Why so few speakers with Passive Radiators?


What are your thoughts on Passive Radiators in speaker design?

I've had many different speakers (and like many here, have heard countless varieties outside my home), from ported, to sealed, to passive radiator, to transmission line.

In my experience by far the best bass has come from the Thiels I've owned - CS6, 3.7, 2.7 which use passive radiators.  The bass in these designs are punchy yet as tonally controlled, or more, than any other speaker design I've heard.  So I figure the choice of a passive radiator must be involved somehow, and it makes me wonder why more speaker designers don't use this method.  It seems to give some of both worlds: extended bass, no port noise, tonally correct.

And yet, it seems a relatively rare design choice for speaker manufacturers.

Everything is a trade-off in speaker design. If there was one "right" way, all speakers would share that design! Each designer weighs the trade-offs to optimize what they think and feel sounds best.

Sealed boxes tend to give the best definition and resolution but at the expense of extension and ability to play loudly. Ported designs, including Thiel's radiators give better extension but at the expense of low bass timing being a bit slow.

One "good" source of info is the Audio Perfectionist Journal, archived on the Vandersteen website (under Resources, Richard Hardesty memorial). In particular, I suggest Volume 3 for trade-offs of vented and sealed enclosures. (I would add hot links but Audiogon's link feature does not work for me).

Thanks, I’m aware that everything is trade offs, but the passive radiator seems such a successful trade off I’m intrigued that it’s not used more, vs the much more common trade off of, say, ported.

Having owned and listened to many sealed designs, I don’t find the passive radiated Thiel bass remotely describable as "slow." It’s completely the opposite.  I've searched in vain for other speakers with as tight, tonally controlled bass as I hear from this passive radiator design.
I agree that it is a "successful" trade-off and I am plenty happy with the bass performance of my CS2.4SE. But I think you need to directly compare sealed vs. vented (including passive radiator) designs to fully understand the what the trade-offs are. Most designers have much more experience with this than me and, presumably, you.

In the case of Thiel, it could be instructive to compare the bass performance of the sealed box CS3.5 with the vented CS3.6 (I've never heard either model). I have no idea what Thiel's rationale for the change was but, clearly, he thought the trade-off was worth it. The dearth of other models with passive radiators suggests other designers hear it differently.
beetlemania the 3.6 is not vented unless you consider the passive radiator as the vent.  It is a sealed enclosure.
I consider the passive radiator a special case of a vented design:

1 Sealed enclosures
2 Vented enclosures
    2a Ported
    2b Passive radiator

Keep in mind I'm just a bozo typing opinions on a forum, not an expert!
Hey man…all bozos are welcome on THIS bus…Well designed passive radiators do fall into the "venting" category although you'd think they imply a sealed box, but the passive radiator is sort of the "unsealed" part part of the thing. I own a discontinued Mackie HR120 sub that was designed for home use (not the "drag it around to gigs" P.A. stuff), and it has a 12" passive on it, unlike the pro stuff I use in some live sound situations that have a vented/ported design (and generally 1000 watts into an 18" driver)…both work well, but the HR120 with a 500 watt A/B amp driving a very beefy downloaded EAW 12" (92 lbs…you just don't want to move it) is flat to 19HZ. REL does well with their passive radiating little subs (my 2 RELs don't have those, but work anyway), as do many others of course, and it's simply another flavor…personally I prefer raw drivers suspended from fishing wire 2 feet from my head, but that's just me. A bozo.
Wait just a minute, you may indeed be a bozo but not as big a bozo as me.  I've only been at this umpteen years and I don't know vented from sealed.....I occupy the front seat on the bus!
I suspect the reason is mostly that ports are cheaper/easier.  I only know of the below three factors.

passive radiator
 - no port noise
 - less sound escapes through the diaphragm than would through a port

 - higher efficiency
@jon_5912 that sounds right. From Richard Hardesty in APJ Vol. 3:
Vented subwoofers offer inferior performance compared to sealed enclosure subwoofers of equivalent quality in the following areas: transient response, phase response, group delay,and low frequency extension. You might ask why anyone would choose to make a vented subwoofer. There is one very good reason—high output. The one advantage that vented enclosure designs offer is a reduction in cone excursion at low frequencies.Reduced cone excursion allows vented designs to play louder. A side benefit is that reduced excursion will also produce lower steady-state harmonic distortion measurements.
and Hardesty in his review of the Thiel CS6 in APJ Vol. 8:
They have vented bass loading utilizing passive radiators rather than ports. Alignment is unusual and bass is tightly controlled with little evidence that the enclosures are not sealed.
So, Hardesty was a big fan of sealed enclosures (and even bigger fan of Vandersteens) yet he implied that the Thiel's passive radiator sounded not too different from that of a sealed enclosure. In turn, this implies that Thiel's passive radiator is a "good" trade-off between sealed and ported enclosures. At least that's *this* bozo's take  :^)
I have a pair of Golden Ear Triiton Reference speakers, each of which have four passive (infrasonic) radiators. Suffice it to say the bass is incredible, IMO. Authoritative, deep/low, and so many other accolades.

I used to own the original Sonus Faber Grand Piano which had a passive front radiator. A small floorstander that was pretty easy to drive but always seemed a bit bass shy to me. Never really fell in love with it and sold it before to long. I know that adds nothing to the conversation and I am at the very back of the bus.
Mainly seen in horn drivers, big woofers Klipsch comes to mind .
Not very accurate that is why most don't use them .

I am almost always impressed by speakers that make good use of passive radiators compared to the competion.

I picked up an 8"front firing Klipsch sub with two side firing passives recently. Was originally looking at much bigger bulkier subs but this little thing competes with the much bigger boys in every way.

I vote for more passive radiators!
A passive radiator is expensive compared to a tube.
It could be a reason why it is rarely used.

With a passive radiator you don't have port noise, but it's not true that there aren't sound at mid frequency that go out from passive radiator.
At mid frequency, passive radiator cone interacts with a woofer cone.
If you use a low pass filter at low frequency you will not have any problems.

With a passive radiator or port you can choose damping of your system and you can get the same result in the listen experience...with an ideal passive radiator!

I don't like passive radiator due to non linearity of the suspension.
Speakers is a really complex non linear system, if we add another non linear component you can loose some performace.

I'm on the bus, too. My Vandersteen 2Cs are a 3-way, with an acoustic coupler on the back of the speaker. Are they considered acoustic suspension? Ported? Sealed?
Thanks for a response, and thanks Prof for this post>

Systems utilizing passive radiators ARE, "vented" systems. The Thiele-Small parameters and calculations, are identical for both. Only the application(vent or radiator) of the results(mass of port air/size and weight of the radiator), varies. https://itstillworks.com/size-speaker-enclosure-passive-radiators-7918995.html   "Tighter" bass can be realized, by adding weight to the radiator(s).
Sorry, forgot to ask is an acoustic coupler the same as a passive radiator?

