Sebastian,
I must admit I did not know much about guitar bracing until I started looking at your links. But the parallel bracing of the JMC speaker made perfect sense to me when I first saw it, as I assumed they were simply bracing the panel in the direction perpendicular to the grain direction, since the stiffness of wood in the grain direction is something like 10 times it's stiffness in the perpendicular direction. Now I see that the JMC bracing is similar to the bracing of a guitar back. I wonder if the complicated bracing of the guitar top might be largely driven by the fact that it has a big hole in it, combined with the fact that it has to be able to avoid buckling under the load of the strings. And hence, that just maybe, since a dml panel doesn't have a big hole, or strings, then maybe the simple perpendicular bracing is all that's really needed for a dml. Just speculating here, of course.
The curvature of the JMC speaker is very interesting too. Certainly the curvature affects the stiffness too, just like the bracing does.
Also, I should add that another crossover between dml's and guitars has also been discussed on this forum before. And that is the concept of "doubletops". As you may know, doubletops are guitar tops made with a nomex or similar honeycomb between two thin skin layers. Interestingly, however, I think "doubletops" were being used for dml's since the 1960's, but are a relatively recent innovation in guitars.
Eric
Christian,
As Sebastian has pointed out, the face layer of veneer can be very thin indeed on some of these "underlayment" plywoods. The ones I have seen and tried (revply, triply, sureply) are all 5 layer plywoods, with relatively thin face veneers. In the case of revply, I would estimate that the face veneers is well under 0.25 mm! (so thin you can actually sand through it very easily if you try). As a result, the stiffness in the apparent "with grain" direction is actually only 1/3 of the stiffness in the opposite direction! The other two (revply, sureply) have thicker (but still thin) face veneers, such that the stiffness in the two directions is actually similar. This is much different from the 3 layer birch plywoods, in which all the layers are about the same thickness, and which are much stiffer in the apparent "with grain" direction than in the perpendicular direction.
Eric
Ha!
This seems to be an impressive panel, Christian!
I can vaguely remember somebody saying that one needs a very stiff surface, that the exciter is glued to, to produce good HF.
I would have considered Balsa rather soft/flexible.
What are the dimension here?
Best wishes,
Sebastian
Bracing Tests:
As I'm in the middle of a DML project using 3mm poplar, I thought it was worth testing out the effect of panel bracing ala Sebastian.
My panels are 900x300, so a lot smaller and 'skinnier' than Seb's but the 3mm ply is quite flexible even after coating so it should be a reasonable test candidate.
My approach:
Lay masking tape out out the proposed patterns
glue 10mmx10mm cedar strips to the tape (basic I know but I'm only rough testing)
This both sticks very firmly to the ply and yet allows easy enough removal after you get it started ( I do the same when testing exciter positions)
Unstiffened and leaning against a stool with a foam separation pad, these panels slope off @ approx 4-5dB per octave down from 100Hz. Above 250Hz they are quite well behaved and capably reach 15kHz. Between 100 and 250 they get a few wobbles but nothing that I can hear. Pretty good for this size panel I think.
So I tried various bracing patterns (W, V, Lattice, W with a fan piece, Ladder, etc, etc).
What did I find:
1/ The bracing had no measurable effect (negatively or positively) from 200 Hz upwards. It was as if it wasn't there.
2/ It caused an effect in the lower frequencies, but it was more of a disruptive effect with more peaks and troughs than with no stiffening.
3/ There was no discernible extension to the bass region response except for a shallow V bracing case where a large (unusable) resonant hump occurred @ about 50Hz.
My conclusion (for my panels at least) is that I can find no acoustic benefit in pursuing this type of bracing. There may be a golden result somewhere but I didn't find it.
Eucy
As I'm in the middle of a DML project using 3mm poplar, I thought it was worth testing out the effect of panel bracing ala Sebastian.
My panels are 900x300, so a lot smaller and 'skinnier' than Seb's but the 3mm ply is quite flexible even after coating so it should be a reasonable test candidate.
My approach:
Lay masking tape out out the proposed patterns
glue 10mmx10mm cedar strips to the tape (basic I know but I'm only rough testing)
This both sticks very firmly to the ply and yet allows easy enough removal after you get it started ( I do the same when testing exciter positions)
Unstiffened and leaning against a stool with a foam separation pad, these panels slope off @ approx 4-5dB per octave down from 100Hz. Above 250Hz they are quite well behaved and capably reach 15kHz. Between 100 and 250 they get a few wobbles but nothing that I can hear. Pretty good for this size panel I think.
