A comparison of OB, H-, M- and U-frame dipole

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Rudolf

A comparison of OB, H-, M- and U-frame dipole
« on: 20 Jan 2008, 05:19 pm »
I believe most of us still wonder how the different dipole frame types really differ from one another and what type to choose. So I built different frames of almost the same size for one driver, the Visaton W250 8 Ohm. All frames are 30x30x30 cm cubes outside, with the I-frame (aka OB) being just the driver mounting baffle for the H- and U-frame.
U-frame


H-frame


M-frame


I-frame aka open baffle


The following measurements where done with the mike on the floor at 60 cm distance from the front plane of the frames. The I-frame was measured at 75 cm distance to put the speaker at the same distance as in the H- and M-frame.
Measurements are not gated, so you see some room response and the influence of the rear wave, but the positions of frame and mike were identical for all frames:



Black line is the "Open Baffle". Red is the H-frame "top down". This means that the H-frame is open to the top. Green is the H-frame as shown in the pictures.
I see this as another demonstration of how much bass energy you loose in "open" bass constructions. Regarding the H-baffle: I know that H-baffles of greater vertical proportions are less prone to bass loss when used with an open top. But for a square H-baffle a closed top seems to be mandatory.
Note that the open baffle looses almost 9 dB compared to a closed H-frame while not having any real benefit up to 600 Hz.

Next diagram compares the front radiation of the H-, M- and U-frame. Up to 200 Hz it is almost the same for all frames, even with the driver in the U-frame being 15 cm closer to the mike than in the H- or M-frame.



Above 200 Hz there are big differencies between the frames. I will discuss them in a following posting comparing the front/rear radiation of the different frame types.
« Last Edit: 20 Jan 2008, 07:15 pm by Rudolf »

JohninCR

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #1 on: 20 Jan 2008, 05:51 pm »
Rudolph,

Wouldn't you agree that these baffle shapes are for use below say 200hz?

Wouldn't you also agree that nearfield measurements of an open alignment in the bass region are pretty much useless, because the rear wave delay distance is significant in comparison to the mic distance?

Where's your damping for the U, since a U-baffle doesn't perform properly without addressing the lumped mass behavior of the air in the cavity?

Don't get me wrong, I applaud the effort and sharing.  I just question the test conditions.

John 

Rudolf

A comparison of OB, H-, M- and U-frame dipole, Part 2
« Reply #2 on: 20 Jan 2008, 07:13 pm »
From the last diagram in my first posting it looks like there isn´t much difference below 200 Hz between the I-, H-, M- and U-frame. So let us look closer how the front and rear radiation vary for each frame:



For the I-frame there is practically no difference between front and rear up to 600 Hz. Equalizing from 500 Hz down and crossing at 2 kHz to a tweeter looks like an easy way, albeit at a very low level of efficiency. But of course that little Visaton driver would bottom out almost immediately. :nono:



The H-frame will peak at 200-300 Hz and needs some more care for those peaks and dips. But the front/rear radiation symmetry is on par with the OB up to 800 Hz.
 


The M-frame peaks at the same frequency as the H-frame, but with a more pronounced peak and following dip. This calls for a much lower X-over point. One could argue whether the better impulse compensation (mechanically) of a two-driver-M-frame is worth the effort. :?
 


At first glance the undamped U-frame looks like the H-Frame, but with much worse front/rear symmetry. But this diagram does not tell the whole story. With the mike 60 cm from the box front, it effectively is 90 cm from the rear when taking the measurement.
With equal distance from the mike to the front and back of the driver the SPL looks like this:



The U-frame calls for detailed compensation of the front/back radiation difference as JohnK has shown. For anybody without the means to tame the U-frame properly, the H-frame looks like the better bass dipole IMHO.
Thanks for your attention. How do you think about it?

opnly bafld

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #3 on: 20 Jan 2008, 07:23 pm »
Rudolf,
Could you make an I-frame with the woofer placed close to the floor @18"w x 24"h ( @45cm x 61cm? ) to measure for comparison to the H-frame?
I appreciate your time and effort to make measurements. :thumb:
                                 

Thanks,
Lin

JohninCR

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #4 on: 20 Jan 2008, 08:42 pm »
Rudolph,

Yes I agree, however, for those of us who use the woofers only to fill in the bass, it's really only the response difference below 200hz that's important.  Damping U's for that purpose is pretty easy.  Some of JohnK's stuff outlines the differences in the alignments, which require mic distances of 2 or more meters to properly quantify.  I appreciate that you're looking at something different, the higher frequency peaks and troughs, which is why you measure that way.

