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-   -   Omnipole, monopole, dipole and...nopole?? (http://www.diyaudio.com/forums/multi-way/104171-omnipole-monopole-dipole-nopole.html)

terry j 26th June 2007 12:08 AM

Omnipole, monopole, dipole and...nopole??
 
It probably won't be long until I find out if this question is a good one or a stupid one....the gales of laughter which may ensue will tell the story!!

Been following Lynn's thread beyond the ariels, and recently it has taken off, exhausting to keep up, and what's more it has been spawning offshoots!!

One thing that has piqued my interest is the one of power response, and whether or not the FR should be flat on or off axis etc. I do like the sound of that one and will check it out soon, will see what new information comes up on it before that.

Obviously, given the nature of Lynns thread the subject of dipoles and the difference with conventional boxes has come up. When reading about dipoles and how they work it all makes perfect sense, but my problem is that in practice the only two dipoles I heard did not impress me at all (one of them was the Orions, so that definitely suprised me).

That experience kinda left me a bit iffy on dipoles, but it was obviously not a very stringent test and probably insufficient to make sweeping general statements about the different alignments.

On the other hand, one thing that became obvious to me (others pointed this out, I'm not smart enough to work it out for myself) is that the back wave of a sealed driver must, to some degree, make it back out of the box through the driver cone itself. That is if we assume absorption internally is incomplete. This must degrade the sound, and possibly quite considerably.

Before I (finally!!) come to my question, whilst I can see the advantage of the dipole 'taking the side walls' out of the equation due to side cancellation, by the very nature of the beast are we not bringing (forcefully) into play the wall behind the speakers?? After all, we do in effect have a speaker deliberately aimed at it.

Lynn mentioned in passing the possibility of a lossy mesh etc, and even Sigfried mentions something like that on his website. It got me wondering if it could be used on 'normal boxes' too.

So, what theoretically to me would be the best of both worlds is a means of having no re-radiated sound coming thru the driver cone, and no sound radiated directly at the back wall.

What if my midrange chamber was built out of something like, I don't know, compressed fibreglass?? Forget for now the bass in a three way etc, ie the pressure not affecting the mid. The 'lossy box' would mean only a slight sealed box effect, yet allow the rear wave to escape and not be coming back through the cone. Any sound that makes it out rearwards thru the cone is hopefully attenuated severely and so does not reflect strongly off the wall.

So there is the question, not a dipole , not a monopole so maybe it is a nopole?

let the laughter begin

BudP 26th June 2007 12:29 AM

Terry J,

I have been EnABLing a pair of Lowther PM6A's over the last two weeks and have had them mounted on no baffle. Just sitting on a platform stand with a sheet of packing material under to help with the one close wall of the stand top plate. The magnet structure is supported by a plastic winding bobbin (from my business) so the cones are perpendicular to the room. I did not expect much in the way of satisfaction. I have been surprised.

There is not a "null" zone to the side and if you step behind the stands it sounds just as it would if you were standing behind the performers. Not muffled or lacking in frequency, but definately not the front side of the energy. As I have progressed and listened I have become quite enamored of this "complete" sound. The Lowthers are about two feet from a brick wall and I notice no back wave problems, just an amazingly full sound down to 80 Hz or so.

So I vote for a no baffle dipole, at least for the range above 120 Hz or so.

Bud

CeramicMan 26th June 2007 12:05 PM

Re: Omnipole, monopole, dipole and...nopole??
 
2 Attachment(s)
Quote:

Originally posted by terry j
... the back wave of a sealed driver must, to some degree, make it back out of the box through the driver cone itself. That is if we assume absorption internally is incomplete. This must degrade the sound, and possibly quite considerably.


I totally agree! People often seem to only focus on making the box solid, while forgetting the most obvious exit point: the speaker cone itself. I've been thinking about ways to make sealed boxes more absorbent than the average "hollow box with bracing". So far, the best I've been able to come with is... (see the pic) and written up some comments about it on my site here.

terry j 26th June 2007 03:09 PM

thanks bud, have been watching your thread, not sure I understand it totally, probably one of those 'just do it' type of things.:)

Lech, yeah thats kinda what i was thinking. Might have to do the maths another day when I'm a bit more awake.

That was one way of trying to remove the rear wave, but I wonder how complete the absorption would be? On another point, but still related I suppose, can the T-S parameters of a midrange be used in box size calculations etc just as the bass drivers are?? I would suppose so, and then pushing this a little further, just as there are recommended qts values etc for a bass driver used in IB, or indeed OB, are there corresponding q values for the midrange as well which make them suitable for any given alignment??

Strangely enough, I would assume the MORE rigid the box is made the worse this particular problem might become, usual 'swings and roundabouts' that crop up in engineering!

So, given your illustration of the concept, and assuming incomplete absorption and so at least some of the sound coming back through the cone, what if (what I take to represent the solid box, ie the white area outside your wedges) instead of going thru the foam, hitting the rigid structure and reflecting back thru the foam and out the cone (albeit attenuated) it simply carried on out of the foam because there is no solid structure there??

In that case have we gained anything at all over going straight dipole?? My only guess is that the sound from the rear of the driver will not have much of a reflection off the wall behind the speaker.

My mains will be 'a la' the Sonus Faber Stradivari, about 750mm wide. Theoretically if this attenuation idea had any sort of merit then it may function kinda like an IB???, an IB in the room if you will.

In any case, have you done any experiments along this line and if so what have you found?

pdan 26th June 2007 09:23 PM

Terry j,

Your idea to come up with a "lossy box" design makes sense to me.

I know of one design that attempts just that. It is based upon a transmission line.
Normally, TL's utilize taperd sections and are lagged and tuned for a balanced bass.
The "lossy" design uses parallel sections, the xsectional area = the cone area.Great
attention was given to deflecting the sound waves round the "bends"; for this the design
used concrete with ceramic tiles.The whole thing was then well stuffed with sound absorbant material.

The idea, is that, by the time the rear wave finds free air it is so attenuated that it
will have little or no effect upon the front wave. The designer also claimed minimal
effect on cone movement due to the use of deflectors and because the constant
xsectional area would not create any backward pressure.

I haven't heard of anyone who's tried it. So I've no idea how effective it is.


Cilla


Bud,

I found your post amazing!

Every time I read something amongst these pages I have my intuitions challenged.
This time they have been confounded!

If I understand you right: are you saying that if an EnABLed Lowther PM6A were to be
simply mounted on some kind of stand , presenting itself in all its naked glory, it would give "an amazingly full sound down to 80 hz or so" ?

Ye Gods! The amount of music that I have in my collection... well, 80hz or so would do just fine.
How about the EX3 ( what I have) , would it work?


Cilla

BudP 26th June 2007 11:17 PM

Hi Cilla,

I will know more about Lowthers as time goes on. Lowther America tells me that a pair of 15 ohm PM6A's and a pair of A45's are on their way to me for treatment. The thread below goes into some detail about EnABL and Mamboni standing wave elimination treatments and long about post 250 a fellow from Taiwan begins some testing that not only supports the practice but adds some facts about treating systemic ringing events that is new knowledge.

http://www.diyaudio.com/forums/showt...0&pagenumber=9

The PM6A's that are here go back to Lowther America on Thursday and are alleged to be slotted in as the main display drivers for Lowther at the RMAF this year. They are pretty amazing sounding, but they had gotten so electron hungry, in coherent small signal propagation, that I had to put a couple of Litz wire electron traps out on the ground lugs of the speakers. The sound was changing abruptly from lushly rich to sharply thin as more and more sources entered the picture, ie. symphony recordings, but were remaining very lush for single piano recordings, like Jessica Williams "Gratitude" CD.

These are the stupidest things I have ever invented. A loop of true Litz cable, 140 strands of #40 AWG magnet wire, 6 inches long, with three 0.6" pieces of shrink tubing shrunk along that length, and then formed into a loop with both ends tied together. The things allow enough electrons to loiter on the ground side to provide a huge increase in tiny signal information and a resultant increase in coherency of all information, but especially reflections, hall sound and internal detail on strong transients, like a piano fortissimo.

This idea did get some input at this thread, much quite dismissive, but it is such a weird idea that anyone with a rigorous understanding of mathematical electronics must dismiss it, as idiocy. It is, unfortunately, very noticeable if the speakers can deal with the coherency it allows.

http://www.diyaudio.com/forums/showt...hreadid=102180

I am not sure if this would be noticeable with an untreated set of drivers, but the EnABL process makes a driver seemingly infinite in it's ability to reproduce coherent, informative detail, along with providing greater transient signal headroom.

So, go read that first thread on EnABL and as you come to the descriptions of how to treat a Lowther, with patterns and step by step pictures, you can decide if the treatment is for you.

