Bipolar Panel Speakers

While searching for some ideas to make myself a set of awesome computer speakers, I came across this link:

I wasn't able to find anything else like it, but I am curious as to how these speakers perform...and if they are as good as the guy said they are, I'd like to build myself a set. They look cool, among other things.

Can anyone provide me with some links to sites with info about building that type of speaker -- a bipolar planar speaker?


-= SsZERO =-
I'll try to get some pics up.

I used a cheap 3" full range 4 ohm speaker firing up. It is sealed around the coffee cup.

I covered the front of the speaker with tape, except the opening between the posts (the sound leaves the speaker, goes through this slot, then enters the baggie). Then attached the bag to the poles and sealed it to the slot with tape.

I drilled a small hole in the coffee cup for the speaker wire to exit.

The posts that support the baggie use the speaker mounting holes for spacing.
here's the coffee cup speaker.


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Thanks for the pic and the info! So how does using this planar setup improve performance over using the driver alone? Wouldn't the sound quality of the planar speaker be limited by the quality of the driver you are using? And if you use a driver rated 50W RMS, does that mean you can feed the speaker 50W or does this configuration alter the driver's power rating?

-= SsZERO =-
Sound change - The speaker used is a low-distortion intercom speaker. It sounds remarkably well for music (200-7K frequency response). The bag limits the frequency response (looser bag gives better LF response, thinner bag gives better HF response)

The biggest change is the bipolar radiation pattern; it makes a more diffuse sound. It still 'sounds' like the same speaker, though. Sound quality of the driver is better in a direct radiating (normal) configuration, but is acceptable in the planar configuraton. I imagine that it could be quite good with some refinement (more attention to aerodynamics for the slot, and more rigid mounting - see the upright section below)

The 'uprights' can be made of anything; I used the metal, because it was already threaded. I would add a horizontal support across the top (you can see the bowing of my supports).

This is another fun one!
Yes, this is going to be an interesting project. I've selected two shielded Audax 3" 'full-range' drivers to be the main drivers. Since I will also be making a sub, I'll tune the speakers for better mid and high frequency performance and leave the lows for the sub to handle.

A few more questions, mostly about dimensions:

- Is it best to have the drivers firing UP or DOWN?

- What size should I make the slot? Should it be equal to the diameter/width of the poles I use?

- Does the size of the enclosure I use affect sound quality, i.e. should I try to "tune" the enclosure to a certain resonant frequency?

- How long should the poles be?

Thanks for all your help!!

-= SsZERO =-
I made the speaker firing up, so that the magnet would be out of the way.

I made the poles the same height as the baggie, but there 'should' be some relationship to the displacement of the speaker vs. the panel area. Maybe just to be safe and limit distortion from the bag stretching 'non-linearly', the total displacement of the panel should be 5 (just a guess) times the displacement of the driver.

I made the opening to the panel the same as the width of the poles, but I think a flared opening into the panel would reduce distortion (turbulance).

If we want to put some sort of scientific measurement into this (sometimes that takes some of the fun out), tuning the panel (bag) to the enclosure (cup) and the speaker should be done. I would guess that we'd want to critically damp the speaker/enclosure/panel system.

What are you planning on using for the panel material. I wonder if a balloon would work better?
For the panel material, I was planning to use long and narrow plastic bags made out of a stronger plastic than saran wrap. I don't know if it would hinder the HF performance or not, but I figured I could try various materials till I found one that sounded good. It is my assumption that lighter material = better HF performance. I just want to use something that is light, durable.

There is also this cellophane-like material that you can find at stores like Hallmark or craft stores. It is pretty thin and light, yet it seems to have a high tensile strength, so it could be stretched without a problem.

For the enclosure, I was thinking about using PVC piping -- one pipe would have a 3" I.D. and another would have a 3.25" I.D. The space between the two pipes on the inside would be lined with Dynamat or some other sound-dampening material. The would be a plug made by cutting circles out of wood, and the inside part of the plug would also be lined with dynamat.

Just a guess, but I think a balloon would be like using rubber guitar'd be hard to tune because of its elasticity....whereas something with less elasticity will be easier to tune since its length/shape remains fairly constant.

-= SsZERO =-
I don't yet have a good enough understanding of how these speakers work to say whether mounting the posts on an inward angle, or any way other than parallel, would be good or bad. But here is my 'best guess'. ;)

By aligning the posts inward, you ar effectively changing the 'tuned frequency' of the panel...the lower part where it is wider would be tuned to lower frequencies, and as the posts come closer together you'll get higher frequency tuning.

Since we are using full range drivers, there is no way to control the frequency distribution across the I would predict that you'd experience distortion when low frequency < 500Hz sounds play back and the top part of the panel can only do sounds as low as say 8000-9000Hz.

That other guy's website said that the tighter the panel material is, the lower the frequencies they can play and the more power they can handle. Perhaps using a spring at the bottom of the posts to keep constant tension on them would be a good idea?

When you use these speakers, what is the sound radiation pattern? Does it go away from the panel linearly? Or is it spread out all over the place?

-= SsZERO =-
The radiation pattern is ideally an '8' with the lobes being in the front and back (perpendicular to the panels). The nulls would be on the sides (parallel to the panel). But the speaker vibrates the opening to the panel which messes with the pattern some, but it is still basically an '8' (maybe an oval or an '0' but not a circle).

I think a good panel material with proper tension and aerodynamics is the key to a good design, here.

Hmm...I bet that lining the whole interior of the base enclosure with sound dampening material would help in keeping vibrations down. Additionally, the panel material could be tensioned both horizonally AND vertically to ensure uniform tension across the surface.

Perhaps making the panel unit separate from the base would be even better...though I don't know how much of a problem the speaker's vibration poses, or if it is even worth the extra trouble to make two separate units.

I think you are right about the panel material and aerodynamics being the key elements in this project.

-= SsZERO =-

Matt MacBeth said:
The radiation pattern is ideally an '8' with the lobes being in the front and back (perpendicular to the panels). The nulls would be on the sides (parallel to the panel). But the speaker vibrates the opening to the panel which messes with the pattern some, but it is still basically an '8' (maybe an oval or an '0' but not a circle).

I think a good panel material with proper tension and aerodynamics is the key to a good design, here.

Ah, Mylar...I think that's the ticket. :) Does that guy Barry have a website? I'd like to get a sheet of mylar.

-= SsZERO =-

phishead8 said:
This project looks rather interesting.
May I suggest some sort of heat-shrinkable material for the membrane? I know Barry Waldron sells 4'x10' chunks of heat-shrinkable Mylar. The stuff works wonders for ESLs, and I don't see why it can't work for these too.
Let me know if these are worth constructing, everybody.
You can try and reach him through on the catalog page. That is your best bet. I've had difficulty reaching him in the past, but eventually he'll reply to you. I've used his Mylar on my prototype pair of ESLs and I loved how well it worked. But, I ran out of the stuff for my first "quality" pair. I hope to get some more. Most Mylar on the market is not the same heat-shrinkable stuff. You'd have to do some deep research at if you want to find out which types are heat-shrinkable. Even then, distributers don't always know what type they sell. It was quite a problem for me. Eventually, I just bought the "normal" Mylar and built a complicated stretching jig. It was much easier to just wave a heat gun for the first pair.
Word of caution: Since summer is nigh upon us, the Mylar may come a little wrinkled and pre-shrunk. Do not worry, the Mylar shrinks alot and will iron out any wrinkles for you.
Good luck with the project.