Take care,

John

Rudolf

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #5 on: 20 Jan 2008, 08:49 pm »
Rudolph,

Wouldn't you agree that these baffle shapes are for use below say 200hz?
John,
By and large, yes. Of course these comparisons are not relevant if one wants his driver to reproduce a range from 50-5.000 Hz.

Quote
Wouldn't you also agree that nearfield measurements of an open alignment in the bass region are pretty much useless, because the rear wave delay distance is significant in comparison to the mic distance?
All diagrams show a dipole roll-of of at least 10 dB. So it isn´t nearfield anymore. And these are just comparable measurements under a given condition. No filters, no EQ, no optimized front wall reflections. They are not intended as building instructions  - just as food for thought.

Quote
Where's your damping for the U, since a U-baffle doesn't perform properly without addressing the lumped mass behavior of the air in the cavity?
You were just a bit too fast with your answer.  :wink: Hope the second part of my demonstration shows that I want to rise attention for the shortcomings of an undamped U-frame.

Quote
Don't get me wrong, I applaud the effort and sharing.  I just question the test conditions.
John 
Hey man, you can´t question the test conditions. It´s my posting, and I´m king here. :green:
But of course you can question the relevance of my findings. That´s up to everybody reading this. I just do what I can. :thumb:
BTW: And certainly I would not mess with chipboard boxes in the rainy winter if I had the means to be in CR like you. :drool:

Rudolf

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #6 on: 21 Jan 2008, 08:28 pm »
Rudolf,
Could you make an I-frame with the woofer placed close to the floor @18"w x 24"h ( @45cm x 61cm? ) to measure for comparison to the H-frame?
Lin,
the gods are with you! :wink: I found an old piece of chipboard 50x70 cm. I allow for ONE cut to size it for your taste. Tell me where. And do you want the OB standing or with the long side to the floor?

@ all else: I don´t intend to make it a habit building and measuring baffles for individual members of this fine forum. :lol:
So please refrain from asking what else chipbord sizes I have in my basement. :green:

JohninCR

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #7 on: 21 Jan 2008, 10:44 pm »
@ all else: I don´t intend to make it a habit building and measuring baffles for individual members of this fine forum. :lol:
So please refrain from asking what else chipbord sizes I have in my basement. :green:

 :scratch: How about measuring the same baffle at a few different distances? aa

John

mcgsxr

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #8 on: 21 Jan 2008, 11:45 pm »
Do I steer the whole thing wrong by asking what you think sounds the best?  I would be interested in that.  I know that the measurements are of significant interest to many, but I would like to try to tie some subjective impressions (yours) to the graphs that I see.

Just my ask!  I will stay away from asking about the various sized boards, unless you happen to have a 90cmx120cm...

opnly bafld

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #9 on: 22 Jan 2008, 12:31 am »
Rudolf,
My thinking was 45cm wide (I just threw out a # on height  :scratch:) because a few OB'ers feel that @45cm (17" to 18" for us odd balls) is a good compromise between bass rolloff and imaging.
I'm curious as to how an I-baffle of this width (your piece of chipboard acting as the lower half of a full baffle with a FR driver on top) measures compared to your H-baffle.

Thank you very much,
Lin

PS- I will PM you all the other sizes to cut.  :lol: 

scorpion

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #10 on: 22 Jan 2008, 12:45 pm »
Let me contribute with a Topless-U measurement. It is my 'Volks-OB' Bass unit A&D R1524 with no crossover that is measured 65 cm in front and back.
The red curve is the front measurement. In room measurement and mic as Rudolf put it. Resolution is 1/24 octave.



I think the Topless-U also deserves a place in this discussion on the basis of this measurements. With a suitable element the Topless-U can also be made
very small as these simulations with MJK's Math-Cad models prove. It is with the actual A&D R 1524 15" speaker, the upper simulation is with the actual
height of 60 cm and 30 cm 90 degree wings from the 40 cm wide baffle, the lower simulation is with 45 cm height and same 30 cm wings.



