Dipole H/W frame using 15" drivers - diyAudio
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Old 17th May 2007, 08:45 PM   #1
arc2v is offline arc2v  United States
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Default Dipole H/W frame using 15" drivers

Here's the rundown on a project I'm looking to start:

Dipole (or dipole'ish) subwoofer to compliment my Magnepan 10's in a home theater (and music). They play nicely down to about 80 Hz, so I only need something to go lower than that.

I was considering this driver:
http://www.partsexpress.com/pe/pshow...number=295-455

15" driver for IB configuration.
Single voice coil (8 Ohms)
Qts = 0.65
Fs = 20.8
Power = 350W RMS/600 Peak
Xmax = 14.3 mm

I was thinking of putting it into a W or H box. I'd like to minimize the size (as much as possible), and of course, have it sound good.

I read about 100 threads here today searching on "dipole" and other terms and think I have a handle on some of the factors involved.

I've always had a hard time picturing people's W and H designs, though.

So a quick list of questions:
1) is this a good driver for a dipole application?
2) any advice on sizing the frame?

I'm sure there will be more, but I appreciate the help.

Thanks,
AC
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Old 17th May 2007, 08:52 PM   #2
arc2v is offline arc2v  United States
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Clarification:
I plan on using 2 of the drivers mentioned above.
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Old 8th June 2007, 06:06 PM   #3
Sjef is offline Sjef  Netherlands
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It's very well posible to build a w-frame with these woofer. I have build a w-frame with two JBL 15" pro woofers and it only measures 42,5*42.5*42.5 cm and is just big enough to fit the two woofers, no more. It goes low to about 30Hz with reasonable output below that and it's usuable to 100Hz at max with at least 24db/oct filtering, usage to 80 Hz would be preferable due to the very strong resonance at 240 Hz with very long decay time. this resonace is not in the woofers but in the design principal and should be avoided at all times, sound really horrible when you don't. Also it does need some low boost eq (mine have a q of 0,31), also by design principal but it's ver easy to apply. With a eq of 0,65 like your woofers and when you only use them to 80hz with 24db/oct you might try it without any eq, might work as well.

Be sure to make the sidewalls very strudy though. The woofers are firing right against it at a very low distance. I have used a sandwich construction of 18mm multiplex, 4mm bitumen and 12 mm multiplex again and this is barely enough, if you use only 18mm multiplex the sidewalls produce about as much energy as the woofers themselves and it sounds terrible. I have build these woofers about 5 years ago starting as a joke with some old drivers I had, just had to do something with these 4 15" drivers and didn't want to build a 260 liter reflex enclosure for them. It turned out to be so good that it's still in use in my system today. go for it.
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Old 8th June 2007, 06:13 PM   #4
arc2v is offline arc2v  United States
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Hey thanks for the response. Glad you like your setup.

This project got put on hold, as I'm now experimenting with an MTM open baffle design for a center channel speaker.

I'm hoping to be done mid-summer and start on the woofers then. Right now the learning curve is steep, as I am learning the FRD tools, speaker workshop, room acoustics, etc.

thankfully I already know the woodworking side

AC
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Old 8th June 2007, 07:24 PM   #5
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i have one of these in a 18"x18"x22"D U-frame and when properly spaced from the walls the f3 is about 24 Hz.

I eventually will have two of them as part of 4 way dipoles, but due to school it is just one unfinished woofer and I am very happy with its sound. Theoretically it should not play this low, so if your room is not like my studio apt. (long hallway and half wall/bar on back wall) then you will most likely have a higher f3 with the above box dimensions.

Good luck

John
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Old 9th June 2007, 09:44 AM   #6
Calvin is offline Calvin  Germany
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Hi,

I rather disagree with Sjef´s opinion.
Your premise is adipole as small as possible!
The problem with smaller dimensions is, that the freeair resonance of the driver sinks considerably (5-10Hz usually, 15Hz possible)
So the right driver should rather feature a Fs above ~25Hz (I usually choose around 30-35Hz) when You plan to reach down to 20Hz!

I don´t like to use drivers with high mass, weak motor and high voice coil inductance. They simply don´t play as precise as I like it. Too my experience showed me that larger drivers are better off with a bit lower Q-factor. While 0.65 is fine for drivers up to 8"-10", I wouldn´t choose a 15" driver with condsiderably higher Q than 0.4!
Together with the elevated Fs, light moving systems and low inductance are often found amongst PA-drivers. There are hundreds of those around and even some long stroke types.
A example with a very good set of parameters is the Beyma 15LX60.
This driver is really well suited to small folded dipoles, because it builds only 14.5cm in depth. So if You want to build a 2-driver dipole -similar to SL´s-W-frame, this could be an option:
Drivers: 2x 15LX60 (facing each other with the membrane sides and sunk in 11mm into their mounting plate)
2x back chambers: 40.6x10x38.8cm (HxWxD)
1x front chamber: 40.6x11.2x38.8cm (HxWxD)
Using 22mm wood results in 45x40x41cm (HxWxD), which is even smaller than the shipping cartonage of the two drivers!!
This is so small, that the magnets of the drivers ´break through´ the sidewalls. Since with the Beymas the magnets and poleplates are really big and impressive pieces this would give optically a quite ´technical touch´
Simulation-data:
Fb: 20.5Hz, Fupper-peak: ~265Hz
Adding the passive filtering to kill the upper peak and to linearize the frequency response -which I always recommend- the fb sinks further down to 18Hz. Frequency response is very linear between 31Hz and 180Hz (-3dB) then (22Hz-205Hz/-6dB).
Excursion limits:
@20Hz/17V/>105dB || @30Hz/26V/>112dB || @40Hz/39V/>117dB (1m).
The filter consists of a series inductance and a parallel Notch
(Lser: 2.7mH/0.4-0.6Ohms | LCRpar: 0.82mH/420µF/0.4Ohms)
This should be a fine response for many uses, but I suggest to add an active subsonic filter which can be configured as a bass boost too. A slight boost of +3dB..+6dB@30Hz (integral part of many sub-amps) will provide for the fun factor and prevents the sub of excessive strokes when playing vinyl sources.


The small build and the filter sink the free air resonance by app. 17Hz! This should proove why the Dayton driver is imo useless for small dipoles. It´s built-in-resonance would be far too low to be of any good. It might be ok for large dipoles like open baffle, but not for small folded ones.

jauu
Calvin
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Old 9th June 2007, 10:37 AM   #7
Sjef is offline Sjef  Netherlands
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Hi Calvin, this design looks a lot like mine, The JBL and the Beyma woofers have quite similar specs and construction. I didn't build the w-grame with the magnets trough the sidewalls though so it is a bit bigger than yours but measures almost the same. I have a low resonance point of 16 Hz and an upper peak at 230Hz.

When you have the chance try and measure the decay spectrum of the w-baffle. I did once. The resonance at 230 Hz in my case has a very long decay wich is very audible. At this point the input has to be attanuated at least by 30 dB, preferably more. This can be done with a notch filter or by using a lower crossover point with steep filters. When done properly bass is very deep, full and tight.
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Old 10th June 2007, 03:13 AM   #8
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Default Worth mentioning...

Calvin has provided the important driver specifications to look for in a dipole woofer. It is worth mentioning that driver noise (rear side) is the first thing to consider as this form of noise (chuffing) will exclude an otherwise excellent selection as far as the numbers go. Vented spider assemblies are worth looking for but don't assume that if a driver has spider vents it will automatically be quiet. Running low frequency sine wave into the driver while you hold it in your hands will tell the storey.
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Old 16th August 2007, 08:40 AM   #9
navin is offline navin  India
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Default Re: Worth mentioning...

Quote:
Originally posted by moray james
Calvin has provided the important driver specifications to look for in a dipole woofer..
one popular OB 15" woofer (Hawthorne's Augie 15) has a Qts 0.92!
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Old 16th August 2007, 10:02 AM   #10
el`Ol is offline el`Ol  Germany
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Not 15", but probably good for a Z-dipole: The Eminence Kilomax 18.
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