Production and Design Principles of Subwoofer Speakers How Subwoofers Are Classified
Subwoofer speakers, commonly known as subwoofers, play a very important role in creating a shocking momentum effect. Most brands sell home theater products in the form of a package consisting of an AV amplifier plus five speakers and a subwoofer. The subwoofer is already an essential configuration. In fact, it is a subwoofer with standard design, exquisite production and excellent effect. Its effect in the audio playback of home theater system is quite charming. It’s a pity that the price of subwoofers on the market with outstanding effects is unacceptable to ordinary people. The effect of the affordable price is unacceptable, and the things in the world are often unsatisfactory. However, audio-visual enthusiasts who are good at hands-on “do it by themselves and have enough food and clothing”. Based on this, this article intends to make a brief introduction to the design principle of the subwoofer for the reference of interested speakers.
Generally speaking, in terms of the composition of subwoofers, bass is also divided into two categories: active and passive. The so-called active subwoofer refers to a subwoofer that includes a power amplifier, and the circuit part is divided into power amplifiers. outside. Usually also has audio frequency filtering (filtering out audio frequency components above the bass), phase adjustment. Volume adjustment and other units; and passive subwoofers are just like ordinary speakers, composed of units and passive power dividers, where the divider is just a low-pass filter. Makes its playback frequency range only for subwoofer audio. The following is an introduction to the subwoofer – large unit speaker and power amplification.
First, the design principle and classification of the subwoofer box
The design principle of the subwoofer can be roughly divided into three categories, namely closed speakers, inverted Phase speakers and bandpass filter speakers.
1. Closed speaker
As the name suggests, this type of speaker box is completely closed, which is different from the general so-called closed box structure. Same as above, see Figure 1.
The characteristics of closed-type speakers are simple structure and better transient response. That is, the sound is deep and clear. The disadvantage is that in the same volume, compared with other types of speakers, the low-frequency dive cutoff frequency is higher than other speakers. Therefore, if you want to obtain a lower low-frequency dive frequency, a larger cabinet volume is usually required. And choose a loudspeaker unit with a larger diameter, and the efficiency of the speaker, that is, the sensitivity, is lower than that of other types of speakers.
There is an engineering design data for reference in the design of box volume. When the resonant frequency Fs of the speaker unit is lower than 50Hz, the volume of the box should preferably be greater than 1.4 liters. When Fs is greater than 50Hz, the volume of the box should preferably be greater than 2 liters.
The closed box usually needs to be filled with a large amount of sound-absorbing cotton during the production and adjustment of the box. The materials are mainly glass fiber and long fiber wool, which can improve the flexibility of the speaker, and can also achieve an equivalent increase The effect of the box volume is theoretically up to 40%. In practice, it can be calculated according to the equivalent volume increase of 15%-24%, which is equivalent to reducing the volume of the box. In addition, filling the sound-absorbing cotton can also improve the efficiency of the speaker. The correct filling amount can improve the efficiency of the speaker by up to 15%. The amount of sound-absorbing cotton usually needs to be repeatedly audited to determine the amount of filling, so that the sound is not turbid (the amount is partial less), dull (too much) is the principle, and the same is true for other types of speakers.
For closed-box subwoofers, the unit requirements are stricter than other types of speakers. It is better to have Fs lower than 40Hz, Qts should be 0.3-0.6, and Fs/Qts≤50. In addition, the unit diameter is preferably greater than 20cm, and it belongs to a long-stroke design.
2. Inverted speakers
It is the largest type of speakers on the market. The speakers are designed with Inverting tube, the so-called bass reflex design, see Figure 2.
Inverted speakers, the cone is displaced when the unit works above the resonant frequency Fs Relatively small, so the power bearing capacity is high, and the resonance distortion is small, but below the resonance frequency, the displacement of the cone is greatly increased, and the resonance distortion increases. Lower bass dive cutoff frequency of the box. In addition, in theory, the efficiency of the inverted speaker can be about 3dB greater than that of the closed box.
Of course, the design, production and adjustment of the inverted speaker including the inverted tube are more difficult than the closed box. The inside of the inverted 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, it is better for Fs to be low at 45Hz, Qts should be less than 0.5, and the value of Fs/Qts should be around 100, and the unit diameter should be greater than 17cm. As with the closed case, a unit with a long stroke design should be used.
3. Band-pass filter speaker
This kind of speaker is relatively rare, see Figure 3 and Figure 4, it can be seen from the figure that it is in the It is developed on the basis of closed box and inverted speakers. It has both the design traces of closed boxes and the characteristics of inverted speakers. The speakers shown in Figure 3 are also called fourth-order bandpass speakers, as shown in Figure 4. The loudspeaker can be called a sixth-order bandpass loudspeaker.
A, 4th order bandpass
in a closed box An open mouth is added, some of which work in closed-box mode, and the other part works in inverted mode. Therefore, this kind of speaker has both the advantages of closed box and the characteristics of inverted-type speakers, and its efficiency is higher than that of pure Closed box, the low frequency dive cutoff frequency is similar to that of the inverted speaker, and a lower dive cutoff frequency can be obtained with a smaller caliber unit. In addition, its bandpass frequency can be adjusted, so the crossover can be simplified, because the speaker itself is equivalent to a natural crossover.
In principle, the unit selection is similar to the closed box, but because the efficiency is slightly higher than that of the closed box, and the displacement of the cone is relatively small, a unit with a smaller diameter and a short stroke can be used.