The 2Ce and 3A use sealed enclosures (I think the Treo has a downward firing port). From Tom Norton's 3A review in stereophile:
A cutaway photo reveals four drivers and three separate sub-enclosures—the bottom of which is the largest, for the rear-facing 10" driver that Vandersteen refers to as an "Active Acoustic Coupler." It is, as the name implies, actively driven [though it shares some of the characteristics of a passive radiator—Ed.]. Since the AAC is designed to cover primarily the range below 35Hz, I would be inclined to call it an integral subwoofer; but perhaps Vandersteen chose to give it another designation to distinguish it from the company's dedicated, outboard subwoofers. 
I think the crossover is designed as a 3-way, so not sure how this thing works - and I ran 2Ce Sig IIs for 10 years! Maybe the filter for the woofer has a sub-network for the AAC?
Hi guys,   Well,  a lot great info, a few minor inaccuracy's... Even the Richard Hardesty post is more opinion and not cast in stone. 
Yes,  A passive radiator is more of a vented or ported design.  When we do a ported speaker,  Lengthening the port lowers the frequency of air moving through the port so changes the frequency the woofer peaks at.  Where Richard was not quite accurate in his description is that it is completely feasible to port/vent a driver to be very smooth and accurate bass, it is then a function of a well damped amplifier to control it. There are many ported speakers out there that are great. A passive radiator works similarly in that as you add mass to the radiator,  it will like a port, change frequency where there is a peak or dip and by how much. Every driver does change spec as it is used over the years and fact is,  a sealed box comes much closer in keeping its original sound as it wears and a ported box is the same.   Oddly enough in my own speakers at home right now, I have 3 pair built. A 10 inch 3 way in a sealed box,  an 7 inch mtm in a ported box and a 12 inch 2 way (big Heil amt) in a passive radiator box.  All 3 are flat, accurate and fast with the right equipment in front of them.  
@beetlemania, thank you!
In the technical section of his Rythmik Audio website, designer Peter Ding makes his case against passive radiators in subwoofers.
In the late 1970s (1977) I had a pair of Koss passive radiator loudspeakers.  The radiator (thick plastic passive cone) had a weight one could add to further dampen the radiator.  I drove them with a Dynaco St-150.  They had the fastest bass attack/transient response I have ever heard.  Traded them for a dining room set and purchased Genesis (old east coast sound) passive radiator floor standing speakers.  I moved on to sub/satellite systems but I fondly recall the Koss.  The cabinets were real oak and the tweeter was a plastic thing in a rectangle faceplate.  EV and Polk also made passive radiator designs in the 1970s-1990s.  They take up a lot of vertical space (unless the radiator is mounted in the back).  
a passive radiator cost much more then a port
A passive radiator speaker may sound slow with a tube amplifier that has a low damping factor. Sealed speakers tightens up the bass by default. 

But, with an amplifier having a good damping factor? A passive radiated speaker can sound as tight as sealed,  but having more dynamics than a sealed can allow for at higher volumes.
As I look back on various speakers I've had over the years, the very best sound, particularly in lower registers, came from sealed designs. This was true with big living room speaker systems; and it's true in the last 2-3 powered monitor set-ups on my desktop/nearfield system. It's also true with subs, where I find sealed to sound best.

Yes, this assessment is 100% subjective; I've done no testing apart from whatever my ears reveal to me.

Currently in my desktop setup, I've just purchased a high-quality 2-way speaker, a sealed design, said to have particularly bass for a speaker this size (ATC SCM12 Pro). That, plus a class D amplifier recently purchased (strongly influenced by positive reviews/comments regarding its musicality) will be my first amp+passives experience on the desktop. I have high hopes. In the case of the ATC, not only do I expect cleaner/clearer/tighter bass than w/previous ported & powered monitors--but hope to also get some of the renowned ATC clarity & detail in the mids & highs (said to be accurate by not bright).

I was relatively interested in several powered studio monitors that feature passive radiators, but feel I might do better with audiophile passive designs (this ATC can be considered a hybrid--it's the professional version of their very similar "hifi" model, the SCM11 v2, but reviewers usually comment on its "musicality" (an extreme rarity in the pro world to see that word).

Wish me luck!
My first pair of what I consider to be good speakers were Klipsch kg 4.2. They have a 10 inch woofer and a 10 inch passive radiator and no port. They blew away my old pioneer 16 inch mush-woffer speakers with deep, tight, and powerful bass. They are definitely heavy on the bass.

Excellent information as above.

I auditioned Vandersteen 2ce and 2ci/signature models starting way back in 1995/96 to 2012. Not a bad speaker at all in either design.  In 2012, I had my first taste of Thiel CS 2.4-  much richer, tonal and  timbre to my ears. In 2013, I auditioned the CS 2.4SE and stopped looking.

