help designing a magnetic planar woofer
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 16th February 2010, 01:06 AM #1 baffleman   diyAudio Member   Join Date: Jul 2005 Location: house 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
 16th February 2010, 12:45 PM #2 båndsei   diyAudio Member   Join Date: Dec 2008 Hey Baffleman. You tune the membrane to a certain frequence,you select yourselves wich one.
 16th February 2010, 10:03 PM #3 baffleman   diyAudio Member   Join Date: Jul 2005 Location: house 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
 17th February 2010, 07:43 AM #4 kavermei   diyAudio Member     Join Date: Jan 2005 Location: Lokeren, Belgium 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 __________________ Never send a human to do a machine's job. --Agent Smith
LineArray
diyAudio Member

Join Date: Mar 2008
Quote:
 Originally Posted by baffleman ... 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
__________________
Oliver, RFZ believer (?)
www.dipol-audio.de

Last edited by LineArray; 17th February 2010 at 09:45 AM.

 18th February 2010, 01:55 AM #6 baffleman   diyAudio Member   Join Date: Jul 2005 Location: house 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 01:56 AM. Reason: spelling
tinitus
diyAudio Moderator R.I.P.

Join Date: Nov 2005
Quote:
 Originally Posted by baffleman 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

LineSource
diyAudio Member

Join Date: Sep 2003
Location: SiliconValley
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
Attached Images
 EminentTech-LFT-8b-4.jpg (42.2 KB, 625 views) EminentTech-LFT-8b-2.jpg (36.8 KB, 618 views) scenium_woofer_3.jpg (36.1 KB, 620 views)

 18th February 2010, 10:04 AM #9 forr   diyAudio Member     Join Date: Dec 2004 Location: Next door 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
 18th February 2010, 10:10 AM #10 tinitus   diyAudio Moderator R.I.P.   Join Date: Nov 2005 It gets expencive very fast I would like to use the superior Rubanoide magnet system, somehow

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