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Old 31st October 2008, 04:59 PM   #1
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Default Hybrid Dipole

Here is a little experiment I'm on with at the moment.

It is an open baffle design with a sub bolted to the base.
Drivers on the open baffle section are:

Goodmans Axiom 401 12 inch lower mid/upper bass
Fostex FE108EZ upper mid/tweeter

Click the image to open in full size.

Click the image to open in full size.

It fits very nicely into a small room.

Steve
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Old 31st October 2008, 05:36 PM   #2
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Hi,

Neat idea, and the baffles/cabinets are simple yet elegant. The rounded tops do make the baffles look somewhat like gravemarkers though - to me at least!

So what kind of subs are you using, and where does the driver face? It looks like it's facing toward the side, where the blue stuff is. How much space is there? The driver needs some room to "breathe" otherwise there can be very clearly audible damping of the response.

If you had a yen to cut another hole in the baffle and move the sub right up to it, it would look rear mounted and integrate well into the aesthetics; in fact to me the sub showing would give the speakers a more poweful, "aggressive" look.

Just my 2c, but it's a nice speaker just as it is. How is the performance?
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Old 31st October 2008, 06:24 PM   #3
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Hi

The driver in the sub actually fires downwards. There is a slot cut into the base of each wing to allow the bass to escape to the sides and of course the sub is free to vent to the rear.

The phase of the sub is arranged so that the bass is in phase with the rear baffle radiation and crosses over to the main array at around 75Hz. There is no direct radiation of the out of phase bass through the front of the baffle.

I have spent around six months working on trying to successfully marry monopole bass with a dipole mid and treble radiation pattern, Very difficult to do but I think I have achieved the objective.

The blue towel stuffed down the side of the sub is a temporary arrangement until I fit the board over the top to isolate the sub from the baffle above. All it does is damp out a slight cavity resonance at around 500Hz that would otherwise intrude with some programme.

As to the sound of this hybrid:
It is a very even tempered sound from top to bottom. The midrange is fast and clean, without a hint of shoutiness and the bottom end is quick and deep.

The subs are a pair of Ruark Vita 50 actives and are fed via their high level inputs from the speaker outputs of my 7W PX25 single ended valve amplifier.

As to their looks: my 23 year old daughter reckons I need to stencil R I P around the inside of the rounded tops But my wife likes the looks.

Steve
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Old 31st October 2008, 07:35 PM   #4
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Ah - I noticed the slots in the side panels and that tipped me off, but I hadn't noticed the legs on the sub enclosure so at first it looked like it was resting directly on the bottom panel.

So, what sort of crossover design did you come up with, and how did you design it? Do you have a rough figure for the cost of the project?
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Old 31st October 2008, 08:52 PM   #5
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The design was evolved over a period of six months, starting with a cardboard mock- up. I'm not much of an acoustician, so it was a case of trial and error with the wing size until I got the sound balance I was after.

I was more into my comfort zone with the crossover. It is just a simple first order crossover with a 10uF polyproplylene motor run cap feeding the Fostex unit and a 4mH inductor in series with the Axiom 401. Crossover point is well clear of the critical 3khz region.

The cost is a bit vague at the moment. The subs were second hand acquisitions at 100 each the wood was 50 the Fostex FE108EZs were 70 each. The missing links are the Axiom 401s which are on loan at the moment from a fellow DIY enthusiast. There's no way I can avoid buying them from him now I've heard them (but then he probably knew that would happen when he lent them to me...crafty bugger) The 401s with the Fostexes are a match made in heaven and it would be tragic to break up the partnership

I would give a rough estimate of around $600 the pair.

Steve
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Old 31st October 2008, 09:45 PM   #6
Rudolf is offline Rudolf  Germany
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Quote:
Originally posted by Steve Cresswell
The phase of the sub is arranged so that the bass is in phase with the rear baffle radiation and crosses over to the main array at around 75Hz. There is no direct radiation of the out of phase bass through the front of the baffle.

I have spent around six months working on trying to successfully marry monopole bass with a dipole mid and treble radiation pattern, Very difficult to do but I think I have achieved the objective.
From intuition I would have said: Let the sub radiate to the front and in phase with the mid front. So the omnipolar radiation pattern of the sub might have merged with the dipole pattern of the midwoofer to some sort of cardioid - at least where both are running parallel. Obviously you found it better the other way round. But did you ever try to run the sub in reverse phase? If yes, how would you describe the difference?
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Old 31st October 2008, 09:49 PM   #7
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Also, have you done any measurements at all, to ensure you have compensated for early roll-off etc? FR measurements?
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Old 31st October 2008, 10:31 PM   #8
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Hi Rudolf,

The sub is in reverse phase to the front radiation ie in phase with the rear radiation. Trying the sub in phase with the front whilst it was mounted behind the baffle resulted in an uneven bass response with audible peaks and suckouts everywhere as it fought with the out of phase signal coming from the rear of the baffle. Also there was a weird discontinuity between the two radiation patterns that was audible from the listening position.

I've heard a few panels where attempts have been made to merge a forward firing cone bass driver with a panel. Without exception the results were not exactly encouraging. Only a pair of Martin Logans came anywhere near succeeding and even then there was still a feeling that the bass was somehow detached from the rest of the spectrum.

Alas I have no measuring equipment to measure the response in-room. Therefore I can't prove that what I'm hearing is any more than self-deception. The point is that it works beautifully and is the most realistic and musical sound I've had in 35 years of playing around with this hobby.

Of course that is not much good for those wanting to reproduce the design in a different environment. I'm not exactly scientific in my approach to speaker design

Steve
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Old 1st November 2008, 12:22 AM   #9
CLS is online now CLS  Taiwan
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How about this:

Making the sub in phase with the FRONT of the woofer, and lifting the whole baffle for a gap to floor -- thus you may get a polar response similar to cardioid -- double up in the front and cancelling at the back.

From the picture, there's a gap (to floor) on the side. And the sub is out of phase with the front of woofer. So it should be major cancelling here. Also, the OB woofer has cancelling on both sides of baffle already, so I guess it's almost silent around the gap now.
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Old 1st November 2008, 12:49 AM   #10
tinitus is offline tinitus  Europe
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Quote:
Originally posted by Steve Cresswell

I was more into my comfort zone with the crossover. It is just a simple first order crossover with a 10uF polyproplylene motor run cap feeding the Fostex unit and a 4mH inductor in series with the Axiom 401. Crossover point is well clear of the critical 3khz region.


10uf and 4mH, sounds like you must be missing a lot of midrange
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