage is the frequency can be lowered through the filter, and the adjustment range is 80Hz-200Hz. The fourth stage is the isolation buffer stage, and the fifth stage is the 0-180 degree phase continuous adjustment circuit. This is also the general subwoofer does not have a functional unit, very distinctive, the last level also belongs to the isolation buffer level, the last for the volume adjustment potentiometer. The manufacturer and its three pieces of double operation amplifier can use the general 4558, the power supply is the regulated power supply ±12-18V, because the current is very small, can be obtained by the power amplifier power supply through resistance step-down, to simplify the design and production difficulty. Those who are interested in this circuit can make appropriate cuts as needed.