Sealed, Ported and Free Air
Free Air
Free air, also called infinite baffle, mounts the sub on a board or wall with a huge space behind it. In a car that usually means the rear deck or a wall between the trunk and the cabin, with the whole trunk acting as the box. The back wave is shut away in the trunk and never reaches the front.
It only works if that wall is solid and sealed all the way round. Any gap lets the back wave leak through and cancel the bass. Because the trunk air is so big, it barely holds the cone back, so free air subs move a lot and need to be built for this kind of mounting.
Sealed
A sealed box traps the back wave completely. The air inside acts like a spring that pushes back on the cone, which keeps it under control. Sealed boxes can be small, are easy to build, and play smoothly over a wide range of notes.
Below the box’s resonance, a sealed box gets quieter at about 12 dB for every octave you go down. An octave is half the Hz, so going from 60 Hz to 30 Hz drops about 12 dB.
Ported
A ported box also traps the back wave, then lets it out through a port at a chosen note so it adds to the front instead of cancelling it. Near that note it is much louder than a sealed box for the same power. Below it, output drops at about 24 dB per octave, twice as fast as sealed, and the cone loses the air spring that was holding it back. Read How a Port Works for the details.
Bandpass
A bandpass box puts the sub entirely inside, between two chambers, and only the port plays into the car. It can be very loud over a narrow band of notes. It is also the hardest to get right, and it gives up range to get that loudness.
Side by Side
| Type | Good at | What it costs |
|---|---|---|
| Free air | No box to build. Takes almost no space beyond the trunk itself. | Needs a well-sealed wall between trunk and cabin. Cone moves a lot, so subs must be made for it. Lower power handling. |
| Sealed | Small. Tight, even bass over a wide range. Easy to build and forgiving if the box is a little off. | Needs more power for the same loudness. Bass falls away gently below the box’s resonance. |
| Ported | Much louder near its tuning for the same power. Cone moves less around tuning. | Bigger box, plus room for the port. Falls off fast below tuning, where the cone is unprotected. |
| Bandpass | Very loud over a narrow band. The sub is hidden inside, only ports face out. | Hard to design and build right. Narrow sweet spot. Highs above the band drop off. |
What Changes in a Vehicle
The charts you see for boxes are measured in open space. A car is a small closed room. Once half a bass wave is longer than the cabin, the sound can’t form a wave inside it any more and the sub starts pressurising the whole space. From there down, the lower the note, the bigger the boost. This is called cabin gain.
Cabin gain climbs at roughly the same rate a sealed box falls off, so the two tend to cancel out. That is why a small sealed box can sound surprisingly deep in a car. A ported box gets the boost too, on top of the extra output it makes near tuning, which is why most of the loudest builds are ported.
A car is a leaky room, though. Panels flex and air gets out through openings, so the gain is different in every vehicle and doesn’t keep climbing forever. With windows, doors or a hatch open, the cabin and its openings can act like the chambers and ports of a bandpass box.