What is the Reason For This Driver Design (Large Dust Cap)

[Chr1]

OK, thanks.

Just remembered I bought a pair of Kef Cadenzas with passive radiators very cheaply in a charity shop a few years back. Got them for a friend for spares, who has Kefs with similar drivers (B200, T27), but with a better box and crossover. Can't recall the model, but he's definitely a fan of the passive radiator designs.
Anyhoo, I digress... Horses for courses presumably. I don't believe that I have ever seen speakers with passive radiators made for higher SPLs but perhaps they do exist?
Like large voicecoils, I have always assumed that ports are advantageous where high power/volume is required.

Apologies if this is wrong/and/or veering off topic here though.

Edit : He had the Kef 104aB. The passive radiator is the B139.

 

[Salt]

As I said, the rabbit hole. No, the T/S parameters determine nothing, actually. We have (DSP) equalizing today. Switch of paradigm! Many tinkerers need decades to get there ...

No.
DSP can correct and refine, but never will substitute the basics of TSP.

 

[Salt]

No, it is based on the same principle as a vented enclosure and has similar properties. The port and the air inside the tube (which is the mass of this resonator), are replaced by a conventional stiff diaphragm. As this one has a suspension with a quality factor of its own, calculating it is a bit different from a classic vented design with a port, but it does something similar: using the air displacement behind the active diaphragm via resonator to create more SPL outside the box, lower in frequency than what the active driver would reasonably produce.

Main advantages of passive radiators are no loss of volume for internal tubes/ports, and no chuffing/diffraction noise at higher SPL. Disadvantages are higher costs and much more area on the speaker walls which are eaten up by the radiator(s). In very very compact subwoofer designs like battery-powered bluetooth speakers, they are mainly chosen for the fact that a port for such a low tuning frequency would not fit into the enclosure no matter what.

Main advantage of PR is where a BR tube would become too long so transmission losses and mid interference by tube resonances would be predominant.

 

[Arindal]

I don't believe that I have ever seen speakers with passive radiators made for high SPL but perhaps they do exist?

Such surely exist, but I saw this concept mostly in very compact, powerful subwoofers. Sunfire, REL, B&O and Velodyne among many others were making it popular.

DSP can correct and refine, but never will substitute the basics of TSP.

Could not agree more. Seemingly there are more and more active concepts neglecting the traditional way of calculating an enclosure and its resonance frequency, because ´everything can be fixed later in the DSP mix´ (That is how a recording engineer would put it). And I do not recall a single compact system which was fully satisfying in terms of lower bass. Maybe I missed one.

 

[Heinrich]

No.
DSP can correct and refine, but never will substitute the basics of TSP.

Yes we do. Think of the Linkwitz transform back in the day ...

Main advantage of PR is where a BR tube would become too long so transmission losses and mid interference by tube resonances would be predominant.

Wind noises?

Could not agree more. Seemingly there are more and more active concepts neglecting the traditional way ...

You got me.

O/k, on topic. There are good reasons to have a big dustcap, there are less rechnically driven decisions on dustcap size. Driver coil and magnet size are less relevant than secondary features neither seen in the looks nor in T/S parameters. Example given available excursion or a limitation blocking cone excursion beyond the magnet's capabilities, and so much more.

 

[Arindal]

You got me.

I did neither mean you personally nor any DIY concept. Everyone is free to break the rules when creating something for himself or herself. Was rather referring to some professional monitor designs employing a lot of power just to DSP the lower bass lacking SPL due to lack of enclosure volume.

 

[ZZdown]

so is the big dust cap acting as extension of the cone in terms of generating pressure waves (sound)? And if not, is reduced cone area affecting performance?

 

[Arindal]

Anything that is attached to the cone or the voicecoil former and more or less sealed/rigid, will act as a part of the diaphragm, so no loss with dustcaps. What is reducing active diaphragm area, though, is phaseplugs screwed into the static poleplate or alike, as well as tweeters/waveguides of a coaxial design, as they are also not moving with the diaphragm.

 

[a4eaudio]

View attachment 450689

looks interesting, do not know where they are sourced from.

That is a Peerless driver.
$32
1,700 in stock

https://www.digikey.com/en/products/detail/peerless-by-tymphany/GBS-135F25AL02-04/6211111

 

[Sokel]

Same drivers can be done with different size voice coils:

https://audiotechnology.dk/files/pricelist/eur_flex__prices2024.pdf

FLEXUNITS from Audio Technology

audiotechnology.dk

There's nothing creating more impact and weight like a big voice coil at a midbass driver does.
Motor strength is everything for it.

