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Old 30th March 2010, 07:47 PM   #1
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Default Open Baffle with Aperiodic bass helper.

Hi all,
I have some nice vintage Isophon drivers and tweeters which I have mounted in some BIB cabs. They sound really nice and sweet, but I have always been a bit disappointed with the bass tightness, and they have always sounded a tad ragged on the upper midrange.
I have been toying with the idea of building a Pair of reverse tapered TL enclosures as a way of improving the bass. I have also toyed with the idea of mounting baffles to the corner of the room.
I came across this article which uses exactly the Isophon drivers I have (though the isophon tweeters are slightly different).

Alnico Saba Speakers Endorphine P18

I have a pair of PA drivers made by Tannoy which are 12" and have enormous alnico magnets which are the same as those used on their dual concentrics. The Tannoys have a really nice tight bass in the TL enclosures they currently occupy.
So my idea was to build the P18 open baffles, but since the Tannoys are unlikely to have enough kick mounted on a pure OB - why not box them in and aperiodically load them with a super large resistive vent. This should greatly unload them but boost the bass response enough to match the 98db Isophons.

My question is, will this work and what are the likely penalties for going from OB to aperiodic. Has anyone else attempted a similar project? Also what am I likely to gain in going from BIB to OB?

All input appreciated.

Shoog

Last edited by Shoog; 30th March 2010 at 07:50 PM.
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Old 2nd April 2010, 01:25 AM   #2
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Are you looking to augment the BIBs or make something completely different using the drivers you have?
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Old 2nd April 2010, 07:44 AM   #3
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The BIB will go into retirement.
I'm happy with the BIB but the one thing they don't do is PA style tight bass, and since I listen to a b it of dance I want that deep tight bass. Trying to add in a sub to the BIB would introduce its own interference problems since the BIB go very low, just not tight.

Shoog
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Old 2nd April 2010, 07:46 AM   #4
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I can't think of any real reason it shouldn't work; monopole LF with dipole mids is often a good way to go -look at Basszilla (albeit that's a BR box rather than aperiodic for the bass driver). I suspect it's probably a better way to employ your drivers than the current BIBs -assuming the Isophons are like most vintage units, they typically need a huge Vb to work optimally in, & you may not have sufficient (?).

As for what you may 'gain' from going from a tapped horn like the BIB to dipole, it's going to be more a case of 'different' rather than 'better' (unless, as noted, your boxes as-is are too small for the drivers, which might be choking the LF gain). Generally you need to position them differently & you won't have any potential box colourations, although you may have some room reflections off, say, a front wall etc. to deal with instead. YMMV.
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Old 2nd April 2010, 09:17 AM   #5
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The BIB boxes are 6ft tall and 18inch front to back, so I suspect that they are adequate for the Isophons. As I said the sound I am getting at the moment is very good, good enough to be the best of about 6 different designs I have tried over the last 7yrs. Its really a case of can I get that realistic bass kick that just doesn't come with a cab that generates almost all its bass from the back wave. What i want is PA bass kick which is what you get with open baffle bass drivers. Aperiodic seems a good compromise.
I once build a little triangular aperiodic box with 10" drivers. They sounded surprisingly good for the quality of driver and small size of the cabs.

Shoog
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Old 2nd April 2010, 11:35 AM   #6
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Quote:
The BIB boxes are 6ft tall and 18inch front to back, so I suspect that they are adequate for the Isophons.
Possibly; then again a lot of 'vintage' units tended to have a high Vas & mostest Q, mandating enormous box volumes. The cab. might also be tuned too low; most of the LF slam-factor is in the ~60 - 120Hz region, give or take a few Hz, so depending on goals, it can sometimes be worth focusing on this at the expense of ultimate LF extension.

Quote:
Its really a case of can I get that realistic bass kick that just doesn't come with a cab that generates almost all its bass from the back wave. What i want is PA bass kick which is what you get with open baffle bass drivers. Aperiodic seems a good compromise.
Sure, and see above WRT the PA style bass kick, respectively. Most PA systems don't actually go that low, it's where they have the grunt & the amount of air that's shifted that matters. A 15in HE woofer per channel, either sealed or aperiodic to make it an easier amplifier load should give you that, with a suitable Qtc.
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Old 3rd April 2010, 02:13 PM   #7
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So i've been thinking about this a bit more. I have come up with the idea of using the existing TL cabs which are housing the big Tannoys, just take off the front baffle with the tweeters and use them with a 2nd order crossover. The cabs are tuned down to about 40hz at the moment with minimal stuffing and the bass is what I am looking for already. TL and Aperiodic both do roughly similar things so I should get similar results.
All I then have to do is give them a lick of paint and mount the new OB onto the front of the cab. Bobs your uncle. This option saves the redundant PA cabs and the work of building new boxes, as well as the cost.

What do you think.

Shoog
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Old 25th July 2010, 12:43 PM   #8
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So all,
I finally had a few days to experiment with my idea. I decided to use the little pyramid shaped aperiodic box to form the bass section. A simple 2nd order crossover at about 400hz. The baffle is 3 1/2 ft tall and 16inchs wide. I took the whole circular baffle off the front of the BIB cabs and mounted it across the new open baffle. Took a few days to get them adequately crossed together. I used a nice Isophon 9inch bass driver for the aperiodic cabs.

Sound wise its a heck of a lot cleaner sounding than the BIB, and it has revealed that there was a significant upper bass suck out with the BIB cabs. As suggested - they were tuned a bit low for optimum bass performance. I had them set up as MLBIB as I always felt that the comb effect was to sever otherwise.
When the aperiodic boxes were first tried they seemed very one note and this would have been underdamping at the 60hz resonance. I stuffed the vent a lot more and this smoothed out and the bass which is now adequately extended below 60hz (as the aperiodic approach predicts it should). Bass on dance music now has much tighter kick. Integration is not perfect yet as on some spoken word there seems to be a bit of hollow boxyness.

I listened to Roksopp and enjoyed it for the first time with the new speakers. This album has a significant amount of deliberate upper midrange distortion which was very grating before with the BIB cabs. It suggests that there was still large amounts of upper midrange coming out of the BIB vent and interfering with the front drivers. There is a level of detail which was absent from the BIB cabs using the same drivers, which has surprised me a great deal.

So far I am very happy with the result of this simple cheap experiment.
I will post some photo's of the cabs when I have tidied things up a bit.

Shoog
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