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Old 1st January 2006, 08:40 PM   #1
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Default isobaric any info on how?

I got the idea of having 2 speakers together to get less space in apolito design.and the problem is if they face back to front they don't need to be out of phase right ?and does the port double in length?I want to use also a double ported chamber,any info on what to check out for?cause I cant test this before I do the box.I will be using the vifa xt18 small magnet,so i will need 4 speakers per side,cause each box will need at least 2 cubic feet and in isobaric only 1cubic think it can work?
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Old 1st January 2006, 10:07 PM   #2
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Well... Yes, if they're mounted so they both point the same way, they would be in phase. In practice the length of the tunnel should be as short as possible to give the best air coupling between the drivers, I should think. But there are some things to bear in mind. The main problem soundwise is that the drivers will probably develop interferance notches towards the top of their passband- assuming they're handing over directly to tweeter. I expect the air coupling between the drivers becomes less 'solid' at high frequencies and cancellation between the drivers and/or reflections from the rear cone through the front one become an issue. Not that this stopped Linn doing well with the Sara back in the eighties or John Cockroft being pleased with the results in Speaker Builder magazine back in the day. Off topic for a moment; I loved Cockcrofts writing style - really amusing in a gentle sort of way, but they were really detailed articles to help the beginner. Always tickled me that he was a nuclear scientist and yet designed speakers largely empirically rather than on a theoretical basis. Anyway... Apart from any possible sonic or technical problems, there are some practical issues too. First and most obvious is cost. Four XT18s a side may not be the best way to go in terms of bang for the buck, either in terms of quality or output, good though the driver is for the money- I have a pair myself. Four XT18s would cost about the same as a couple of SEAS Excels of the same size or a couple of Scanspeaks. Not that this necessarily solves the cabinet size issue of course- unless you find you can drop the MTM configuration and just use one woofer a side of course, in which case you could perhaps buy just one pair of startling wonderful drivers for the same money or have a lot of change. Also, and potentially most upsetting, will you really save that much cabinet real estate? for a small midwoofer, the ratio of the size of the isobaric/ compound tunnel to the main enclosure doesn't look nearly as good as with some vast subwoofer box. It won't be two cubic feet plays one, closer to 2 vs about 1.3-1.4 or so, given enough clearence and an extra internal baffle. I'd certainly take a serious look at alternatives before going ahead. As you probably already know, although the box size is less for a given bass extension you don't get any more overall volume output for an isobaric pair and IIRC, you need to take advantage of the doubled power handling to feed twice the watts in to do it. At least if you've got four a side you can keep the impedance pretty much the same as a single driver.
I think the best way of using XT18s is just use one a side- unless you're desperate for every last decibel, in which case maybe a three way system with a woofer or subwoofer would be a better bet. Incidently, Madisound sell crossovers for the Selah SA-1 kit, which combines the XT18 with the Fountek ribbon, which I'm looking forward to trying once the group buy comes through. Just a thought... What tweeters are you using BTW? If you haven't got any yet, best value are probably the cheapish SEAS drivers that Zaph likes (check out the Zaph Audio site or search this forum if you've not run into Zaphs stuff already) or the XT25s perhaps?
WinlSD seems to think (for example) a 47 x 62mm long port for a single driver and 47 x 157mm for two drivers used as an isobaric pair, so rather more than twice the legth. Oh- I ran that for the WH version of the driver. That's the one I have and I forget which is which... OK, so the WO comes out at 69mm long for a single driver and 173mm for the iso for the same nominal 47mm diameter port. Enclosure sizes are virtually identical for the two XT18 variants (as you suggest, about 1cu ft for single drivers and half for the isobaric) according to the manufacturers specs I've entered into WinlSD- of course real drivers may be slightly different!
All the best.
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Old 1st January 2006, 10:55 PM   #3
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I thought sticking them back to back was the best way to go - cancells distortion etc. I'd thought of doing a system using different drivers - the rear driver acting as a "compressor" being driven by a cheapo amp enabeling you to tune the outcome. Although the "inbetween" airmass gets a little squidgier as you increase the seperative volume, it also becomes part of the effective driver component lowering the resonant frequency of both units - I suppose it's all down to what you want to acheive.
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Old 1st January 2006, 11:00 PM   #4
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To get the distortion cancellation of an isobarik you need to use identical drivers driven from the same amp. In my experience, the volume of air between the drivers isn't all that critical, but you don't want it too big or you will lose the size advantage that you get by going isobarik in the first place.
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Old 1st January 2006, 11:08 PM   #5
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Sorry Al, didn't make myself clear - I know the bit about the cancellation but I'm still pondering over what to do with my dads elipticals (there's only two of 'em & I won't find anymore!). Was thinking about some kind of isobaric/TL combo using a 3886 to power the rears & his old JLH for the elipses.
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Old 1st January 2006, 11:13 PM   #6
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I have pondered isobarik TLs before, just never got round to experimenting. Make sure you keep us updated!
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Old 2nd January 2006, 10:25 AM   #7
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Yep, back to back mounting is probably a better implementation, all else being equal, but as pinkmouse says, the tunnel can become big enough to virtually cancel out the size advantage, particularly for a relatively small box like the one under discussion. With back to back mounting the volume would be up to about 1.6 cu ft or so- not much of a size reduction. The main application for isobaric loading is for subwoofers or the bass section of multiway speakers I reckon, since you avoid cancellation problems in the midrange and the tunnel becomes fairly small compared to the main box volume. These days drivers like the Peerless XLS series and cheap(ish) high powered amplifiers or complete subwoofer plates, along with the use of EQ make compact bass systems possible without the expense of two drivers.
BTW, eagle-eyed members may have noticed I referred to a non-existant Fountek ribbon group buy. In fact it's a pair of Aurum Cantus G2sis I'm waiting for, but these are essentially the same driver as the Fountek used in the Selah SA-1, as demonstrated by measurements on Troels Gravesons site...
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Old 2nd January 2006, 12:02 PM   #8
rinox is offline rinox  Italy
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Isobarik it' s not so different from push-pull with the pros and the cons.
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Old 2nd January 2006, 01:31 PM   #9
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wow thx guys!! ,well first my box in total for 2 xt18 is 2.30 cubic feet counting the braces and all,with a port of 3"x7" for both woofers.but I thought that using iso would cut that in half but it's true I forgot the space use in between the woofers.figured I'd cut that part holding the other woofer in circles about an 1" thick take less space in between the woofers and in the box,hope I'm clear... I was told that you get a quicker response but at the expense of db's of the woofers.but if I use 4 per side what would be the the result in db's considering it's 87db per woofer
? here is a picture to show the wood around the apeakers.

oh and I'm using the tweeter seas 27 tdfc no ferro in it.
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Old 2nd January 2006, 02:40 PM   #10
Toaster is offline Toaster  United Kingdom
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It depends how you wire the drivers, and the amplifiers to hand. If you parallel connect the paralleled pairs together you'll end up with a very low impedence around the 2 Ohm mark, which may be unrealistic. If you connect up the parallel pairs in series, you end up back at 8 Ohms, still at 87dB (I think!) but four times the power handling of a single driver. Ultimate loudness depends on how much amplifier power you have handy. If you had or could build two pairs of amplifiers for the midbass you could use nominally smaller amps- albeit able to drive 4 Ohm loads, rather than one big one...
All else being equal bass or any other part of the spectrum won't actually be quicker, but the 'double glazing' of the drive units may result in less reflections from inside the cabinet getting out through the cones, and give a cleaner quicker sounding result. This may be offset somewhat by reflections off the rear cone as the air coupling breaks down at the top of the XT18s passband. You pays your money...
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