Edit: site

 

[Salt]

Same drivers can be done with different size voice coils:

https://audiotechnology.dk/files/pricelist/eur_flex__prices2024.pdf

FLEXUNITS from Audio Technology

audiotechnology.dk

There's nothing creating more impact and weight like a big voice coil at a midbass driver does.
Motor strength is everything for it.

Edit: site

That's the audiophile DIY source ...
Of course they know how and why they do their job and that very professional

 

[Sokel]

That's the audiophile DIY source ...
Of course they know how and why they do their job and that very professional

The one thing you can't accuse Ejvind Skaaning for is not knowing what he does.
Scan-Speak and Dynaudio are his children

(they can bang, seriously)

 

[Salt]

The one thing you can't accuse Ejvind Skaaning for is not knowing what he does.
Scan-Speak and Dynaudio are his children

(they can bang, seriously)

Just saying ... not?

 

[Sokel]

Just saying ... not?

I'm more of a fun of its old company, so...

(not that the comparable Scan-Speak ones are cheaper though)

 

[somebodyelse]

I found an example of a 5.25" woofer being labelled as ´dome woofer´.

looks interesting, do not know where they are sourced from.

It's a Peerless as @a4eaudio pointed out. It's an unconventional driver design to meet shallow mounting requirements, as can be seen from the rear view.

 

[dfuller]

So is it a rule that bigger dust caps mean bigger voice coil? And are bigger voice coils always advantageous?

Usually but not always.

 

[Arindal]

It's an unconventional driver design to meet shallow mounting requirements,

Thanks for sharing, very unconventional design indeed. The super shallow motor explains why a dome makes sense.

Unfortunately many of these shallow designs are not really useful for home subwoofer application in terms of TSP. Many show comparably high Fs and Qts calling for a bigger enclosure. Xmax capabilities are limiting the use of an electronically boosted sealed box. Seemingly many of them are optimized for automotive application or in flat, huge enclosures like a flatscreen TV or wallmount.

 

[Heinrich]

... not really useful for home subwoofer application in terms of TSP. Many show comparably high Fs and Qts calling for a bigger enclosure. Xmax capabilities are limiting ...

I don't know if I should say that. This assessment is not entirely correct. First of all, the Qts is proportional to the resonance frequency if the diaphragm mass / motor strength remains constant. The Xmax of the driver shown is relatively high.

Obviously, very conservative ideas about speaker crafting are mixed up here with the actual topic. All that can be said on the subject is that the size of the dust cap does not indicate anything to the layman. Sometimes their size or shape seems to lead even experts a little astray.

But that's not necessary if you simply judge the industry's offerings on the basis of the measurements on the finished product. I would like to expressly promote this instead of indulging in half-informed speculation.

Otherwise, the bass section of the KEF R series would also be of outstanding interest - the membranes are half-shells, but curved inwards. And, the surround is tilted by 90°. Yes, what is that, a production error? The data looks very good, actually excellent. Can this be allowed?

 

[Arindal]

I don't know if I should say that. This assessment is not entirely correct.

Of course you should. Speaker design is all about compromise, setting priorities and finding an unusual solution. I might be completely wrong with my assertions, and invite everyone to disagree.

First of all, the Qts is proportional to the resonance frequency if the diaphragm mass / motor strength remains constant. The Xmax of the driver shown is relatively high.

Just looking at the specs, this is accurate. There is nothing wrong with them.

I was merely referring to the practical capacities of such a dome/full-dustcap shallow design. If you try to build a speaker making a lot of SPL below 45Hz, it will need a lot of enclosure volume due to high Qts and Fs, and might fail nevertheless due to limited power handling or max air displacement.

the membranes are half-shells, but curved inwards. And, the surround is tilted by 90°.

What do you mean by 90deg tilted?

Such ´inverted-dome-shaped´ bass diaphragm is usually part of a double-cone concept involving a steep inner cone, mainly for the sake of rigidity.

 

[Heinrich]

I was merely referring to the practical capacities of such a dome/full-dustcap shallow design. If you try to build a speaker making a lot of SPL below 45Hz, it will need a lot of enclosure volume due to high Qts and Fs

... What do you mean by 90deg tilted? ...

This doesn't relate to the original topic, that somehow was lost when again posting the iron rules in speaker DIY.

Please get yourself an R type KEF speaker and investigate how the surround is mounted. It may (partly) mitigate the probs with surround originating intermodulation.

R type woofer, maybe this helps->

On topic, the KEF bass drivers show, that questioning the dustcap size as such isn't reasonable. What counts is the overall design, and finally, what it does in the ready made speaker box one buys, judged by the outcome.

Spoiler { Qts ~ fs ~ sqrt(Vas), the latter determining box volume, while high fs is beneficial for bass reflex, as it mitigates the problem with excursion explosion below tuning }