When a loudspeaker moves in one direction, creating a compression in front of itself, at the same time it also creates a dilation behind it. The two generated waves would tend to nullify each other, being out of phase, and this would greatly limit the diffusion of the acoustic wave into the environment. To avoid this, the loudspeakers are fixed onto panels whose function is to split the two waves so that the external one can be free to propagate. A set of loudspeakers are then set up on one of the four faces of what is called an acoustic box, which keeps sealed the wave generated behind the loudspeaker, therefore making sure it doesn't interfere destructively with the external wave.
With this system the wave diffuses but it is decidedly inefficient, seeing that the entire posterior wave remains unexploited. It is possible however, to use the energy of the posterior wave to increase efficiency; the three traditional types of boxes designed for this scope are: the bass reflex box, the drone cone box, and the rear horn box.
The following diagram shows a cross section of this kind of box:
Thanks to its opening, the box behaves like a Helmhotz resonator [Helmotz resonators ] which resonates with frequencies that are close to the cone's resonance frequency (the box's dimensions are designed accordingly) and therefore it returns the same frequency emitted by the cone at the front opening. What is particularly interesting is that this wave, having completed its route in the box arrives at the opening inverted in phase, and therefore in phase with the front wave generated by the cone. This way the wave behind the cone contributes in strengthening the front wave, thus improving the box's efficiency.
A second cone, lacking any coil or magnet, is set up next to the main cone. The box is hermetically sealed and when the main cone moves, the posterior wave travels through the box and makes the secondary cone move in phase with the main one. This way efficiency is increased. All this is illustrated in the following figure: