What is a Passive Radiator in a Speaker

I am interested in figuring out something about the Vimberg Amea speaker. It has a passive radiator woofer in the rear. I was wondering what that meant. Here is a quote from a the Canadian Vimberg distributor.
The woofer at the back is actually a passive radiator. It’s using the air pressure inside the cabinet to move so you can put it close to the wall without any problem.
Anyone care to chime in with an explanation of a passive radiator woofer. What is the benefits and negatives of this design. What are other speakers using such a design? I was under the impression Thiel used this design.
Most of the early Polk speakers used a passive woofer.  This design extends the bass response and you don't have to worry about port chuffing.

The only negative I can think of is that the speaker needs to be airtight.  If someone messed with the gaskets or was sloppy putting the speaker back together it will effect the bass.   You can easily test for this by gently pushing in on the passive and hold it in.   The drivers should move out and take a few seconds (the longer the better) to return to normal.   If they push out but immediately start to go back in you have a leak somewhere.
Thiel has used this design, as has/does Klipsch.  No doubt others.  I've always thought of it as a kind of half-way compromise between sealed box and ported/vented.  Achieving in theory a sort of half-way mix of the advantages and disadvantages of both of those designs.
Owned several pairs of the original Polk 10 Monitors and thought they were the bees knees, great bottom end driven by my GAS Son Of Ampzilla, thought I was in heaven. Didn't know that about pushing the cone in and watching for the drivers to reset, that is interesting to know and makes a lot of sence.
Ohm delivered various great sounding box design speakers that used passive radiators like the H and CAM 42 models for many years. The H was top of the line box design and my favorite back in the day.

I have a pair of brand new Vanatoo Transparent 1 Encore speakers that leverage passive radiators among other technology tricks to get a lot of good bass out of a very small box and these sound amazing especially for the size.

I also have a Klipsch sw308 sub I use with KEF ls50s that uses a forward firing 8" main driver and two side firing (one each way) 8" passive radiators to deliver a lot of great bass again out of a smaller box. Love it!

So as you can tell I am a long time passive radiator fan. More speakers should use them.
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