[terry j]

good news on the driver

had to cut away the dustcap, simply doing all I could to chase back the connection line, found I had results putting the battery directly on to the leads to the VC, so a quick re-solder of the connections under the dustcap and voila!.

Luckily had some black mastic that I could use to re-attach the dustcap, looks acceptable.

VERY quick muckaround last night, I'm pleased indeed with the results of trying to integrate the sub with the existing woofer.  Looks like I can indeed get a reasonable fit of the sub (running from the analog out of the bass on the deqx, into dcx 2496 to amp to sub0 tailor the raw response of the sub using the dcx to dovetail with the bottom end of the 18, run them both at the same time and the deqx does indeed pick up both drivers output as one and then performs the correction filter as normal whoo hoo, I've now got a bodgied up four way using a single deqx.

Next step is to do some outdoor measurements - of course that means it rains today doesn't it! - with the 18 roughly mounted on an OB.  I can then see what sort of eq is required to get down to hopefully 80 hz using the 18 OB. It only has an xmax of 8mm from memory.

So, my son, I accept your epiphany on the looks of a wide baffle in my cathedral, and will gleefully accept you back into the fold, amen.  Blessed are those that stray, yet return, as they surely have learnt a powerful and yet personal lesson, halleluiah brother.

[terry j]

Well, I just fired up my first (and only to date) test baffle.

It's something like 750 by 1400 at the moment, I can always cut wood off easily enough, more of a pain to add it!!

I have two xls 10 in a sealed box right at the bottom, then the PHL 18, PHL 1660 and tweeter above them all centred on the midline of the baffle.  I would presume some sort of asymmetry of baffle shape (thereby 'offsetting' the drivers) would be preferrable, if that's indeed true then I may consider it as well.  Having said that, the deqx's verification measurement of it's correction looked pretty flat.  Maybe the problems associated with regular shaped baffle are real problems if you have to try and passively correct?? With the deqx it's a doddle by comparison. Of course there may be other issues than simply getting a flat response, if so I'd love to hear them.

I only have one baffle, the other side is still the old sealed boxes that I'm thinking of switching from.  My idea was to be able to mute each side in turn and then compare the sound of the two, not that I have a mono recording!

So at the moment it's a bit of a nether world, neither one way or the other!

The only real OB's I've heard are the orions, I fell in love with the dipole bass but was not super impressed with the rest of the package.  Sure, I could see the attraction but I felt they delivered quite a bit less resolution than I was used too.  I then realized that unless I placed MY drivers in OB I couldn't 'write-off' the concept of OB itself, all I could conclude that I preferred my sealed system to the orion, which is a different kettle of fish than saying OB is no good, if you can follow that (I thought it would be clear when I started typing, but I've given up as a lost cause being clear in that sentence!!)

So, my main concern was in losing the detail, as I noticed diminished on the orions.

Jury is still out, I must admit I can't hear any real obvious loss of detail, which is certainly encouraging. I've not done any room eq yet, not sure how it will affect two different topologies each side.  I only did speaker correction down to 80 hz (as my subs come in under that, and only one side has subs at the moment) so of course in the comparison I turned the sub off. 

There was no real knock out blow for either alignment yet, although I thought I could clearly detect the 'boxy' sound people talk about.  That was using an old measurement tho, and with eq in the deqx that flattened the dipole but of course also acted on the sealed as well ( the single eq tab affects both speakers), so that is inconclusive yet.

There is one test track I know well, a single female vocal left of centre with a solo electric bass guitar to the right of centre.  The test baffle was the left speaker so must be reproducing most of the voice.

Not sure if I'm kidding myself, but I'm not sure that I've ever heard that vocal so clean and lifelike before.  Promising signs.

On the other hand, and once again I could be imagining things, the soundstage (whilst still large) could be slightly less extended out from the left speaker. Strangely enough, on these particular tracks I use to test that, it might not be a bad thing!!! as the soundstage is SO wide it almost is unsettling and you are concentrating on how wide it is!! ie it can be slightly unreal. So perhaps it is not a bad thing ha ha.

Again, only by comparing instruments in the left vs in the right, it seemed that the instruments were not as 'point-like', sort of occupied a larger point in space, yet having said that they could very well have sounded more real as a result.

Of course, early days yet and I'm sure the entire presentation will change once the second baffle gets made, and really only then could I start to make REAL conclusions.

I'll try and attach the verification measurement, but if it fails (read if I muck it up!) across the midrange (96 db slopes at 300 and 3000 hz) you can see a slight residual undulating shape, a result I assume from driver response within the large baffle.  In any case, the worst result within that pass band is a difference of about 2 db and that lessens in the undulations as the frequency rises.

Anyway, lets hope the graph gets up. no, what I thought I had to do - didn't.

will have a look for a sticky on posting an image, but till then this will have to do.

[JeffB]

Can anybody explain what might be the cause of the big frequency response dip on the wider baffle?
It is not at all intuitive to me.

[Rudolf]

Can anybody explain what might be the cause of the big frequency response dip on the wider baffle?
It is not at all intuitive to me.

The back radiation of a dipole driver is 180° out of phase relative to the front. If the next way from the driver backside to the front - which obviously is half the baffle width - is half a wavelength (same as 180°) those 180° will add up to the back wave 180°. At the baffle edge the back wave is 360° in phase with the wave originating from the driver front. Both add up. Result is the peak around 400 Hz.

If the next way from the driver backside to the front is a full wavelength, at the baffle edge the back wave lurking around will be still 180° out of phase compared to the wave originating from the driver front. Both are subtracted. Result is the dip around 800 Hz.

The above is only strictly valid for a point source. If the driver area gets wide with regard to the baffle width, one has to look at it more like a cluster of distributed point sources - each with its own set of peaks and dips. So the red response only is a sum of many green responses. You can easily see this by gradually changing the driver area in an EDGE simulation.

[D OB G]

I'd like to point out that using the series inductance method of "An OB design", the output of the speaker that is used in a linear mode is the freqency response BELOW the turnover frequency, in this case 400 Hz.
That is, the sloping straight line of 6 dB per octave below the turnover frequency is compensated for a flat response, and the "lumpy" response above the turnover point is rolled off at 6 dB per octave, low pass.
This can be done for a midrange as well as a woofer, reducing the nulls and peaks in the overall output.

David

[D OB G]

Here is the response of the PHL 1660 using Basta (which uses the Edge to determine open baffle responses), by the series inductance method.

The lower crossover in this case is determined by a 1.5 mH inductor.
The total resistance of the inductor, plus any extra  resistance is 0.3 ohms.

These figures may differ from real world values (for example, I wouldn't be surprised if the inductor needed to be larger).  Test gear, including an inductance meter, and ideally a variable tapped inductor (or numerous small inductors)  is really required.  Yes, a pain in the arse.

The upper crossover is determined by the baffle size, in this case pretty much the free driver itself.
To raise the upper crossover frequency by this method requires a smaller baffle (smaller driver).

Note the nature of the peaks and troughs.

I hope this alternative technique may be of some interest.

David

[terry j]

hi David

gee thanks! for simulating the 1660.

Luckily for me I don't need to worry about components, I just use the deqx.

Look, could someone point me to a sticky on uploading graphs please?  I tried searching for 'how to upload graphs', not much came up that was of use.

I'll have to re=do a whole bunch of measurements in a day or two, I could then post the measured response of the raw driver before correction.

I tried doing it outdoors, started off OK but then the wind came up....

So I cut my losses and did it indoors, not the best but enough to get me started.  I measured from a distance of two metres, hopefully enough for any dipole effect to be in full force.  Is that sufficient for both the bass and mid??


EDIT  thought of another quick question (which I'm sure has been answered before...sorry) is there some rule of thumb regarding how far off the wall the speaker needs to be??

I've recently had to re-organise my room, and for now that meant I had to bring my seat forward, and therefore the speakers back towards the wall behind them.  They are just on a metre out from the wall, before the reorganisation there were closer to 1.5 m.

Is one metre sufficient ? or does that 'strangle' them.

[kyrill]

as i understand Terry you must put yr photos on the web, so they have a web address then  highlighting it and then apply "insert image " icon under the "italicized icon"  ( top left icon)

[terry j]

I'm afraid that running the 18's in OB turned out to be a fizzer, which was what I'd feared ll along

They bottomed a bit too easily, so with much reluctance and sadness I took the saw to them and chopped it up boo hoo.  I did that because the sub enclosure was built into it, at least I salvaged that bit and can run the sub under my original sealed boxes (in a FR sense that is).

That has produced some advantages at least, even with insane boost (which didn't really cause much problems) I could only get the 18's down to 29 hz in room, with the new sub (4 peerless xls 10's) I now have a minus 3 db point around 13 hz.  That does not make a huge difference with music, but enough to notice (music only here).

However, what I have done is when I do the correction on the mains with the deqx, I set the lower limit to 80 hz now, which is the expected roll off point for the 18s, in other words they have no boost.  This must help with distortion, though I don't really know yet because I'm still mucking about with the whole sub integration thing.  It is basically working (the old splitter to a dcx 2496 trick ha ha) and of course the dcx has a lot of parametric power in itself that enables me to get a good match up to 80 hz where it then meets the 18's.  That leaves me with the full complement of para points in the deqx to then handle the full room anomaly problems, and also lets the deqx remote eq also affect the subs.  Pretty neat really.

I won't be doing much fiddling now till probably mid october, I have a bunch of guys coming for a shindig and so for now I'll just concentrate on dialling the whole thing back in.  I will probably build the second sub though (same as before but for the other side).

After that I suppose I can at least put the mids into open baffle and see what happens, I'm just quite dissappointed that the woofer didn't work.

I do however have one xls 12 laying around, and I might get three more of them and use them in OB.  After all the xls 10 is used in the orions as OB, it would not defy logic that the 12 would also be suitable for that would it???  I could then use the xls subs to come in under that to get the extension.

That is however a bit further away time wise than trying the 1660's as OB.

Will leave any graphs or pics on the web for now, it's all I can do to master the computer to be on a forum ha ha.

Thanks to all for the help and interest so far, will let you know asap regarding my further adventures!!

Ciao