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Why so few speakers with Passive Radiators?

Folks,

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.

Thoughts?
prof

Showing 5 responses by beetlemania

beetlemania
841 posts
 
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
841 posts
 
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).
beetlemania
841 posts
 
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!
beetlemania
841 posts
 
@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  :^)
beetlemania
841 posts
 
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?
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