Isobaric: two drivers, half the box

Mount two identical drivers one behind the other, with a small sealed pocket of air between them, and they move as a single cone. Either cone to cone with the pair wired out of phase, or cone behind magnet wired in phase. Both work; the first is more common because the two suspensions then flex in opposite directions and cancel some of each other's distortion.

The air trapped between the cones does nothing acoustically. It only couples them, which is where the name comes from: the pressure between them stays constant.

What it does

The pair moves twice the mass on twice the suspension, and presents one cone's worth of area to the room, because only the outer cone faces it.

The consequence is that Vas halves while Fs, Qts and Xmax stay exactly where they were. Half the Vas means half the box for the same alignment: the same Qtc, the same F3, the same shape of curve, in a cabinet half the size.

That is the entire attraction. If a driver wants 40 liters, an isobaric pair of them wants 20.

What it costs

Two drivers to get one driver's output. Efficiency halves, which is 3 dB.

Where that 3 dB goes depends on how you wire the pair, and this is the part people get wrong.

• Parallel. The pair plays exactly as loud as a single driver on the same amplifier, and halves the impedance doing it. You have not lost 3 dB of loudness; you are drawing twice the current to get the same loudness. Check the impedance curve before committing, because a nominal 8 ohm driver pair becomes a nominal 4 ohm load.
• Series. The impedance doubles instead, and at the same voltage the pair is 6 dB quieter than one driver. The amplifier is working easily; it is simply not being asked for enough.

Either way you have bought cabinet volume with amplifier power, plus the cost of a second driver that produces no extra output.

When it is worth it

When the cabinet is the binding constraint and nothing else is: a subwoofer that has to fit under a seat, or a speaker whose shape is fixed before its acoustics are. Also where you already own two of a driver and cannot get a third.

It is rarely the cheapest route to a given response. Two drivers side by side in a bigger box give you 6 dB more output for the same money. Isobaric buys space, and only space.