/Erling
« Last Edit: 22 Jan 2008, 01:55 pm by scorpion »

scorpion

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #11 on: 22 Jan 2008, 02:30 pm »
Lin,

No estimation but the simulation of my A&D 1524 on a 45 x 60 cm baffle looks like this (Bass-center 25 cm above floor):



/Erling

Rudolf

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #12 on: 22 Jan 2008, 05:15 pm »
:scratch: How about measuring the same baffle at a few different distances?
John
Good idea, John. I will make that part of the agenda.

My thinking was 45cm wide (I just threw out a # on height  :scratch:) because a few OB'ers feel that @45cm (17" to 18" for us odd balls) is a good compromise between bass rolloff and imaging.
I agree. 45 cm seems to be a common width. Hopefully I can get at it before the weekend.

Do I steer the whole thing wrong by asking what you think sounds the best?  I would be interested in that.
I have not listened to anything but test tones with these frames. But I believe that it depends totally on the proper x-over and EQ, whether those frames sound well or not.
If you look at the comparison of the different frames (second diagram of my first post) there is practically no difference below 150 Hz. Everything that will make them sound different stems from the different response above 150 Hz. If you put a 24 dB/oct low pass filter at 80 Hz, you hardly need any compensation above 150 Hz and all frames will sound the same.
Without a x-over, the H-frame is +/- 5 dB from 200-2000 Hz, which isn´t too bad sounding. The M-frame will have +/- 10 dB, which IS bad. The U-frame has some peaks around 200 Hz but doesn´t look too bad elsewhere.

opnly bafld

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #13 on: 22 Jan 2008, 06:06 pm »
Lin,

No estimation but the simulation of my A&D 1524 on a 45 x 60 cm baffle looks like this (Bass-center 25 cm above floor):


How close to the floor is the bottom edge of driver?

Thanks,
Lin

scorpion

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #14 on: 22 Jan 2008, 06:12 pm »
Just 5 cm (2") from the floor both in the real OB and the simulation.

/Erling

Rudolf

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #15 on: 22 Jan 2008, 09:19 pm »
Hardware is on location. Measurements will have to wait till tomorrow:



Bob in St. Louis

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #16 on: 22 Jan 2008, 10:58 pm »
Rudolf, - After you test Lins baffle with the driver on the bottom would you flip it "upside down", and see what it does with the driver on top?
Thank you!

Bob

EDIT: And move the mic vertically to correspond to the center height of the driver too please.

opnly bafld

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Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #17 on: 22 Jan 2008, 11:09 pm »
Rudolf, - After you test Lins baffle with the driver on the bottom would you flip it "upside down", and see what it does with the driver on top?
Thank you!

Bob

EDIT: And move the mic vertically to correspond to the center height of the driver too please.

It will lose a lot of output.

Lin

D OB G

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #18 on: 23 Jan 2008, 01:29 am »
Seconded from Bob in St. Louis.

Rudolf, I suppose Bob's request is OT, but it would be very instructive, and in particular for those of us who use a separate midrange baffle, either for isolation reasons, or to optimise the step response, to see what happens.

Having done a lot of simulating in The Edge, there can be advantages in midrange reponse in having the speaker near the top of the baffle for the midrange/full-range (lots of qualifications).

If you can see your way clear......?

Thank you.

David

Rudolf

Re: A comparison of OB, H-, M- and U-frame dipole
« Reply #19 on: 23 Jan 2008, 10:24 pm »
My results comparing these dipols: H-frame of 30x30x30 cm and OB of 70x45 cm.
 


Please note that all measurements were done with two drivers. I did not exchange one driver between H-frame and OB. All  measurements were done with the dipoles at the same position. All measuring distances are from the driver plane. Since the measurements are not gated you see a lot of room influences. But they are absolutely identical for both dipoles.

I start with a direct comparison of OB and H-frame. Mike positions were at 60, 120 and 260 cm distance and 0, 70 and 110 cm height from the floor respectively.







Thank you Lin for suggesting the 70x45 cm baffle. It looks like it has just a little less bass efficiency than the H-frame.

Next two pictures show the response for each dipole with rising distance:





Next I turned the OB upside down. The driver is now at 56 cm height. Measurements at 120 and 260 cm show what is lost and gained when comparing to the woofer at the bottom:





My last diagram is a comparison of the "high" driver measured closely (20 cm) on axis and at 120 and 260 cm distance. It will give you an idea how the dipole roll of becames more severe with rising distance.



Interpretation is up to the forum community. :scratch: