Passive radiator: a port with a suspension

A passive radiator, sometimes called a drone cone, is a driver with no motor: a cone, a surround and a spider, and nothing to drive them. It sits in the wall of the box and is moved by the pressure the woofer creates inside. It does the same job as a port.

The difference is what the two are made of. A port is a plug of air, so it is a mass and nothing else. A radiator is a mass on a spring, because its suspension pulls it back to center. That one extra spring is the whole of the difference, and everything below follows from it.

Why you would use one

A port needs length. Tuning a small box low means a tube so long it will not fit, or so narrow it chuffs. A radiator tunes by mass: to go lower you add weight to the cone, and weight takes no space. That is why small subwoofers and bookshelf speakers use them and why the radiator is often on the side or the bottom.

You also get no port noise, because there is no moving air, and no pipe resonance part-way up the band.

What the spring costs you

A notch. At the radiator's own free-air resonance, fp, the suspension and the moving mass resonate against each other and the radiator stops contributing. The response drops sharply at that one frequency. A port has nothing like it.

fp sits below the passband, so the notch normally falls where there is little music. It is visible on the plot as a narrow spike downward, and it is real: it is not a simulation artefact.

A steeper rolloff. Below the notch the output falls away faster than a vented box of the same tuning, closer to fifth order than fourth. This is mostly harmless and slightly helpful, since it means less cone motion on subsonic rubbish.

Tuning

The box air and the radiator's suspension are two springs in series, so the system always tunes above the radiator's own resonance:

f_b = f_p\sqrt{1 + \frac{V_{ap}}{V_b}}

Vap is the radiator's compliance expressed as a volume of air, the same idea as Vas for a driver. A stiff radiator, small Vap, drags the tuning well above fp. A floppy one, large Vap, tunes close to fp and behaves most like a port. Floppy is usually what you want, which is why radiators are built with soft surrounds.

To lower the tuning, add mass. Doubling the moving mass drops fb by a factor of √2, the same as for any mass on a spring.

The limit that actually binds

The radiator moves further than the woofer does. Near tuning the woofer barely moves, because the box is doing the work, and it is the radiator taking up that motion. A radiator is normally specified with at least twice the driver's linear travel for exactly this reason.

So the excursion curve to watch is the radiator's, not the cone's, and Baffle plots it separately. A radiator that hits its limit makes the same ugly noises a driver does, and it will do so while the woofer's own excursion plot still looks comfortable.

Choosing one

• Match the displacement, not the diameter. Area times travel is what matters. A small radiator with long travel can replace a large one.
• Aim for a low fp and a large Vap. Both keep the notch out of the way and the tuning close to the radiator's natural behavior.
• One radiator is not a rule. Two smaller ones share the motion and each moves less.