Open-air - extreme acoustics
Let's try to figure out what the acoustic design of the type is - Open Air. Not to be confused with free air, meaning an acoustic screen. A free air design would be possible if the subwoofer were cut into the wall separating the cab from the cargo compartment, the wall would be airtight, and all that was behind it would not give a damn if there was no acoustic short circuit between the front and rear sides of the diffuser.
What would happen would be shown schematically in the figure. However, this type of acoustic design is rare in SPL machines. If you make the volume behind the diffuser closed, in the form of a hard box with a volume comparable to the equivalent volume of the speaker, you get a classic closed box. This is the best design for music, but not for SPL.
A much more efficient bass reflex. We made a phase, everything according to the rules, here is the speaker, here is the output of the tunnel tuned to the desired frequency. Now let's see what happens if you start to increase the box size while maintaining the tuning frequency. The tunnel will be needed shorter and shorter, and if you increase its cross section (usually this leads to an increase in length and causes a headache to the creators of music systems), then all the same, sooner or later it will turn out that its length is comparable to the distance to the speaker. Thus, an Open Air type housing is born, where the phase inverter tunnel is part of the internal space of the body, with a huge (and therefore contributing to the efficiency of the phase inverter) cross section.
For the “greater best” in such cases smooth transitions are smoothed out, so that a giant waveguide is obtained from the back of the diffuser to the front wall. The main problem associated with this type of design is the complete impossibility of calculating it. All existing methods and models suggest a phase inverter tunnel that is an order of magnitude or two smaller than the body volume, but it is not clear where the box volume ends and where the tunnel begins.
Free air. The subwoofer works on the entire volume of the body, much tens of times higher than the equivalent volume of the Vas speaker. Sound pressure is maximum at those frequencies where the movement of the diffuser becomes uncontrolled, and this threatens disaster and material losses. A subwoofer in a closed enclosure of a volume comparable (in order of magnitude) with Vas. For music - excellent, but for SPL the effectiveness of this design is insufficient.
A phase inverter is the way to victory. But just what will happen to him when tuning to “espiel” characteristics? With an increase in volume, returns increase, and the tunnel becomes shorter and shorter. Its diameter does much more than musical subwoofers, but it helps only for the time being ...
The tuning frequency of the “espiel” subwoofer is higher than that of the music subwoofer, so the tunnel must be made so wide and short that its proportions approach those of the rest of the box. This is how Open Air is born: a design in which part of the internal volume of the body works like a reflex tunnel. To reduce turbulence losses, the contours of the phase-inverter channel are smoothed.