Double tuned reflex: two chambers in series

Take a vented box and, instead of letting its port open into the room, let it open into a second vented chamber. That chamber's port opens into the room. The driver still faces outward and still radiates directly.

So there are two chambers and two ports, and one of the ports is internal: nobody hears it, it only couples one chamber to the other.

This is sometimes called a double bass reflex or a dual chamber reflex. It is not a bandpass box, where the cone is sealed away and only a port is heard.

What the second chamber buys

One resonator gives you one tuning. Two give you two, and they sit roughly an octave apart.

A single vented box reinforces the bass over a fairly narrow span around its tuning and then falls away. Two tunings spread that reinforcement over a wider span, so the useful bass region is broader than a vented box of the same total volume gives.

The more practical benefit is cone excursion, and it is worth knowing exactly where it applies, because the obvious version is wrong.

A vented box puts its one deep excursion minimum at its tuning, so right there it is hard to beat. A double-tuned box has two minima and places them elsewhere, one below and one above. The result is that it protects the driver in the region below where a vented box does: against a vented box of the same total volume and the same outer tuning, the cone travels about a third as far two thirds of an octave down, and less across that whole region. At the vented box's own tuning it moves roughly twice as far.

For anything asked to play below its tuning, which is most subwoofers, that is the trade you want. Less movement down there means less distortion and more headroom before the driver runs out of travel.

Look at the impedance curve to see the difference plainly. A sealed box has one peak, a vented box has two, and this has three.

What it costs

A steeper rolloff below the lower tuning. More resonators mean a faster fall once you drop below them, and the cone unloads below the lowest tuning just as it does in a vented box. A high-pass filter is at least as important here as there.

A more complicated cabinet. An extra panel, an extra port, and an internal port whose length has to fit inside the box.

Two things to tune instead of one, which interact. Changing either chamber moves both resonances, because neither chamber is alone once they are joined. That is why the tuning figures shown for each chamber describe that chamber with its own port, taken by itself. Treat them as handles to turn. The finished system resonates somewhere else, and the impedance plot shows where.

Port noise in a place you cannot hear it coming. The internal port can be the faster of the two, and it is inside the box, so watch the port velocity curve rather than listening for it.

A reasonable starting point

Two chambers of similar volume, with the outer port tuned about an octave below the inner one. From there, enlarging the outer chamber pushes the lower tuning down, and shortening either port raises that chamber's tuning.