Why so few speakers with Passive Radiators?

Lots of good information here about the various trade-offs of bass frequency production. I'll add one missing element of high consideration for some designers, which is system efficiency / sensitivity. The hard limit of bass output in a system establishes the overall system efficiency. If you want deeper bass, you must lower the efficiency, etc. etc. If a designer (like Jim Thiel) wants overall higher system efficiency with maximum bass extension, then he chooses a non sealed box configuration of some sort. The passive radiator takes less internal cabinet volume, doesn't chuff, minimizes internal cabinet noise, and generally behaves more predictably than an open vent.

In the particular case of Thiel Coherent Sources, the active woofer must time-align with the midrange along the baffle . . . whereas deep bass benefits from near-the-floor coupling. The radiation plane of the passive radiator can be both time-aligned with the woofer and floor-coupled. By the way, years of work went into optimizing all the elements of the vented enclosure, eventually replacing the port of the O2 and O4 with a double-surround precision passive radiator of our own design and manufacture in the CS 3.6. 

By the way, Jim Thiel (and all of us at Thiel) preferred the active equalizer solution present in the O1, O3, O3a - b, CS3 and CS3.5, which provides sealed-box precision and rolloff characteristics while extracting the needed power curve from the driving amplifier, not to exceed power required for midrange transient peaks. The active equalizer was abandoned not on technical grounds, but on persistent resistance from audiophiles. Thiel's execution was qualitatively different than Bose's, but the audiophile community didn't see that. Also, the cost to do it well was judged by the market as too high. If I were designing (which I am not) a Jim Thiel Tribute Model, it would include sealed box bass with convoluted chamber (some form of transmission line) and an active electronic equalizer. 

Your room is the most critical element of bass enjoyment. Enjoy. 

tomtheil wrote:

• whereas deep bass benefits from near-the-floor coupling.

This is one reason I’ve viewed some of the many boutique speaker platform/feet with a bit of suspicion. Many report sonic change in their speakers upon using these feet, but it seems obvious to me that raising the speaker off the ground is going to change the relationship of the bass with floor coupling and...change the sound. Not to mention raising the mid/tweeters higher in relation to the listener’s ears.  This is changing the, I presume, from the floor relationship the speaker was designed for (placement of woofer height relative to ground, etc).

So it’s not surprising sound will change...but...for the better? Haven’t you changed the relationship the designer created with the floor to something else?

I recently tried some iso-acoustic iso-puck footers under my Thiel speakers and found pretty much those changes - a change in bass and the high end/mids. The bass actually because a bit more "overhang"-like and fluffy, and tightened right back up again when I just put the speakers directly on the floor. (I’m not using spikes).

Though, on the other hand, I’m wondering if speakers are generally voiced with spikes underneath? If so, I suppose as long as new footers don’t alter the height beyond what you’d have with the spikes, it should be a detriment to the sound.

Tom,
I got lost with sealed box and transmission line. How do you have both? Aren't all transmission lines open box?