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Old 16th February 2010, 12:06 AM   #1
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Default help designing a magnetic planar woofer

Hi, i am part way thru designing a magnetic planar woofer like an apogee.
Using the efficiency formulas supplied by linesource, i have come up with a design
that i would like to build, however there are a few details i dont understand.

How can i calculate the Qes of the speaker when i havent built it yet?

So far i have Sd= 0.9M^2
Re= 8 ohms
mms= .03Kgs
BL=3
The only formula i have found wants Fs as well.
How to find Fs in an unbuilt panel??????

Any help appreciated
Tony
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Old 16th February 2010, 11:45 AM   #2
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Hey Baffleman.
You tune the membrane to a certain frequence,you select yourselves wich one.
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Old 16th February 2010, 09:03 PM   #3
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Bandsei thanks for that, i was hoping to be a bit more "scientific" and try to work out all the thiele small parameters before i buy lots of magnets.

As this is an open baffle, i would like to compensate for the 6db rolloff by making the woofer q=2 , like what carver did with his amazing.

Tony
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Old 17th February 2010, 06:43 AM   #4
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Hey Tony,

I think, what Bandsei meant, you can tension the membrane to any Fs you want. Thus, you can work out what Fs you need if you want a certain Qes. While tensioning, you can measure the membrane's Fs by tapping it. Or by exciting it with an external sound source (function generator+amp+speaker) and checking for maximum response.

Kenneth
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Old 17th February 2010, 08:26 AM   #5
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Quote:
Originally Posted by baffleman View Post
...
As this is an open baffle, i would like to compensate for the 6db rolloff by making the woofer q=2 , like what carver did with his amazing.

Tony
Hi Tony,

Qms is also affected by the structure behind the membrane.
The perforation of the plate/sheet holding the magnets will
affect damping.

Qts=2 seems a fairly low Q for a tensioned membrane.
Mechanical part of the damping will mostly be provided
by the structure behind.

If damping turns out to be too low, a layer of fabric fixed
on the rear perforated plate might be a way to go, or chose
a plate/sheet with less free area.

The right fabric fixed directly on a more open perforated
sheet might yield better results (even if it might not look as cool).

Kind Regards
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Last edited by LineArray; 17th February 2010 at 08:45 AM.
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Old 18th February 2010, 12:55 AM   #6
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thanks to all who answered, some good ideas there.

Linearray i wasn't aware that the magnet structure could influence Qms so much.
It appears i have to build a test panel first, to find out all the numbers and then build a "real" panel with the results of the tests.

Many thanks
Tony

Last edited by baffleman; 18th February 2010 at 12:56 AM. Reason: spelling
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Old 18th February 2010, 02:38 AM   #7
tinitus is offline tinitus  Europe
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Quote:
Originally Posted by baffleman View Post

As this is an open baffle, i would like to compensate for the 6db rolloff by making the woofer q=2 , like what carver did with his amazing.

Tony
Hmm, Carver ? that was a dynamic woofer with a long ribbon, wasnt it ?

I suppose a big planer woofer module already on hand has high Q

But true, its very clean and detailed, and misses some of lower midrange/midbass richness
I expect it will always be that way

And be aware of the so called "drumskin effect"

There have been a very innovative and funny one
A big plastic film streched out in front of a woofer, with closed air in between
Strangely, It actually sounded ok
Maybe not 100% correct, but I remember it being very "musical"
Back then, I just didnt want to pay the money
Well, I also thought it was a bit suspect, at least fore a commercial product
But to DIY it, could be fun
I wouldnt be surpriced if mr Lyngdorf holds a patent on that one
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Old 18th February 2010, 03:55 AM   #8
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These Theil/Small equations might be helpful for a paper design you can prototype and measure.

The Large Apogee panel had a Qts varying from 2 - 2.4. Fs ~ 24 Hz. The panel was made out of Kapton, and clamped to a wood perimeter frame. The wood frame had adjustable springs to connect to the exterior cabinet. Adjusting the springs allowed setting the Q, and more importantly, allowed Q variation over the length of the frame to control Q and Fs. This avoids resonance and puts the highest Q near the floor for bass gain and a lower Q on the top area for clarity.

The push-pull frame used by Eminent Technology looks like a good design to study. A push pull motor with NdFeB magnets would be worth modeling in FEMM.

To validate your work and cost, you can look at AE Speakers 15" Infinite Baffle woofers @ $125 each to compare air volume moved. There are many free tools like the Edge that allow you to model flat frame and H-frame dipoles. You will find people on this forum that stack 3-4 dipole woofers to get monster bass.


Vas = 1.4E8 x Cms x Sd^2
Cms = Vas \ (1.4E8 x Sd^2)
Sd = the square root of {Vas \ (1.4E8 x Cms)}
Fs = 0.159 x the square root of (1000 \ (Mms x Cms))
Mms = 1000 \ (Cms x (Fs \ 0.159)^2)
Cms = 1000 \ (Mms x (Fs \ 0.159)^2)
no = 9.78E-10 x Vas x (Fs^3 \ Qes)
Sensitivity = 112.2 x 10log10 "no"/log10
Rms = ((6.28 x Fs) \ Qms) x (Mms \ 1000)
Qes = (6.28E-3 x Mms x Re x Fs) \ Bl^2
Qts = (Qes * Qms) / (Qes + Qms)
Vas is in liters
Cms is in meters per Newton m/N
Sd is in meters squared
Mms is in grams
Re is in ohms
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File Type: jpg scenium_woofer_3.jpg (36.1 KB, 548 views)
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Old 18th February 2010, 09:04 AM   #9
forr is offline forr  France
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Here are some details of a planar subwoofer called Infraplanar, designed by Claude Lacroix. Resonance is about 10 Hz.

INFRAPLANAR haut parleur isodynamique plan pour grave

INFRAPLANAR : mesurse, écoute ...etc

INFRAPLANAR
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Old 18th February 2010, 09:10 AM   #10
tinitus is offline tinitus  Europe
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It gets expencive very fast

I would like to use the superior Rubanoide magnet system, somehow
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