Production of subwoofer

Subwoofers, commonly known as subwoofers, play a very important role in creating a shocking effect. Most brands sell home theater products in the form of a package consisting of an AV amplifier, five speakers and a subwoofer. Subwoofers are already an indispensable configuration. In fact, subwoofers with standardized design, exquisite production and excellent effects have a very charming effect in the audio playback of home theater systems. It's a pity that the prices of subwoofers with outstanding effects on the market are unacceptable to ordinary people. The effects of affordable ones are unacceptable. Things in the world are often not satisfactory. However, audio-visual enthusiasts who are good at hands-on "do it yourself and have enough food and clothing". Based on this, this article intends to make a brief introduction to the design principle of subwoofers for reference by those who are interested.

Generally speaking, from the perspective of the composition of subwoofers, bass can be divided into two categories: active and passive. The so-called active subwoofer refers to a subwoofer that includes a power amplifier, in which the circuit part, in addition to power amplification, usually also has audio frequency filtering (filtering out audio frequency components above the bass), phase adjustment, volume adjustment and other units; while the passive subwoofer is no different from a general speaker, consisting of a unit and a passive power divider, in which the divider is just a low-pass filter. Its playback frequency range is only for super bass audio. The following is an introduction to the large unit speaker and power amplification of the subwoofer.
1. Subwoofer box design principle and classification
In terms of the design principle of the subwoofer, it can be roughly divided into three categories, namely closed speakers, bass reflex speakers and bandpass filter speakers.
1. Closed speakers

As the name implies , this type of speaker box is completely enclosed, similar to the so-called closed box structure, see Figure 1

The characteristics of closed speakers are simple structure and good transient response. That is, the listening experience 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 a larger-diameter speaker unit, and the efficiency, that is, the sensitivity, of the speaker is lower than other types of speakers.
In terms of box volume design, there is an engineering design data for reference. When the resonant frequency Fs of the speaker unit is lower than 50Hz, the box volume should be greater than 1.4 liters. When Fs is greater than 50Hz, the box volume should be greater than 2 liters. When
making and adjusting the closed box, it is usually necessary to fill a large amount of sound-absorbing cotton in the box. The material is mainly glass fiber and long-fiber wool, which can improve the flexibility of the speaker and achieve the effect of equivalently increasing the box volume, theoretically up to 40%. In practice, it can be calculated as an equivalent increase in volume of 15%-24%, which is equivalent to reducing the volume of the box. In addition, filling with sound-absorbing cotton can also improve the efficiency of the speaker. The correct filling amount can increase the efficiency of the speaker by up to 15%. The amount of sound-absorbing cotton usually needs to be determined through repeated audition, with the principle that the sound is not turbid (too little) or dull (too much). The same is true for other types of speakers.
For closed-box subwoofers, the requirements for the unit are stricter than other types of speakers. It is hoped that Fs should be lower than 40Hz, Qts should be 0.3-0.6, and Fs/Qts≤50. In addition, the unit diameter should be greater than 20cm, and it should be a long-stroke design.