So I tried various bracing patterns (W, V, Lattice, W with a fan piece, Ladder, etc, etc).
What did I find:
1/ The bracing had no measurable effect (negatively or positively) from 200 Hz upwards. It was as if it wasn't there.
2/ It caused an effect in the lower frequencies, but it was more of a disruptive effect with more peaks and troughs than with no stiffening.
3/ There was no discernible extension to the bass region response except for a shallow V bracing case where a large (unusable) resonant hump occurred @ about 50Hz.
My conclusion (for my panels at least) is that I can find no acoustic benefit in pursuing this type of bracing. There may be a golden result somewhere but I didn't find it.
Eucy
I was also in the idea of a bracing perpendicular to the grain as in piano soundboard to get the stiffness.
The number of bracing for the top of guitar is simply incredible. Reading a bit around, it seems not being only a question of resistance but also of sound.
Looking about double top, I read some guitar designers have even not bracing in that case, others have...
Christian
This is a canvas panel where the canvas acts like a membrane more than a plate. The canvas is a linen one, 41x33cm. At center, a pad of balsa is glued (PVA): one layer 15x10cm, rounded corners and then a second layer 10x10 grain in the perpendicular direction of the first layer. The exciter is epoxy glued at the center.
What is impressive is the 1st mode which gives bass to this low dimension panel. 40Hz from a 41x33cm speaker without rear load. There is a lack of level at 150Hz and 220Hz.
Even if I prefer the sound of my 3mm plywood panel, it is a good wide range.
Christian
Cool tests. I suspect a wider panel would show more effect. Also, your cedar strips have a very different geometry from traditional bracing. Your tape is very thin I assume? Like the VHB and not thick for tape?
Yep - Masking tape is paper tape - so paper thin - very useful for testing stuff out
Eucy
A couple of things I got from the two videos about guitar bracing.
First surprising thing is that the majority of brace tuning is done with a free soundboard, after which they glue it to the guitar!! This would change all sides from no support to clamped support, but it works. Something deep going on there - maybe some sort of duality in vibration modes?
Second thing I noticed is the second video, where he described the cross-dipole mode as the most 'projective' mode for a nylon strung guitar. This is a mode with + and - areas of supposedly equal pressure. These modes tend to cancel on a panel. Maybe the fact that the guitar is most highly curved around those 2 regions helps it to escape to an edge and get launched in different directions, before cancellation kills it.
Cross-dipole mode:
First surprising thing is that the majority of brace tuning is done with a free soundboard, after which they glue it to the guitar!! This would change all sides from no support to clamped support, but it works. Something deep going on there - maybe some sort of duality in vibration modes?
Second thing I noticed is the second video, where he described the cross-dipole mode as the most 'projective' mode for a nylon strung guitar. This is a mode with + and - areas of supposedly equal pressure. These modes tend to cancel on a panel. Maybe the fact that the guitar is most highly curved around those 2 regions helps it to escape to an edge and get launched in different directions, before cancellation kills it.
Cross-dipole mode:
If anyone is truly interested in knowing what the natural frequencies of their panels actually are, it's not hard to do. And it might even help you understand why your panels work well, or not. For simple designs, it's not too hard to figure our which vibrational mode corresponds to which frequencies. It would be harder for more complicated panels (with bracing for example). But it would probably be interesting for many of you, in any case.
https://www.diyaudio.com/community/...r-dml-design-and-analysis.383567/post-6954047
Eric
https://www.diyaudio.com/community/...r-dml-design-and-analysis.383567/post-6954047
Eric
I really enjoyed the video, even got my wife (a novice guitar player) to watch it with me. But I caught those same two points. With regard to the first one: my guess is that with enough experience, one can learn what sound in a correctly braced (but unmounted) guitar top, corresponds to a good sounding fully constructed guitar, even if they are not the same.
With regard to the second one, my guess is that what he thinks is happening, isn't really what's actually happening. But I could be wrong...
The thing that made me laugh a bit was when he got to a part when I thought to myself "now this is getting interesting" and then he continued by saying something like: "some people are interested in this, but not me"!
Eric
Check out Mayht technology it a true bipolar driver https://www.mayht.com/technology
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About bracing...
