The inverting box can be said to be the largest market share of the box, if you can not have a good understanding of the principle of the inverting tube, then it is difficult to design the inverting box, because this seemingly simple box is actually difficult to design.
    Before we begin, let me tell you about a phenomenon. That is, if you hold a syringe in your hand, the kind of hospital injection, then you will find a phenomenon, the kind of needle with a large hole, when you give the injection, your hand can easily push the liquid out. If the opening is as thick as the needle, then it is easiest, and the smaller the opening, the more difficult it is to push. This point is intuitive and understandable. What I want to say is that the situation of the speaker in the inverting box is similar to that of your injection, except that the injection is liquid and the compression rate is small, and the box is air and the compression rate is high.
    So let's analyze the speaker in the box. It can be said that when the speaker diaphragm moves backwards, (backwards refers to the box) we know that the air in the box is compressed, just think, at this time the speed is very fast, the compression of the air in the box is the back of the speaker is first compressed, and then transmitted to other places. This will cause a problem, the air compression brought by the speaker, in the box led to the imbalance of the air pressure in the box, and we know that the air leakage of the inverting box, only the inverting hole this path, because we can know that the conduction of pressure is transmitted to the near the inverting hole, and the box at this moment, is the speaker near the high pressure, the inverting hole pressure is low, The air moves towards the inverting hole. The fatal problem came when the speaker moved backwards to the maximum, the disobedient guy moved forward again. In fact, any moving object on the earth has inertia, that is, when the air moves forward in the speaker, he still continues to move in the pilot phase hole, which creates a problem, the pressure of the air behind the speaker at the moment suddenly becomes smaller, producing a pull to pull the air back. Then the poor air couldn't help it, and when the inertia wasn't enough to resist the pull, it was pulled back through the inverting hole. Over and over and over again. The most important problem is that we can see that the movement of the speaker is always faster than the movement of the air, that is, the movement of the air is always lagging behind the movement of the speaker. Let's stop there and just understand that the movement of the air lags behind the movement of the speaker.
    Let's talk about sound waves. We've all learned about physics. There's an important theory about the propagation of sound waves in the air that if two sound waves are superimposed in the air, if the phase is the same, then the crests and crests will be superimposed, then the amplitudes will be superimposed, and if the phase is 180 degrees different, then the crests and troughs will be superimposed, and they will cancel out, or even zero. Now that we know how this works, let's look at speakers. The reason why the speaker that is exposed to free vibration in the air will not feel the bass is because the phase of the sound wave that is vibrating in front of the speaker is exactly the opposite of the phase of the vibration sound wave on the back, then they cancel each other, so they can not feel the sound.
    Knowing this, we know that the phase of the sound wave vibrating in the speaker box is opposite to the phase of the sound wave vibrating in the front of the diaphragm, so the task of our inverting tube comes, and it needs to reverse this phase. Just now we know that due to the imbalance of air pressure in the box, resulting in the movement of the air, the movement of the air has inertia, this inertia leads to the movement of the air is always lagging behind the movement of the speaker, now we turn the focus to the inverting hole this place, when the speaker vibrates backward, the air is discharged from the inverting hole, which is supposed to be so, we imagine the same, But the key fact is different. When the speaker diaphragm moves backward, the nearby air is first compressed, and then transmitted to the guide tube, this transmission takes time, you go to the street to buy a pack of cigarettes also takes time, when the pressure is transmitted to the phase tube, it has lagged the speaker, then the speaker diaphragm may have begun to move forward, and on the back of the speaker led to a negative pressure, the negative pressure to the air back suction, At this time, the pressure of the inverter tube is greater than the back of the speaker, the air goes back, and it takes a certain time to come back, and by then the speaker may have moved in the opposite direction, which leads to a phenomenon, that is, there is always a certain frequency of sound waves, his delay time is just when the speaker moves outward, which part of the inverter hole is the most pressure, Therefore, the inverting hole is also radiating sound waves outward, when the speaker moves inside, the pressure near the inverting hole is minimum, and the inverting hole absorbs steam from the outside box, which is an interesting phenomenon, you will find that the direction of air movement of the inverting hole is completely consistent with the speaker, that is, the inverting hole phases the sound waves radiated from the back of the speaker inside the box to each other. It becomes a second woofer, and the sound waves are in the same phase, superimposed on each other, and the amplitude increases. However, it should be noted that only a certain frequency of sound waves is like this, and the phase of the frequencies above and below this frequency will not be 180 degrees, so the effect of superposition is different.

With that said, let's summarize.
  1, due to the pressure difference caused by the vibration of the speaker, the sound wave moves to the only drain hole - the inverter tube.
      2, the poor air is inertial, it started to run, want him to stop running back it takes time, therefore, the movement of the air in the box and the movement of the speaker there is a significant lag.
      3, there is always a frequency of sound waves whose hysteresis time is just enough to achieve the maximum amplitude of the speaker diaphragm reverse, but the air has just reached the maximum or minimum pressure near the inverting hole.
      4. The phase reversal hole then reverses the sound waves.
      This is the principle, then, what effect does the parameter of the inverting hole have on the function? This problem is really very complex, involving many disciplines. I don't really understand, just a rough analysis of it.
