Question about speakers

[Lawrence001]

Since I bought a pair of Kef 104abs years ago, I've always wondered how passive radiators work, and now I've got a pair Audio Physic Virgos that have them, I want to understand. I initially thought it effectively increased the size of the bass speaker, so basically you want a strong magnet on the active cone, and the pressure inside the speaker drives the passive cone. But then I thought surely the passive is out of phase with the active so there will be cancellation rather than an increase in bass. The air pressure in the speaker should travel at the speed of sound and the distance between the cones is not far enough to mean the back pressure from the active driver is at 180degs at say 50hz once it gets to the passive so it will push it out as the other driver is pulled in by the magnet. Then I read it does the same thing as a port and that got me wondering how ports increase rather than cancel the bass. So who can answer the question, in terms of what the air molecules are actually doing?

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[struth]

Got a set of kef caras that have them. Its basically same as a reflex port, just that you can go deeper and not need a long tube in cabinet. The bass driver probably doesnt have to move wuite so much either. I like mine

[walpurgis]

It's called the Hemholtz effect. The air in the cabinet is excited by the rear radiation from the bass driver cone. At the point to which the speaker's system bass resonance is tuned the air in the cabinet is resonating at its max and this resonance will drive the ABR in phase with the main cone. (think I explained that right :scratch:)

[spendorman]

You are right about cancellation, below a certain frequency, the port (or ABR) will reduce the output of the speaker.

[Lawrence001]

Thanks I'll research the Hemholtz effect

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[Haselsh1]

From 2013 to 2017 I was using KEF Q500's with two ABR's per cabinet and I couldn't get my head around those ABR's. I was also convinced that their output would be completely out of phase with the main driver. You live and learn.

[walpurgis]

From 2013 to 2017 I was using KEF Q500's with two ABR's per cabinet and I couldn't get my head around those ABR's. I was also convinced that their output would be completely out of phase with the main driver. You live and learn.

Have you considered what may happen if you use two ABR's in a cabinet, one mounted directly behind the other? ;)

Or, if you use a regular bass driver as an ABR and put a series capacitor across the terminals?

[Firebottle]

... put a series capacitor across the terminals?

Wouldn't that be a parallel capacitor old man?

[sumday]

I once had a scruffy pair of Kef Kalinda's which some charming children (not mine) had kicked in the ABR'S.
to get them working in a spareroom system I took out the busted units and replaced them with plywood blanks.

to my ears they sounded not half bad...a huge cab for just the 8" B200 to drive.
I did keep my eyes open for a pair of the "driven" B139s with the idea of making a bodged 2 way kalinda but never happened....probably would have sounded ace!!

this was in the days when Klassic Kefs of this era sold for peanuts...the prices charged for the drivers today make me laugh.

NIGE.

[walpurgis]

Wouldn't that be a parallel capacitor old man?

Quite right! :)

[walpurgis]

I once had a scruffy pair of Kef Kalinda's which some charming children (not mine) had kicked in the ABR'S.
to get them working in a spareroom system I took out the busted units and replaced them with plywood blanks.

to my ears they sounded not half bad...a huge cab for just the 8" B200 to drive.
I did keep my eyes open for a pair of the "driven" B139s with the idea of making a bodged 2 way kalinda but never happened....probably would have sounded ace!!

this was in the days when Klassic Kefs of this era sold for peanuts...the prices charged for the drivers today make me laugh.

NIGE.

You could have replaced the ABR with a tuned reflex port in your plywood blank. It would have achieved the same end.

(I have two pairs of those KEF ABR's on a shelf right in front of me at the moment actually)

[Macca]

Ithis was in the days when Klassic Kefs of this era sold for peanuts...the prices charged for the drivers today make me laugh.

NIGE.

Funny you should say that as I just sold a near perfect pair of Calindas for fifty quid.

I know I could have made a lot more by breaking them but that would be criminal. They are now in someone's main system and being used and appreciated every day, hopefully they will run for at least another 40 years.

Comparable modern speakers bought new would cost well over a grand.