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Thread: Question about speakers

  1. #1
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 917
    I'm Lawrence.

    Default Question about speakers

    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?

    Sent from my NEM-L51 using Tapatalk

  2. #2
    Join Date: Feb 2013

    Location: W Lothian

    Posts: 36,608
    I'm Grant.

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    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
    Regards,
    Grant ....

    I've said it before and I'll say it again: democracy simply-doesn't-work
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  3. #3
    Join Date: Apr 2012

    Location: Southall, West London

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    I'm Geoff.

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    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 )
    Mr. Tact!

    Main system: MMs/ADCs/Low output MC's/One rare Japanese SUT/One scarce British phono stage/various tonearms/hefty Japanese DD TT and hefty Japanese BD TT and small British BD TT. 4 CD players/2 jitter buster/2 DACs/Valve buffer. TVC stepped attenuator or valve pre-amp or solid state pre-amp. Current dumping power-amp or either of two Class A SS power-amp or Class A EL34 valve monos or big Japanese (part Class A) integrated. Big dual concentric speakers/Smaller dual concentric speakers/Two way British compacts and full range speakers, amongst others. And too much more to list!

  4. #4
    Join Date: Feb 2008

    Location: Middlesex, UK

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    I'm Alex.

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

  5. #5
    Join Date: Jun 2015

    Location: London/Durham

    Posts: 917
    I'm Lawrence.

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    Thanks I'll research the Hemholtz effect

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  6. #6
    Join Date: Dec 2008

    Location: Lincolnshire, Home by the Sea

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    I'm Shaun.

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    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.
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  7. #7
    Join Date: Apr 2012

    Location: Southall, West London

    Posts: 28,572
    I'm Geoff.

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    Quote Originally Posted by Haselsh1 View Post
    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?
    Mr. Tact!

    Main system: MMs/ADCs/Low output MC's/One rare Japanese SUT/One scarce British phono stage/various tonearms/hefty Japanese DD TT and hefty Japanese BD TT and small British BD TT. 4 CD players/2 jitter buster/2 DACs/Valve buffer. TVC stepped attenuator or valve pre-amp or solid state pre-amp. Current dumping power-amp or either of two Class A SS power-amp or Class A EL34 valve monos or big Japanese (part Class A) integrated. Big dual concentric speakers/Smaller dual concentric speakers/Two way British compacts and full range speakers, amongst others. And too much more to list!

  8. #8
    Join Date: Oct 2012

    Location: The Black Country

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    I'm Alan.

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    Quote Originally Posted by walpurgis View Post
    ... put a series capacitor across the terminals?
    Wouldn't that be a parallel capacitor old man?

  9. #9
    Join Date: Mar 2013

    Location: nottingham

    Posts: 218
    I'm nigel.

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    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.

  10. #10
    Join Date: Apr 2012

    Location: Southall, West London

    Posts: 28,572
    I'm Geoff.

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    Quote Originally Posted by Firebottle View Post
    Wouldn't that be a parallel capacitor old man?
    Quite right!
    Mr. Tact!

    Main system: MMs/ADCs/Low output MC's/One rare Japanese SUT/One scarce British phono stage/various tonearms/hefty Japanese DD TT and hefty Japanese BD TT and small British BD TT. 4 CD players/2 jitter buster/2 DACs/Valve buffer. TVC stepped attenuator or valve pre-amp or solid state pre-amp. Current dumping power-amp or either of two Class A SS power-amp or Class A EL34 valve monos or big Japanese (part Class A) integrated. Big dual concentric speakers/Smaller dual concentric speakers/Two way British compacts and full range speakers, amongst others. And too much more to list!

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