As for how it sounds, well the electron traps are still "filling up" so ever greater levels of detail are being reconstructed (takes about 4 hours or so to complete, just by playing music) and I may even find I have too much trap space. The quality is just stupifyingly good. These are without doubt the best sounding speakers I have ever heard, once EnABL'd, and not at all shabby before treatment. The bass is quite good, deep into the piano left hand and allows all of a cello's bass string when Yo Yo Ma attacks on his Silk Road SACD.

it is obvious that bass viola's need an auxiliary driver and the Eminence Beta 8 looks like a very good addition, but on an open baffle, down to 40 Hz or so. In addition I am using a pair of Wright Sound 7 watt 300 B prototypes, with our power and SE output transformers, so that is also upping the information content while allowing safe Lowther power levels.

So, yes, naked Lowthers, but with a little EDGE developed OB for the Beta 8 below it.

Bud

pdan 27th June 2007 12:12 AM

Bud,

Thanks for the detailed response 'n ' links.

My heads spinning.....think I'll lay down.


Cilla

Drew Eckhardt 27th June 2007 12:43 AM

Re: Re: Omnipole, monopole, dipole and...nopole??
 
Quote:

Originally posted by CeramicMan


I totally agree! People often seem to only focus on making the box solid, while forgetting the most obvious exit point: the speaker cone itself.
The Linkwitz Pluto has both drivers (the Aura mini-driver used as a tweter is open backed) mounted in damped transmission lines with a measured 40dB return loss for the mid-bass driver.

I own both Plutos and Orions and the Plutos sound surprisingly similar.

terry j 27th June 2007 01:14 AM

cool Drew, will check it out.

You obviously have some experience with both, and bear in mind I haven't checked out what you've just said, but the pluto is not a true dipole as the orions are?ie more along the lines of 'removing' the back wave etc.

It may simply be a matter of terminology, but I always think of a transmission line as a means of tuning and deepening bass response, does it have that effect in the midrange as well or is it used in a different type of sense?

I will however get on over there and have a read a bit more closely on the pluto.

Does anyone have any data on whether or not you can nilly-willy throw any old midrange driver into an OB alignment?? or indeed a no-pole :cool: :cool: :cool: ie could it lead to excursion problems etc etc etc? (ie is it only suitable for some mids and not others).

CeramicMan 27th June 2007 01:30 AM

Quote:

Originally posted by terry j
That was one way of trying to remove the rear wave, but I wonder how complete the absorption would be?

Hi Terry, yeah it's hard to say, I was looking for information about anechoic chambers and they generally seem to be rated for reflecting about 1% of the incident power, or about 20dB down. But that's still very good.

Quote:


On another point, but still related I suppose, can the T-S parameters of a midrange be used in box size calculations etc just as the bass drivers are?? I would suppose so, and then pushing this a little further, just as there are recommended qts values etc for a bass driver used in IB, or indeed OB, are there corresponding q values for the midrange as well which make them suitable for any given alignment??

It's an interesting problem because a low Qts seems to be better as it means that the speaker's motor provides a higher back EMF, which provides a larger opposing force to external influences such as sound pressure from inside the box. A stiff cone should be very helpful in that regard too.

However, a stiff cone combined with a low Qts is also a stronger 'nodal point', giving resonances a higher q-factor and placing greater reliance on damping materials inside the box. So there's a bit of give and take.

Quote:


Strangely enough, I would assume the MORE rigid the box is made the worse this particular problem might become, usual 'swings and roundabouts' that crop up in engineering!

That too!

Quote:

So, given your illustration of the concept, and assuming incomplete absorption and so at least some of the sound coming back through the cone, what if (what I take to represent the solid box, ie the white area outside your wedges) instead of going thru the foam, hitting the rigid structure and reflecting back thru the foam and out the cone (albeit attenuated) it simply carried on out of the foam because there is no solid structure there??

Actually I thought that the wedges themselves could be made of wood. They could be layered with felt or something like that, but the idea is to have a smooth horn-like transition between the air and the solid edges of the box. My theory is that the backwave would be funnelled into infinitesimally small points, where the extremely high SPL pressures and tiny displacement would be easily converted into heat.

Quote:


In any case, have you done any experiments along this line and if so what have you found?

I want to try it out in practice as soon as I can! But at the moment it's just researching and theorizing.

With lossy boxes I think the hard part would be creating a good acoustic filter. Maybe a similar 'wedge' design could work, but you'd have to tune it somehow.


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