B, sixth-order band-pass loudspeaker
Another opening is added to the closed part cavity of the fourth-order band-pass loudspeaker, that is, there are two Open mouth, one of which works at a lower frequency and the other at a higher frequency, the two combine to have a frequency response with a certain bandwidth. Compared with the above-mentioned fourth-order bandpass speakers, the efficiency and bandwidth can be adjusted. The performance is more flexible, and a lower and deeper bass effect can be obtained with a smaller diameter driver, and at the same time, the cone displacement is smaller and the resonance distortion is lower.
In terms of unit selection, it is basically similar to the inverted speaker, but the Qts should be controlled at about 0.4. There is basically no strict requirement for the unit diameter. If you want to obtain high sound pressure power and low distortion Output, of course, the unit aperture should be as large as possible.
Because the air flow and sound pressure of the port of the band-pass speaker are usually relatively large during operation, especially in the case of large dynamic and ultra-low frequency signals, therefore, whether it is a fourth-order band-pass speaker, It is still a sixth-order band-pass speaker, and the inverted tube should be as large as possible to avoid airflow sound during work.
In terms of box design, the value of its volume is not based on theoretical calculations in practical applications. Especially for commodity boxes, it is mainly based on the coordination of appearance and size, and the electro-acoustic indicators are leaning The phase tube and sound-absorbing cotton can be adjusted to achieve the best level. Of course, the closer the volume is to the engineering calculation value, the better the performance will be. In addition, in the production of the box, the role of the internal reinforcing ribs cannot be ignored. Adding some reinforcing ribs at the joints of the box and the middle of the large plate is beneficial to reduce the resonance of the speaker, so it is always beneficial to have a lighter box.
Second, the composition of the circuit
The subwoofer is widely used in home theater systems, and the reason is that the video audio decoding and restoration process obtains A subwoofer signal, no matter in the analog Dolby system or the very popular digital surround system, since there is a subwoofer signal, it is bound to need special speakers to reproduce.
As far as the subwoofer circuit composition is concerned, it is generally composed of pre-amplification, low-pass filtering, phase adjustment, power amplification, protection and power supply. p>
The output subwoofer signal is further amplified to an amplitude sufficient to drive the full power output of the power amplifier part, because the voltage of the subwoofer signal provided by each brand of AV amplifier is different, generally ranging from 0.3-1 volts, so the front Stage amplification is still necessary, and the pre-stage circuit also plays an important role in isolating and buffering. Because there are differences in the super bass signal output by each brand of power amplifier, some manufacturers steal materials in the design, resulting in a very high output internal resistance. High, if you directly drive the power amplifier unit of the subwoofer, the effect may be very bad; low-pass filtering is a relatively important unit in the circuit part of the subwoofer, and its function is to mix the low frequency in the super bass signal output by the power amplifier. The above signals are further filtered. Generally, the design will be 80-180Hz (many high-end products design the low-end cut-off frequency of the filter to be continuously adjustable). If it is a fixed-frequency filter, the general value is about 110-150Hz. Too low speaker is easy to produce mixed sound, too high, it is easy to mix into the music signal that can be discerned by the human ear; there is no unified regulation on whether the super bass signal output by the AV power amplifier of each brand is in reverse or normal phase. Therefore, the phase adjustment is in the When placing the subwoofer, use the subwoofer in normal phase or reverse phase according to the needs of the system connection, depending on the effect, and it is generally necessary; the power amplifier unit does not need to be wordy, it is the core of the active subwoofer. Similarly, in order to protect the subwoofer to work safely and protect the equipment from damage to valuable components or amplify the fault in the event of an abnormality, a protection circuit is generally necessary; the power supply is the driving force for each circuit unit to work, and it is the basic component. What needs to be added is that some subwoofers have recently been designed with an automatic power control function, which enables the subwoofer to automatically turn off the main power of the subwoofer when there is no signal.
This article provides a relatively well-designed subwoofer pre-stage signal processing circuit, in which the first stage is signal amplification, and the amplification factor of this stage can be adjusted according to needs, and the second stage is The boosting circuit for super bass below 50Hz, which is not available in general subwoofer circuits, the third stage is a frequency-adjustable low-pass filter, the adjustment range is 80Hz-200Hz, the fourth stage is the isolation buffer stage, and the fifth stage is 0 -180-degree phase continuous adjustment circuit. This is also a functional unit that ordinary subwoofers do not have. It is very distinctive. The last level is also an isolation buffer level, and the last level is the volume adjustment potentiometer. The manufacturer can use the general 4558 for its three dual op amps. The power supply is a regulated power supply ±12-18V. Since the current is very small, it can be obtained from the power amplifier stage power supply through a resistor step down to simplify the design and production difficulty. Those who are interested in this circuit can make appropriate deletions as needed.
As for the power amplification and protection, the power supply circuit is no different from the general power amplifier. It is omitted to save space. However, it is used for subwoofer work and super The low frequency band consumes a lot of power as far as the power amplifier is concerned, and the power amplifier part is required to provide sufficient power output. According to the efficiency of the speaker, the output power is generally required to be greater than 80W. Likewise, it is necessary to have sufficient power reserve. The output distortion increases and the output power is restricted, which affects the effect of the subwoofer. As for the discrete components or the power integrated block, it should be possible. There is no doubt that for the subwoofer, the transformer and power The higher the amplified output power, the better.
It needs to be added that the production of speakers seems to be simple, but it is really not an easy task to make speakers with excellent effects and sound. It is even more difficult under amateur conditions. If the unit parameters provided by the manufacturer are relatively standard and a reference cabinet design guide is provided, it is relatively easy to make speakers under amateur conditions, especially for subwoofers. Of course, this does not affect the spirit of some senior audio professionals, through trial and error, debugging and amazing hearing, to produce excellent speakers.
