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Thread: Digging deep, 4th order bandpass subwoofer build

  1. #21
    MartinT Guest

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    Mark - was the Graham Holliman infra-bass design or elements of any use to you?

  2. #22
    Join Date: Aug 2009

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    I'm Martin.

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    I think some of the big B&W use ports and ABRs - 683 is a recent one I think
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  3. #23
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    Quote Originally Posted by MartinT View Post
    Mark - was the Graham Holliman infra-bass design or elements of any use to you?
    I found a huge thread on the subject Martin, did some prying but the jury is out right now. I have more investigations to do yet

    As promised the Blue Peter ABR I'm hoping it's pretty self explainatory, but if you have questions then feel free to ask. I built this back in about 1996 It's 26" along the longest section & yes i made a round 16" innertube fit an oval













    Tuning is simply done by increasing the air pressure in the innertube & or adding more mass - simples
    Bests, Mark



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  4. #24
    MartinT Guest

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    Nice one! Must say, I'd never contemplate trying to make something like an ABR. I would probably try going ported the way of REL's resistive matrix design.

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    The more difficult something is the more i often rise to the challenge I'll look into that Martin, something tells me that i'll probably come back to something like this again though. If i do then they'll be round this time, i had to make it that shape due to the enclosure size & shape.

    Round should be a damn sight easier to!
    Bests, Mark



    "We must believe in free will. We have no choice" Isaac Bashevis Singer

  6. #26
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    Quote Originally Posted by MartinT View Post
    Nice one! Must say, I'd never contemplate trying to make something like an ABR.
    Well i have just discovered something of significance that is forcing my hand to make a couple of these big passive radiators

    That big oval PR has a resonance frequency when mounted to the box of 40Hz. Now i was just giving the thing a good shake when i realised how low the unboxed resonance frequency was, it's approximately 2Hz give or take.

    I just did some simple math & worked out that the PR has a VAS of over 5000L, this might not be significant in itself but it means that in any box the factor governing it's resonance frequency will always be the box & air inside of it. There is virtually zero mechanical losses unlike any other passive radiator i know of, in fact modelling it it behaves in almost precisely the same fashion as a ported enclosure. A standard PR will have significant mechanical losses due to the PR suspension & the whole speaker will suffer because of this

    I'm off hunting for bits
    Bests, Mark



    "We must believe in free will. We have no choice" Isaac Bashevis Singer

  7. #27
    MartinT Guest

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    Is what you are saying that the ABR will flop uselessly due to the effective volume being too large giving it virtually no stiffening?

  8. #28
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    No, the Exodus Audio PR15 which i was going to use has a resonance frequency of 20Hz in free air. When it's put in any box (just like any driver or PR) the resonance frequency will be raised. To get the Exodus PR to resonate at 11.4Hz or so in the enclosure a lot of mass has to be added. The Exodus has a VAS of 245L.

    Now you can see that because the free air resonance is 20Hz with no box that the PRs suspension will have a big effect on proceedings as it's already way above what i want it to be boxed. The big oval PR has a free air resonance at about 2Hz which is way lower than i'm after, which means that the box air stiffness & not the PR suspension will have a vastly greater effect & to all intents & purposes it can be factored out of the equation

    I'll get some graphs up after i have finished buying stuff to do the job I always wondered why the heck that sub with the oval PR sounded so good

    Obviously i'll make these two new ones look a lot prettier
    Bests, Mark



    "We must believe in free will. We have no choice" Isaac Bashevis Singer

  9. #29
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    Interesting.

    The first thing that strikes me is the difficulty of tuning. Given you have enclosed air as part of the suspension as the unit moves the air will heat up, tube expand etc.

    Next, looking at the construction, it looks quite heavy compared to say a paper cone mounted on a traditional paper/pulp spider and that means a larger than practical moment of inertia I would have thought so response speed may suffer and from what I can see it looks difficult to control.
    While you can tune roughly to a certain frequency roughly, in a sealed enclosure at least, ABR’s tend to vibrate within a range of frequencies and careful spider and suspension matching is needed to keep that range useful.
    Because of the weight of what passes as a cone, won’t the unit tend to sink into the lower air tube?

    Also, what sort of pressure wave will you get from a flat surface? Will such a wave propagate in a useful manner?

    500 liters is a bloody big box and for digital media certainly any response below 20 Hz is wasted. An extra couple of Hz can be achieved by careful internal box configuration perhaps by using a pressure wave matrix but those who have tried this out haven’t met with a great deal of success without having a driver that goes down to the same frequency.

    Anyway, I’ll be interested to see how it develops.
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  10. #30
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    Quote Originally Posted by Welder View Post
    Interesting.

    The first thing that strikes me is the difficulty of tuning. Given you have enclosed air as part of the suspension as the unit moves the air will heat up, tube expand etc.
    Easiest thing ever to tune John, get it roughly right with mass & then final adjustment with a bicycle pump
    Quote Originally Posted by Welder View Post
    Next, looking at the construction, it looks quite heavy compared to say a paper cone mounted on a traditional paper/pulp spider and that means a larger than practical moment of inertia I would have thought so response speed may suffer and from what I can see it looks difficult to control.
    While you can tune roughly to a certain frequency roughly, in a sealed enclosure at least, ABR’s tend to vibrate within a range of frequencies and careful spider and suspension matching is needed to keep that range useful.
    Because of the weight of what passes as a cone, won’t the unit tend to sink into the lower air tube?
    Well i worked out that the aluminium weighs in at about 2Kg & considering the size of it that is naff all. Consider that i'd need to add 1.65Kg to one of the PR 15 drivers which has a smaller surface area now which do you think the air in a 300L box would have better control over? A bit more mass & over double the area or a small area & nearly as much mass? The diaphram does not sag at all fella, in fact if i turn it so it's horizontal it only sags a few mm. My guess is that the Exodus PR 15 with 1.65Kg would sag like a 80 year olds boobs
    Quote Originally Posted by Welder View Post
    Also, what sort of pressure wave will you get from a flat surface? Will such a wave propagate in a useful manner?
    As good as a cone chap, there are one or two speakers out there with flat diaphrams (Kef, Technics) & lots of PRs (Celestion, Kef).
    Quote Originally Posted by Welder View Post
    500 liters is a bloody big box and for digital media certainly any response below 20 Hz is wasted. An extra couple of Hz can be achieved by careful internal box configuration perhaps by using a pressure wave matrix but those who have tried this out haven’t met with a great deal of success without having a driver that goes down to the same frequency.

    Anyway, I’ll be interested to see how it develops.
    I have a good deal of music with sub 20Hz high power signals, a good bit sub 15Hz. If you go to hometheatre shack you'll find a list of films with deep bass & a good deal of those have strong signals right the way down to 10Hz I like my movies & my music. This will be a pair of 300L speakers that will happily reproduce 123Db @ 20Hz & 115Db @ 10Hz (if required).
    Bests, Mark



    "We must believe in free will. We have no choice" Isaac Bashevis Singer

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