Let me talk about common sealed boxes first
The speaker cone, front and rear are two non-ventilated spaces, one is a cabinet and the other is an infinite space (of course it is not in the car. The equivalent space in the car determines the lower limit of the low frequency. Generally, a hatchback is better than a three-box. The reason why car bass is better) In terms of eliminating acoustic short circuits, this is indeed a very ideal environment, but unfortunately, the low frequency response of the speaker on the infinite baffle depends on the resonance frequency of the speaker, and the closed cabinet will cause resonance Frequency increase (although sometimes this is not a bad thing) first refutes a mainstream view that half of the energy of a closed box is consumed in the subwoofer. This sentence is not wrong, but the infinite baffle consumes half of the energy of the horn spirit. The closed box is equivalent to an air spring, which increases the stiffness of the speaker vibration system.
In other words, the box body becomes smaller and the speaker resonance frequency increases, resulting in an increase in the low-frequency cut-off frequency. The big bass must be matched with the big box to be effective. However, once the volume of the box body is finished, it is difficult to change, and there is no way to change the supplementary box body or finished box according to the requirements. What if the cabinet volume is not suitable?
The volume is too small. Placing sound-absorbing cotton in the speaker can increase the volume of the cabinet through physical action (this is a thermodynamic phenomenon) and can also weaken the standing wave in the cabinet that is unfavorable to the sound quality (the standing wave mode is weakened and higher Frequency shift, good for sound quality) In theory, it can be equivalent to an increase of 40%, but the actual test only increased by about 20%. Adding sound-absorbing cotton is not a good way to solve the standing wave (similar topics will be written later) The volume is too large, you can fill the box (medium fiber board) to reduce the volume.
However, a closed box is not a panacea, although a rubber-edged speaker can make a small volume to obtain a better low-frequency response. The most obvious is that at the resonant frequency of the speaker, the cone amplitude is very large. When the same unit listens to electronic sounds, it is not enough. At this time, an inverted subwoofer (also called an open-hole box, In fact, the opening is for a closed box. The labyrinth box can be regarded as an open box. As the name implies, the speaker is directed toward the inverted sound wave in the box, and through the acoustic components, it is reversed in a certain frequency band. , Let it radiate out in phase with the sound waves from the speaker. This kind of speaker can increase the sensitivity of a certain frequency band by about 5db (equivalent to an increase of 3.2 times the attack rate of the power amplifier). In addition, this kind of speaker gives the paper cone sound load near the resonance frequency. And maximum impedance.
Although the sound pressure of this frequency is the highest (mainly output by the inverted hole), the amplitude of the paper tray is the smallest (the amplitude of the paper cone is the largest at the closed box f0) to avoid the distortion caused by the large amplitude of the speaker, and the inverted subwoofer can adopt resonance Loudspeakers with higher frequencies. Will the speakers pick the cabinet, or some speakers are designed to have an open-hole cabinet to be effective?
This problem has been solved. From 1972 to 1973, Dr. Richard Small quantified the factors of designing a closed or open box. The judgment is based on the efficiency bandwidth product (EBP) EBP=Fs/Qes (Fs is the resonance frequency of the speaker , Qes represents the Q value of the speaker circuit, and Qms represents the mechanical Q value) This is not an absolute basis for judgment, but it is very helpful to guide the design.
When EBP≤50, it is suitable for designing closed box. When EBP≥50, it is suitable for designing open box. If you do not know the specific parameters of the speaker, there is also an empirical method.
To make EBP greater than 50, Qes is generally small, which usually means that the magnet is relatively large, and the bass of the small magnet is usually a closed box. Note: The efficiency of the open box is higher than that of the closed box. This is the credit of the speaker unit itself, not the high efficiency brought by the box design itself. There are many types of cabinets on the market, and there are some other types of speakers, such as labyrinth (tornifer type or transmission line type) and horn type, passive radiation (empty paper cone), band-pass speaker cabinet, and rear-loaded design (folding The horn, this efficiency is very high, but to design a 35hz suitable horn, the volume needs to be about 500 liters), plus the combination speaker, it will exert several times the power of the power amplifier, even if it is only the principle