I just started some tests by adding some flexible bracing to the canvas panel (see post #5419, 33x41cm panel). For now it is "just a X cross" bracing. The consequence on the FR seems in the way Eucy described : the humps at 55 and 400Hz are higher, the dips are less pronounce. But it seems not more "flat".
Does somebody have a deterministic approach for the next steps? Should I add "fingers" to get a flatter FR?
Christian
I just started some tests by adding some flexible bracing to the canvas panel (see post #5419, 33x41cm panel). For now it is "just a X cross" bracing. The consequence on the FR seems in the way Eucy described : the humps at 55 and 400Hz are higher, the dips are less pronounce. But it seems not more "flat".
Does somebody have a deterministic approach for the next steps? Should I add "fingers" to get a flatter FR?
Christian
Sebastion
I apologise in advance but having seen this example I do believe that the SurePly/RevPly is not the best choice for a DML panel and the excessive flexibility shown in the photo is the reason a lot of bracing is required and why the HF response is subdued. It seems a shame to go to such a lot of work to overcome shortcomings in the basic panel material.
A stiffer base material will afford significant benefits
I was (very) unconvinced about using plywood for DML panels. I tried locally available 3mm building ply from our equivalent of Lowes (Bunnings) and it was bad- floppy and very dull.
My first build was therefore made from solid Western Red Cedar boards (stored for over 20 yrs in my garage) laboriously thinned down from 12mm to 4mm and they sound excellent.
After watching @typicaben's bamboo ply panels demo on YouTube (
The material Ben used is called LaserPly (https://plyco.com.au/collections/laserply)
I ordered a sample pack and tap tested them and then tried each with an exciter. The bamboo Ben used was I thought perhaps the dullest but others were worth a try.
In possibility order the first 4 were Tas Blackwood, Poplar, Hoop Pine and Birch. Surprisingly, the poplar sample, which was 6mm thick (2x the others) was very bright and the loudest under excitation and way different from my Bunnings ply material.
I subsequently ordered 3mm Blackwood (5ply) and 3mm Poplar (3ply) sheets and cut them into 900x300 panels for testing. The Blackwood (Acacia melanoxylon) is 2x the density of poplar and very stiff. The 3 ply poplar has low transverse stiffness due to the 3ply construction.
Under testing I found that the Blackwood gave a nice tone and a little more even frequency response than the poplar but because of it's density, it was harder to drive with my exciters (DAEX25SHF4 Steered High Flux 25mm Exciter 20W) even though they pack a sizable punch for their wattage. So currently I'm focussing on the poplar even though it isn't as visually appealing as the Blackwood.
Get to the point Eucy!!
My (longwinded) point is that based on my tests, I reckon a good quality 5 or even 6mm 5 ply genuine poplar panel like the laserply (ie your local equal) should give a significantly better result than the RevPly with far less need for support and for the low cost should maybe given a test before you install the current panels. The poplar appeals because it is a lot lighter than the other species which is important for a ceiling install.
Best Wishes
Eucy
Attachments
My exciters arrived!
Unfortunately, this week is very busy for me and I won't be able to work on the panels before the weekend.
However, based on Eucy's post I realizedthat something must have gone wrong when I copy&pasted the url from my browser:
This is the panels that I got. I am so sorry Christian:
https://www.lowes.com/pd/RevolutionPly-5mm-Poplar-Plywood-Application-as-4-x-8/50121135
And as an additional note, the 4 ft version does not bend as much as depicted in the picture.
Sebastian
I haven't even tried plywood yet, so I could be wrong, but I would have thought a marine plywood would give the best stiffness to weight. They are bonded with phenol formaldehyde under heat and pressure, and generally use A grade veneers for both the interior and both sides. Laser ply, AFAIK, is non-structural and used for creating laser-cut decorations, signs, wedding invitations etc, and the facing layers are used for appearance only.
Interesting feedback Eucy.
I am happy you come also to the conclusion that poplar is a good candidate.
The advantage of its low density is for me mainly in the efficiency.
As you made tests with different stiffness and thickness for the same dimensions, you should have different results also in bass extension.
I don't understand how you can conclude from the picture that the revply is not a good material... The results from other revply DML builders are if remember quite good. The picture shows simply the bending of a low thickness plywood.
If you have measurements to share, it would be interesting for all to see the behavior of different woods.
Christian
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