Foam Plugs

Trelja...Kindly explain what you mean. And. please. no pontification.

Eldartford, "no pontification." Do you have something against us Catholics?

Seriously, though, what I mean is that your statement, "A true "acoustic suspension" speaker system involves a woofer with a free air resonance of about 15 Hz" is incorrect. It really is as simple as that.

AS woofers were not always 12". They were often 8" or 10", with an Fs of 25 - 40 Hz. While certainly not high, not the 15 Hz you stated as being a requirement, either. Fs in and of itself was not what made the driver suited to the task - or not. Instead it was some of the other things you mentioned.

Trelja...I concede that the Fs of 14 or 15 is what I would expect for a 12" or 15" AS driver. 10" or 8" would be higher, which is perhaps why such drivers are less successful as AS drivers.

Note that my point is not that you can't achieve a rational sealed box alignment with a tight driver, but rather that the linear quality of the cone spring is degraded, affecting distortion, about which T/S has little to say.

My affection for acoustic suspension goes back to the AR1. This was a woofer-only box. The better-known AR2 was created by adding a tweeter.

Pontification is not unique to Catholics. In fact it refers to people who are not the holy father talking as if they were.

HSU ported subs currently are double ported. They also come with a single foam plug. You run the sub in 'Max Extension' or 'Max Output' mode, depending on plug insertion or not. There is also a backpanel switch to be set according to the plug insertion condition.
I have never experimented with this, since even in Max Extension mode, it goes plenty loud enough, even in a room which is on the large side for the sub.
The plug is not intended to 'convert' the sub to true AS, but just vary port tuning, as near as I can 'figger.