The working principle of the inverter box
Each speaker has its own resonant frequency point. When this frequency appears in the electrical signal, it will cause the speaker to resonate, resulting in increased distortion. If the resonant frequency of the resonant cavity is adjusted to be the same as the resonant frequency of the speaker in the speaker, when a resonant frequency signal appears in the electrical signal, the speaker and the resonant cavity will resonate at the same time. The two ends of the resonant wave: one end is the diaphragm, the other end is the box The wall and the inverted tube, the resonance wave in the box compresses or expands at the same frequency, and the box plate with low intensity also vibrates. The air layer in the inverted tube and the diaphragm vibrate inward or outward at the same time, radiating the same frequency For sound waves in the same phase, the radiation impedance of the diaphragm at this time is greatly increased compared with the case without resonance, so the distortion is correspondingly reduced. When there is no resonant frequency band in the electrical signal, the resonance stops. At this time, the inverter tube becomes a microphone, and the transmitted sound wave is a harmful sound wave, which will have a negative impact on the sound.
Prevention of harmful sound waves
(1) Prevent air leakage from the cabinet. The effects of air leakage are manifold: first, it weakens useful resonance waves, causing low-frequency sound pressure to drop. The greater the amount of air leakage, the worse the low-frequency strength; second, the resonance frequency is changed. It causes resonance imbalance, changes the original best coupling state, causes the output of the inverter tube to decrease, and the resonant frequency shifts; the third is to cause harmful sound waves to be transmitted.
Some articles recommend using rubber pads as speaker seals. In fact, rubber pads can cause distortion in high dynamics at high volume. When the diaphragm moves, the diaphragm pushes the air to move, and the force of the diaphragm is equal to and opposite to the air reaction force. If the loudspeaker is connected to the cabinet through elastic rubber pads, the anti-vibration on the surface is actually the opposite. Especially in the resonance frequency band, the reaction force of the back radiation is very large, which will cause poor transient response when the resonance frequency band is large and dynamic. The best way is to put a small amount of glass glue on the edge of the opening and install the speaker in place before curing. Leakage ensures the accuracy of installation and is easy to disassemble.
(2) Ingenious placement of sound-absorbing materials. The sound-absorbing material has two major functions in the speaker: one is to reduce the reflection of sound waves by absorbing sound waves. Due to the close reflection of the inner wall of the sound box, the back-radiated sound waves will produce strong reflections and standing waves (including various harmonics) in the box. These harmful sound waves act on the diaphragm, causing additional vibration of the diaphragm, making the front face The radiated sound wave is also distorted; the second is to adjust the damping of the speaker, the sound-absorbing materials increase, the damping is enhanced, and the resonance is weakened. The quality factor Q, which reflects the damping characteristics of the resonance peak, is reduced, and the output of the inverter hole is weakened. The response in terms of actual hearing is the decrease in low-frequency strength, but strong damping can reduce the after vibration and overshoot of the diaphragm, making the transient response better.
For the inverter box, it is better to choose a speaker with a Q value of about 0.4. A too high Q value can easily cause unnatural low frequencies. After packing, the Q value can be adjusted with sound-absorbing materials. It is usually better to adjust the Q value between 0.6 and 0.7. At this time, the low end of the frequency response curve is relatively flat, and the transient response is also good. Although the damping state reflects the strength of resonance, it will also affect the effect of the woofer. The more sound-absorbing materials, the more back-radiated sound waves are absorbed, and the less harmful sound waves interfere with internal and external energy, and the better the effect .
Sound waves radiated to the back cannot be muffled blindly, but should be treated separately according to different frequency bands. The method of adjusting the damping by shielding the inverted hole with sound-absorbing material is unscientific, although The damping is adjusted, but the sound efficiency of the inverted hole is reduced, which will increase the influence of harmful sound waves in the box.
(3) Put the inverter tube on the back of the box. Some people mistakenly believe that there will be 180 when the inverter is installed on the front panel and when it is installed on the rear panel. The phase difference of is actually not the case. Although the directions of radiated sound waves before and after installation are opposite, they are still in phase. Regardless of whether the nozzle is in front of or behind the box, it will not affect the actual sense of hearing. Therefore, the inverter tube should be installed on the back panel and as far away as possible from the woofer, so as not to affect the low-frequency resonance wave output, but also to reduce the impact of harmful sound waves in the box on the front audience.
(4) Replace the inverter tube with an empty basin. Replace the inverted tube with an empty basin. This is the traditional empty basin box (also known as a passive radiation box). Its working principle is exactly the same as the inverter box, but the design parameters and debugging methods are different. The advantage of the passive radiation box is that it prevents harmful sound waves in the box from coming out and improves the sound quality, but the harmful sound waves still have an impact on the interior.
(5) Prevent the panel from vibration and sound. It is difficult to avoid the vibration of the box plate. Thickening the plate and setting the stiffener can be improved, and the thick sound-absorbing material can also reduce the vibration. The main consideration should be from the box shape and material. Boxes with unreasonable box shapes and materials will produce strong box plate vibration under resonance wave oscillation. The larger the area of the board, the greater the pressure on the board. Therefore, the appropriate box size should be selected, especially the shape and size of the panel. The panel should be as narrow as possible to reduce the total area. How to reduce the distortion at each harmonic is a problem that must be considered in speaker production.
Multi-stage resonant box
The usually referred to as inverter boxes are single-stage resonance boxes. They have only one cavity. Although there can be more than one The resonant wave output by each inverter has only one common resonant frequency band, that is, a single peak centered on the resonant frequency. It is difficult to adjust to the best condition if multiple inverters are installed on one speaker. If a first-stage resonant cavity and an inverter tube are added in series, it becomes a second-stage inverter box.