      We know that when making a inverting box, the area and length of the inverting hole are important design indicators. The larger the area of the inverting hole, the longer the required length. And vice versa. Why is this so?
      We analyze that if the speaker is not closed, we know that the unbalanced pressure in the box will soon leak, that is, it will soon be passed outside, and on the contrary, it will soon suck into the speaker, which means that the air will leak fast, the inertia is small, the lag time is short, and the short time means that the frequency is high. As the opening becomes smaller, the pressure in the box is "gathered", forming an air cushion that is held in, the energy is released from the hole, it will be slower, and the reverse pull of the speaker will offset the energy of the air cushion, the time will be longer, that is, the lag time will be long, the time will be long, and the frequency will be low, which is if the opening is larger (do not consider the length of the inverter), Then the higher the frequency point of the inversion, and vice versa.
      The product of the length of the inverting hole and the cross-sectional area determines that the inverting system has a certain volume, and the air in a certain volume has a certain mass. The air inside the box needs to be drained, and this part of the air must be pushed, which allows us to understand another concept. We can understand that when the inverting hole is large (in order to avoid the area of the inverting hole is too small to cause airflow sound), the discharge speed is too fast, in order to slow down the discharge, we should not increase the air quality of the inverting system, thereby increasing the pushing resistance of the air in the box, in order to slow down the discharge, extend the lag time of the air in the box, and reduce the resonance frequency. We must make the length of the inverting hole longer, so that we can achieve the same resonant frequency as the small-area inverting hole. This is the reason that the smaller the area of the inverting hole, the shorter the length, and the larger the area, the longer the length. We can understand that the speaker is a power source, the compressed air generated to be released, the speed of release, determine the system lag time and resonant frequency. The inverted hole is a load, which provides a certain damping or load for the compressed air body. The size of this damping is determined by the area of the opening hole and the hole depth (length). The smaller the hole, the larger the damping, the slower the discharge, the longer the lag time of the air in the box, the lower the resonant frequency, and vice versa. In addition, the structure of the inverting hole determines that it is a resonance cavity, and there is a certain quality of air in the resonance cavity, and the frequency of the resonance cavity can be quantitatively calculated through the knowledge of acoustics.

Now we conclude:
  1, the speaker and the box and the inverting hole constitute a system, the system speaker is the power source.
    2. Unbalanced pressure is generated in the box, and this pressure should be conducted and discharged through the inverting hole
    3, the inverting hole system is actually the load system of the loudspeaker power source, it can provide the box unbalanced pressure of the discharge damping, adjust the size of the damping, you can change the compressed air cushion of the discharge speed, by changing the discharge speed, you can adjust the 180 degree phase lag time of the air in the box, corresponding to the resonance frequency point.
    4, therefore, since the box has been set, the speaker has been set, the focus of the inverting box design is to change the size of the leakage damping to control the corresponding resonant frequency. Qualitatively speaking, small area, small length, large area, large length.
    Why the resonant frequency of the inverter system should be the same as that of the speaker. This is well understood and is determined by the characteristics of the speaker. The speaker has the largest amplitude near the resonant frequency, and the inverting system has just said that at the resonant frequency, if the speaker is moving outward, then the air flow of the inverting hole is also moving outward, then we can see that the speaker is moving outward, behind it is negative pressure, pulling the air flow to the speaker, the inverting hole air flow is also moving outward, behind it is negative pressure, Pull the air flow towards the inverted phase hole, and the forces generated by the two air flows make each other restrain and resist. Therefore, at the resonant frequency, the pressure of the inverting hole is the same as that of the loudspeaker, but in the opposite direction, it can effectively restrict the excessive increase of the amplitude of the loudspeaker, so as to improve the characteristics of the loudspeaker at the resonant frequency, so that the frequency characteristics are flat, and the inverting hole is superimposed with the loudspeaker at the resonant frequency, so that the sound pressure at the resonant frequency can be improved. Expand the low frequency characteristics. We can also know that below or above the resonant frequency, the amplitude of the speaker decreases rapidly, but although the phase of these frequencies is not in phase, it is at least superimposed, so the frequency point near the resonant frequency can also enhance the sound pressure, especially below the resonant frequency, which can be said to expand the low frequency characteristics.
    Talking about the box, the box volume we all know is an important indicator of the speaker. Let's analyze, the box is a place of unbalanced pressure, the speaker produces compression power, the size of this pressure, and the box has a direct relationship, in principle (actually not this, but also see the speaker parameters, etc.), the smaller the box, the greater the pressure generated, the greater the energy of the air cushion, then with a certain inverted hole damping, Lower resonant frequencies can be generated. Under the same conditions, if the box is larger, the pressure is smaller, the air cushion energy is small, and the resonant frequency is higher under the same guidance system. This is a small speaker, if equipped with a large box, then the bass will not be good reason. The specific design can not only rely on this, to integrate many factors, especially the parameters of the speaker and so on.
    In short, the inverting box is by no means what some people think through the reflection of the rear panel of the speaker to change the phase, it is a complex unbalanced pressure cushion conduction delay system, with unique acoustic characteristics, which is also the inverting box inverted hole opening is not necessarily in the front panel of the truth, here the analysis is still very superficial, please criticize the shrimp!