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Old 19th April 2007, 05:40 AM   #1
JinMTVT is offline JinMTVT  Canada
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Default dipole baffle idea ...

will probably look completly stupid
but i have some design i have to expose
( well ..cna't say if i can call it a "design " )

please don't laugh at me

http://img185.imageshack.us/img185/2...esigna1uy5.jpg


just trying to remove all reflections within an as compact as possible open baffle design and trying to remove front baffle diffraction ??
the outside could be all rounded ..

let me know if i am waisting my time completly
( probably ) or would be worth the effort over a front flat baffle

thanks again all
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Old 19th April 2007, 05:47 AM   #2
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What is the round thing in the middle?
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Old 19th April 2007, 05:51 AM   #3
JinMTVT is offline JinMTVT  Canada
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it is a block or cylinder...keeps area constant without leaving any paralell walls
( i guess that a driver can't push out at back more air at a time then what it's castin/basket allows to pass through ??? )
and it has aesthetic purpose as open backed loudspeakers don't look too good from the back
( wife tested already ... )

mmm ..other than that i don't know what it does there!



but seriously i thought about rear dispersion for higher fs ..but i can't say that it would work ( like waveguide?)
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Old 19th April 2007, 06:19 AM   #4
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I'll have to brush up on it, but I don't think you'll get the classic dipole radiation pattern from it. In fact, due to there being some restriction on the flow out to the rear and a less restricted flow 'up' (unless it's designed to be from floor to ceiling), you might find it's behavious to have some aspects of horn and/or t/line enclosures.

Not sure. Have you built one? Not for real, but out of (say) an old refridgerator cardboard box. It's give you some idea of dispersion pattern...
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Old 19th April 2007, 06:52 AM   #5
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It would probably work if you use the front half. That is actually something in my mind.

Are you going to do the wood work yourself?
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Old 19th April 2007, 10:44 AM   #6
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i would think that this would be benificial at a frequency with smaller wavelength than the width of the cabinet.


below that i think you might just end up with a phase or dispersion issue at with the rear as the waves are pressured again at the mouth you have there.

not founded at all just a thought.
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Old 20th April 2007, 12:08 AM   #7
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I agree with your comments.

I think that dipole speakers we usually see are either flat baffle or flat baffle with wings. If we cut the posted drawing and use the front half, it would work in a way similar to flat baffle with wings but with rounded edges at the front. Of course, the radiation pattern, phase would be different from a standard dipole with flat baffles.

The idea is to reduce the on axis, negative effect of diffraction as much as possible. I have been playing with Baffle simulation programs a lot lately. I hate to see the frequency response irregularities (+-2dB typical around the 1k - 4k region where human ears are most sensitive) and delay / phase errors generated by the wide, flat baffle, though according to John K, the diffracted source may be -15dB comparing to the direct radiation from the driver.

I have just got the A B C Dipole from John K and started playing with it. I already found it highly enlightening. I would recommend anyone to get a copy. It has a lot of dipole theory packaged in simple descriptions with illustrated graphs to make them easy to understand, plus John K's open baffle simulation program and more. It is really a bargain for USD$10.95.

I found the only way to reduce the diffraction effect is to make the front baffle as narrow as possible, which has an additional advantage of pushing the first, usually the largest peak and dip higher to the XO point so that it can easily be dealt with by the XO. Of course, most commercial speakers do just that. But to go one step further, a 2 inch radius rounded corners will reduce the peaks and dips to within +-0.2dB, and I would expect the sound be a lot more smoother and detailed. This is what I am aiming at. Of course, this generally applies to closed boxes.

For open baffle, narrow baffle is not practical. Adding angled wings will introduce a lot of unknown problems to be resolved and it does not reduce diffractions. So I am thinking about the narrowest baffle for the midwoofers, then using a 2" large radius rounded corners. The woofers will be centred horizontally. The tweeter will be offset slightly up to 1". With this baffle, the dipole source distance between the front and back are effectively increased over the standard flat baffle, pushing the dipole peak and dipole roll-off lower, making it possible to have less EQ and less demands on the midwoofers. The diffaction effects are hopefully reduced substantially due to the large transitional surfaces. It may no longer be a standard dipole, and the dipole null would be a bit behind the 90 degree axis. The radiation patterns will be changed a bit, and I can expect a stronger, narrower rear radiation, which may not be a bad thing. Below is a quick drawing I just did with MS Paint to illustrate the concept.

Click the image to open in full size.

I don't know if I could still use the A B C Dipole to model this baffle by calculating with an equivalent D. I think I will email John K and invite him to shed some lights on this thread if he has the time. Hey, I have to tell you honestly I am not a salesman for John K. I just admire his work and has built his NaO with much satisfaction - still tuning and playing with the XOs for my experiments.

Don't count on me. I am still learning. If this turns out to be good, where can I find a cabinet maker / wood worker to make the baffle for me!!!!! Any forum mates around Sydney area can help? I will do the electronic part for you for a swap.

Regards,
Bill
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Old 20th April 2007, 01:12 AM   #8
JinMTVT is offline JinMTVT  Canada
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Your drawing could be done in alot of things other then wood, why do everybody stick to wood as the ONLY material ?

neway ..if you want the exterior to be almost sphere like, you could work probably 40% of you angle on a big lathe turning ... and do the rest with hand

you could also use the glued pannels technique and make all the pannels from 0.75" mdf with the perfect shape and then glue them together from bottom to top


so on to the design ..
the reason why i started this thread, is that i was wondering about the implication of using wings on a dipole

i undestand that it pushes the center of the radiation pattern backward and changes the front SPL pattern,
does it affect dispertion of the drivers also?

Then as said, consdering only the idea and not the actual drawing i made in a few seconds ..
If the area at the back between the "cyllinder" and the drivers output is more than what the driver needs i don't see why it could influence it much ( won't be restricted and shouldn't do some loading ) ????


Lets get a few things straight ..
( well for me, cause you all might know this already)

on a sphere like enclosure as we've all seen on few designs and Apple desktop speakers, is there any problem associated with diffraction ?

Then if we only use a slightly curved surface as per NutNut drawing, what is the implication on the refraction??


then what is the implication on the radiation pattern and accoustics if using wings on a dipole wich alters the dipole center offset ?

do we absolutly want a perfect centered pattern ?
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Old 20th April 2007, 02:06 AM   #9
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JinMTVT,

What are you intending to use these for? Is it a single driver, 2, 3, more? MTM, WMTMW?

I was thinking (although in actual execution it would be hard) of something that looks a little like an hour-glass from the front, but is open backed. The shape, when looking from the top would always be hyperbolic, but would vary depending on where in the hourglass it is.

Sorta like this picture (but I'm lousy at pictures).

But, as I'm lousy at wood-working I wouldn't be able to do this anyway. Maybe if I make friends with someone who does formed plywood moldings...
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Old 20th April 2007, 03:46 AM   #10
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JimMTVT,

You can download the Baffle Diffraction Simulator from the FRC site and play with it. Of course, that is only for closed boxes. For dipole baffle simulation, get the A B C Dipole.

For woofers with larger radiation area, diffraction is usually not a problem especially at lower frequencies. The problem is mainly with the tweeter that has a small radiation area.

Rounded corners reduce diffraction. The radius of the rounded corners is related to the frequencies. If you have a radius of 1", the response is a lot smoother for frequencies above 10k roughly, but it does not do much for frequencies below 10k. But if you have a radius of 2" / 50cm, frequencies from 2k upward can be made very smooth. Larger radius is not necessary.

Regards,
Bill
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