\Why does the speaker have a wooden box?

- Apr 11, 2020-

When the speaker vibrates, it's not only the sound wave on the front, but also the sound wave on the back, as we can imagine from the exposed diaphragm on the back. But the sound waves on the front and back of the speaker are different - imagine that when the diaphragm vibrates forward, from the back, it vibrates backward. In this way, in acoustic terms, the "phase" of the sound wave before and after the loudspeaker is opposite.

Because of the diffraction effect of sound waves, we can realize that if there is no barrier, when the sound waves on the back of the speaker are diffracted to the front, they just offset each other with the sound waves on the front of the speaker, then the sound will not exist naturally. This phenomenon is called "sound short circuit".


The way to solve this problem is to use a large board and place the loudspeaker in the center of the board. In this way, the sound wave can't come around. The name of this board is "baffle". With this board, the speaker can sound normally.

But there are also problems. That is, the baffle is big and ugly, and the baffle can't be infinite, so some of the sound waves can come back.

Now let's imagine:

Suppose there is an infinite baffle. Let's bend it up, down, left, right Bend... Next to each other! At this time, the infinite baffle becomes a closed box, and the sound wave at the back of the speaker is completely closed in the box - this is a simple speaker, a closed speaker.

In order to increase the sound damping of the speaker box as much as possible, the sound absorbing cotton material will be installed in the speaker to absorb the sound waves in the speaker. In some hifi speakers, asphalt is even coated inside. However, there are not many sound-absorbing cotton used in multimedia speakers due to cost considerations. At the same time, due to the national security regulations on speaker manufacturing, sound-absorbing materials are not allowed to be used in the box with power amplifier, so as to prevent heating and fire after long-term work.

In order to reduce the resonance of the box, there are also special requirements for the design of the box. Generally speaking, there are three kinds of reinforcing beams and ribs, namely, the reinforcing diaphragms in the center of the box (this design is related to the acoustic design of the box, and needs to be considered as a whole), the 45 degree ribs at the side feet, and the irregular shape ribs inside the large area of the side, so as to change the resonance frequency of the box 。 The stability of the sound box with stiffeners is obviously better than that without stiffeners, but it is rarely used in the low-end box due to the complex process.

The closed box has the advantages of clear sound and good effect, but its sound playback is completely borne by the front speaker diaphragm, which causes its playback frequency will not be too low when the speaker is small. The inverted phase box is to solve this problem.

But in fact, most of the multimedia speakers we usually buy are so-called "inverted phase boxes" with an opening, so what is the inverted phase box?

The theoretical basis of the so-called inverted phase box is that when the sound box is strong enough, the box can be regarded as a mechanical "rigid body" (i.e. an object that will not produce deformation). At this time, the air inside the sound box will be compressed under the vibration of the back of the speaker to produce a common vibration. At this time, an opening will be opened on the box and a pipe will be connected, and the air will be high in this pipe It vibrates fast and makes sound, just like the sound principle of horn in music.

Here we can see in particular that the real sound is the vibration of air in the box and pipe, not the loudspeaker. The loudspeaker just plays a "driving force", just like the diaphragm rather than the coil is the real sound on the loudspeaker. Because of this, the sound from the inverted phase tube is much lower than that from the loudspeaker and has nothing to do with the size of the loudspeaker, because the frequency of the tube is much lower than that of the diaphragm.


So what does "inverted phase" mean?

Inverted phase refers to the reflection through the box, and the sound phase from the inverted phase tube is the same as that of the sound wave on the front of the loudspeaker diaphragm, that is, the phase is opposite to the sound wave on the back of the loudspeaker, and the phase is "inverted". But it should be made clear that these sound waves come from the vibration of the air, and should not be understood as the sound waves on the back of the speaker are "reflected back".

Because of this, the inverted phase box has the following advantages compared with the closed box:

The volume is small, the lower limit of low frequency is 0.7 times of the closed box, the sensitivity is 3dB higher than the closed box, and the sound pressure is higher. So in the small volume of the multimedia speaker, the inverted phase box is used very much.

However, the requirements for acoustic design of the inverted phase box are much higher than that of the sealed box, because there is an inherent problem in the inverted phase box, that is, when the playback frequency is close to the natural frequency of the box, the inverted phase sound wave will offset the front sound wave of the speaker, resulting in a sharp drop in the sound pressure. The specific performance is that although the low frequency lower limit of the inverted phase box is relatively low, the sound pressure drops very fast when approaching the lower limit, and the transient response speed of the inverted phase box is often worse than that of the sealed box. This leads to the poor design of the phase inversion box. Although the bass is relatively low, it is easy to be muddy and the sound quality is poor.

Many manufacturers just open a hole in the speaker and add a pipe, which is considered as "inverted phase". The sound quality is terrible. For this kind of box, it's better to use a sponge to block up the inverted phase hole, which may be better.

Some speakers use special hyperbolic phase reversal holes. This is because in order to achieve good bass effect, the inverted phase tube should be longer, but the long inverted phase tube will produce friction sound under high-speed air flow. The hyperbolic pipeline has the least fluid resistance, which can achieve the balance between high-speed airflow and airflow noise as much as possible.

In the common bookshelf type multimedia speakers, there are two types of inverted phase structure: front inverted phase and back inverted phase, and the subwoofer has another side inverted phase. The number of rear inverted phase speakers is the largest, because the box design is smaller and more beautiful. A rear inverted speaker with a wall at the back and at least 30 cm from the wall makes sense. If it is too far away from the wall, the inverted sound cannot be heard at all, and if it is too close, it will form standing wave (the sound wave will be reflected after encountering the barrier and superposed with the original sound wave, which will offset or enhance each other at some frequencies, which is actually manifested as the enhancement or weakening of some frequencies