Happy Listening!

In each brand the passive radiator (Thiel) and acoustic coupler (Vandy) really adds flavor to the musical presentation, IMO.  I feel Thiel has the better energy.

Happy Listening!

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.
I got lost with sealed box and transmission line. How do you have both? Aren't all transmission lines open box? 
Do you mean a sealed box, equalized within a transmission line? I guess that would make sense?
This would be only for the very, very low frequencies I assume.
I have always liked transmission line bass. Very relaxed and effortless.
I have had virtually every flavor on the market, since 1956. Planar, electrostatic, loaded, ported, open air. It is my feeling that, while the loaded are power shy I find the ported boxes offer some Chuffing. I find this disconcerting. Currently I am happy with the 3 individual enclosures per side, 2-6 inch Scan speaks per box, for a total of 6 small woofers per side to be quite rewarding. Quick, powerful,authoritative and natural. No lumbering giant cone, no vent. I may stay with this configuration forever.
Speaking of the east coast sound pat5jfet referred to, I have a pair of EPI 200C's I picked up for cheap a few years ago from the original owner that bought them in the 70's, like new grills and cabinets. I can't remember the details of the history, but I recall Genesis factored into the lineage somewhere. A fellow named Huh Powell was once associated with one or both of the companies and still provides parts and service for the brands in New Hampshire. I bought updated jack panels/crossover caps for the 200C's couple years ago. With a big passive radiator, 12" IIRC, and the inverted dome tweeter they are very nice sounding and doing active duty in my third system now.

I agree, it seems like a very good way to deal with the air, but sometimes it makes no sense to me.

The KEF Q series floorstanders for instance seem to me to be a complete engineering disaster.  You have two chambers, one on top that holds the uni-q array (tweeter and midrange) and one passive radiator, then the bottom chamber holds an active woofer and passive radiator.

SO..  When the active woofer is moving OUTWARD the passive radiator is moving INWARD.  180 Degress out of phase.  How can you deliver a clean wave to your listener when BOTH the top and bottom passive radiators are 180 degrees out of phase??? They should have been place on the back, or bottom IMO.
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Legend- With a little research, you’ll find that both a properly designed/implemented radiator, and the mass of air in a vent, act as resonators and reinforce the system's TUNED bass response.  Below the tuning point, yes- cancellation occurs(and is irrelevant).  That understanding should remove the why and how. Third subject, here: http://audiojudgement.com/bass-reflex-speaker-design/ Then: http://www.accuton.com/en/technology/passive-radiator/

It seems to me that if one had a cabinet big enough for the size of many of the passive radiators used, why not just add a cross-over and add another woofer? Or just use a larger woofer to start with? Sure there would be some added expense, but I'd much rather have a proper sealed box than any ported design, passive radiator or not.

With DSP and digital EQ having made such strides recently, I wonder why someone has resurrected the eq'd sealed box designs  like the earlier Thiel's up to the 3.5's with active eq bass boosting compensating for the natural roll off? With updated DSP and EQ one could adjust the bass so that not only in anechoic chambers but one's actual room, the most troubling frequency range, the bass, could be optimized.

 True transmission lines are ginormous.

You wouldn't use them unless you need to use them, typically in smaller speakers to increase bass response. If you are designing largish floorstanders with large multiple woofers...why'd you need to use em?
^What specifically are you referring to?
Plenty of modern stuff:

Legacy Audio Signature SE and higher level Legacy speakers
Endeavor Audio E-3 MK2
Magico various speakers
Focal Sopra 2, 3

These are all ported or sealed twin woofer designs that go very deep, there is no need for passive radiators.
^I’d more often than not have a passive radiator than a port.
I should have posted this link in my 01-25-2018 5:24 post instead of or in addition to the link to the wiki article: