To smoothly transition impedance you can accurately machine a profile in thickness in both materials, then glue them together, or (easier) you can interdigitate both materials so that the transition takes place on the X-Y axis rather than the Z.
I'd like to try something similar with 2mm acrylic and EPS. The idea is to mount an exciter to the acrylic and observe the hf response, then estimate by trial and error (or FEM?) at what radius to begin a transition (with triangles or smooth fingers). Sort of a physical crossover.
What's your reason for wanting to go from acrylic to EPS? To cover different frequency ranges? Another difficulty might be the mismatch of efficiencies,
I'm thinking more in terms of the impedance mismatch at the panel termination. How to terminate a panel with a fixed boundary (simple or clamped supports or in between) but avoid the reflection at the termination. Would a "quasi-clamped" perimeter work where the "clamp" was made of a wide band of very soft but high damping foam?
Maybe this is similar to your foam that varies from thin to thick.
Eric
Thomas,
I would be interested in more details about specifically how you implemented the stepwise concept in your speaker.
Also, can you share how you made the Goebel "cuts" in your carbon/balsa panel. I would like to try that strategy myself.
Thanks,
Eric
Until he tells us, maybe you'd like to revisit this post #8,460 and watch the video?
I was thinking why not use Cling film to cover the balsa panel? Maybe, two or 3 layers? That is, after adding the pore closer, and any other layers on the panel. The Göbel speaker was see through.
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I also like the bendy 3mm ply from the crates, it is more efficient than the more rigid 3mm ply.
it also has a more vibrant sound ,but obviously suited for use in smaller panel designs.
If I remember correctly ,goebel said he used end grain balsa wood but also a flexible cloth and resin to maintain a small amount of flexibility in the panel ?
Steve.
To fill the pores in balsa or some wood ply, maybe one can use water based primer, or oil for parquet, or decking oil, or linseed oil for cricket bats, or some alternatives for linseed oil. By the way, why not use willow as the (ply)panel?
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I was looking for ply panels, and found this flexible or bendy plywood. Actually, I have one sheet taken off a very old cabinet, used as the backing, but it is quite dry, and cracks when I try to cut it. Maybe, I'll apply parquet oil on it to see, if it softens. I doubt it, though. Its still few months to summer.
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Nice build!
2 questions:
How have you attached the exciter to the frame is it flexible or fixed?
The black ribbons on the panel, what are they made of and how to they work?
Thomas
I trying to find my notes from when I was experimenting with this it was in 2018.
But as I recall it, I spent quite some time to search and find different material to use.
I ended up with a stack
Panel CF
Aluminum
Wood
Tile fix cement based
plastazote I also tested with foam made of epdm.
one major flaw in the design is that the ”stack” is exposed thus generating sound as it is vibrating, ouch.
I intend to explore this further.
The cuts are copied from göbel and is cut using a coping saw with a very fine blade.
Thanks,
In both DML builds, there is a block of EPDM foam between the back of the exciter and the spine that connects the exciter to the foam. The foam has adhesive already on one side, and I added a thin pressure sensitive adhesive strip to the other side. I have been using the foam there just to give the connection a bit of compliance in case of some warping of the panel or spine over time. I can't say for sure if it's any better or worse than a rigid connection, but I don't really think there is much effect.
By black ribbons I presume you mean what's on the perimeter of the panel in the image below. That is simply the same EPDM foam (3/8" x 3/4"). Likewise I used some thin pressure sensitive adhesive to attach the back of the EPDM foam strips to the face of the frame. Similar to yours, I think.
Eric
Cardboard is warmer more accurate (less coloration) sounding with less echo/reverb. XPS will sound more lively and dynamic with more coloration.
EPS has a slightly warmer tone then XPS.
AF,
Could be. I concluded some time ago that it made little difference but I can't recall what tests I did or even if I really had any idea of what to look for at the time! I suspect the most of the reaction comes from the mass of the exciter, at least at high frequency, but where is that transition and what happens below it? I don't know.
Coincidentally, just a few days ago I was thinking about this question again. I began to prepare another test of that, but never really got started. Maybe I'll get back on that if I can't think of sopmething better to do.
What's your typical mounting like?
Eric
Yes, to get highs which I’ve not seen from the thruster exciter with XPS. I have to try high density eps first, but I’ve seen on the Typical Ben channel he had a rising hf above 10k with acrylic.
I’ll be a bit scarce next few days as I have finally gotten Covid
The idea of putting some type of foam dampening behind the exciter was from Shelly Katz Podium speakers in which he used it to tune the sound of his panels.
Using any type of damping behind the exciter will reduce reflections and make it sound darker. Its no different the Acoustic Room Treatments but instead of the damping material being on the wall/room its directly on the speakers magnet. This technique is even used on Open Baffle type of speakers that are to close to the wall.
My mounting is like yours except I use Epoxy adhesive if its going to be permenant but if temporary for experiments I just use double sided tape.
This is not free air they’re talking about. Are you proposing to embed the listener into a metamaterial lattice in their living room? Talk about the hard-sell.