Passive radiators

Docfoster

Have noticed these on a number of older speakers from the 70s.
Basically they look like a woofer but don't have a voicecoil or any electrical attachment. They just move in sympathy with the pressure generated within the cabinet by the actual drivers.
Never heard one tho. Anyone out there have any direct experience...?

PAC

They're an alternative to ported speakers and used to be very common place (eg Celestion loudspeakers).  I've never been convinced by those I've heard but then again. Ditton 15s were incredibly dull things anyway IME.  There's good and not so good just like everything else.  They operate just the same way as a the plug of air does in a port but have advantages.  They work by moving with the internal pressure changes in a sealed cabinet caused by lower woofer frequencies.

This movement take some of the energy from the woofer, limiting driving woofer excursion thus limiting both distortion and excessive woofer movement, protecting the woofer, increasing bass output and smoothing LF response.  The type/size needs to be calculated as for a port but positioning in the cabinet is less critical plus you can get away with a shallower (smaller) cabinet than with some ported designs.  Downsides are great expense and with age, larger passive radiators change their output characteristics slightly (unlike a port which doesn't) as suspension softens, plus many require refoaming just like woofers if using foam surrounds.

Docfoster

Paul, thanks for that.
Really appreciate the detailed response.
Hadn't equated them with ports before, but that makes perfect sense...

Yomanze

My problem with passive radiators is the inevitable phase shift that occurs i.e. the woofer is pulling in when the radiator is pushing out, so you will get 180 deg phase shifts unless am wrong on that side & someone can correct me?

uglymusic

Those I've heard seem to have a stodgy lumbering sort of bass.

Docfoster

My problem with passive radiators is the inevitable phase shift that occurs i.e. the woofer is pulling in when the radiator is pushing out, so you will get 180 deg phase shifts unless am wrong on that side & someone can correct me?

To my mind that sounds right.
It can't be otherwise.
Can it?
I certainly can't see how they could move in concert with the drivers...

PAC

Whoa there chaps!  Not exactly. 

There is phase shift across the working spectrum of the woofer but at the tuning frequency, the output is in phase with the woofer in terms of output, the woofer's excursion being limited to practically nil with most of the bass coming from the PR (or port).  The problem with the PR is that it takes a little more time to get it going than the mass of air in a port due to inertia, and a little more time to slow it down hence it can, if not carefully selected sound lagging with a little overhang.  The real benefit though is that you can extend bass much more in a small enclosure using a PR than a port because to match it's output, the mass of air in a port would demand quite a long port, longer in fact than practicable for a small cabinet, hence why many designs of yesteryear used them in compact cabinets.

Look at the phase response from either a port or a PR.  Cabinets that use this tuning tend to have quite a high "Q" at resonance meaning that their operation is like a bandpass function, ie they are easy to get going at resonance but fall off quickly above and below, perhaps only putting out any meaningful output for one half octave above and below.  Therefor the output from port or PR is the dominant driver where they are operating at resonance, but there is phase shift above and below, rapidly falling off to inaudible levels. 

Above or below that bandpass function, the cabinet behaves more like a sealed box and the "Q" reduces.  Ports do not "leak" significant output above the bandpass function and PR's do not output anything above the bandpass function (or below it).  The main driver does the work there.

The way to think about it is that both a PR and a port are "system" phase inverters, so that at resonance, there is no cancellation with the woofer (if there was, we would have zero output at those frequencies from the "system"), rather the PR or the port are the primary bass drivers.

If the system was tuned, say, to 40Hz, the bass output would be inverted 180 degrees at that frequency but as there are no meaningful relative phase issues, we don't perceive the sound as being out of phase.  We are not susceptible to detection of absolute phase, just relative phase.  By say 100Hz, the port or PR would be putting out very little in comparison with the main driver so whilst the phase curve shows a phase shift, it becomes largely inaudible.

It's only when the bass driver starts to become the dominant output driver that we could detect any phase issues but as the PR or port output drops so quickly, that is rarely the case.

To my mind the advantages of PR's are outweighed by their disadvantages of inertial delay unless carefully designed.

uglymusic

...

To my mind the advantages of PR's are outweighed by their disadvantages of inertial delay unless carefully designed.

I think that's the technical rationale for my subjective observations.

Docfoster

To my mind the advantages of PR's are outweighed by their disadvantages of inertial delay unless carefully designed.

Surely you can't ignore that self-challenge?

prevster

Well i have owned both KEF 104ab and Calinda. And i thought they worked very well indeed. Better than ported. But i am biased towards KEF of yesteryear.

Docfoster

Thank you Prevster. :-)
Having never knowingly heard such a design myself, your exprerience is most welcome.
I appear to have moved away from ported to older sealed designs too, tho haven't heard yours.

cj66

I used to have some Gale GS302. They were not a popular design but I loved them! I'd buy some again if I could find them, although they'd definitely need refoaming by now.

Docfoster

They look of good size. :-)