https://www.diyaudio.com/community/...s-as-a-full-range-speaker.272576/post-7233820
I have allready shared this in the DML thread.
I think its usefull i.e. makes it more feasible to work with impedances.
I have allready shared this in the DML thread.
I think its usefull i.e. makes it more feasible to work with impedances.
Burnt
I had doubts seeing the equation. Going a bit further into it, it seems not correct from units homogeneity. The mechanical impedance is the ration force (N) to speed (m/s) so Ns/m. 8.sqrt(B.µ) is correct about that with B the bending stiffness in Nm and µ the mass per surface unit similar to rho in the equation you proposed in kg/m². I have a doubt about E being the young modulus or the bending stiffness but in both case seems not match.
Is the impedance at the edge different to the impedance at the driving is just a question. Coming back to the idea of force to speed ratio, I wonder if they are the same. At the driving point, the force has to push against the membrane material all around (full plane), at the edge, the force "sees" less material (half plane)? May be wrong... Nevertheless technically I see here a possibility of evaluation by simulation. By FEM I don't have enough knowledge at the moment but with FDM (Finite Difference Method) seems possible. At the driving point why not, at the edge, which conditions to consider? Free edge? If somebody has an idea...
Christian
No I dont beleive its infinate at the edge. Its the difference in impedance of the diaphragm and the "surround" that causes reflection. If the "surround is air then the difference is huge and the reflection be close to 100%. If it is EPDM then its a closer match thus less reflection.
There "two" waves to take care of the incident wave from the exciter carrying the music and potential standing waves i.e. the natural frequencies.
Thomas
Hi Thomas,
I have seen this paper in the "main" thread but I don't really what to do with it... ok I went through it quickly twice, not deeply but my skills with differential equation is in a certain way limited so if I don't see something directly related to plates, I am a bit lost. By analogy with electricity or propagation of waves in tubes, horns I have some ideas but not for a plate. Perhaps it is because my understanding of mechanical impedance is limited.
Christian
i don't think one can separate both things, as the incident wave becomes a bending wave in the panel, both driver response and panel nat freqs are correlated. Because of the modal behavior of the panel, u can play with driver position (each quadrant of the panel will have the same modal distribution), to excite or non excite some modes, or to cancel some waves with the edge reflection. That's the reason why one doesnt put the exciter in the middle of the panel, as u would be cancelling f0 (this is a guess) for both axial dimensions, because the exciter would be at a "fixed" max or minimum displacement determined by nat freqs.
An analogy may be the position of a subwoofer in a room, which is highly determined by its position. U usually put it in a corner, not only because of the directivity factor resulting in overall louder response, but also to excite as much max pressure modal lines as possible (the corner is the fixed max pressure point for most room modes). In the panel one wants to have the highest modal density possible, and then play with the exciter position to make those modes result in the smoother response one can achieve.
Another may be a TL where the driver position helps to cancel some of the natural frequencies of the line.
About the impedance on the edge, i said in the previous post that the damping will result in a non infinite impedance boundary... I would't call it closer to a match, as still the impedance jump will be huge, but yep, some of the energy will be absorbed by the damper. Yet, those boundary conditions are so hard to represent, that we dont have much to do than trial and error with driver position, or the incident wave characteristics.
An analogy may be the position of a subwoofer in a room, which is highly determined by its position. U usually put it in a corner, not only because of the directivity factor resulting in overall louder response, but also to excite as much max pressure modal lines as possible (the corner is the fixed max pressure point for most room modes). In the panel one wants to have the highest modal density possible, and then play with the exciter position to make those modes result in the smoother response one can achieve.
Another may be a TL where the driver position helps to cancel some of the natural frequencies of the line.
About the impedance on the edge, i said in the previous post that the damping will result in a non infinite impedance boundary... I would't call it closer to a match, as still the impedance jump will be huge, but yep, some of the energy will be absorbed by the damper. Yet, those boundary conditions are so hard to represent, that we dont have much to do than trial and error with driver position, or the incident wave characteristics.
Okay, I'm thinking out loud here so bear with me here.
To eliminate a reflection at the intersection of two plates, to eliminate a reflection, one should match impedance. That means, mainly (I think) to match D (bending stiffness and areal density. But can the same kind of "match" actually be made at the termination of a panel (boundary)?
In the first case (panel to panel transition), it seems damping might not be necessary. But in the second, it seems essential.
For panel to panel, you can transfer momentum and energy from one panel to the other. But for panel-to-boundary, is there any way to transfer that momentum and energy, except to absorb it? Somehow it seems like a different thing then, to transfer from a panel to a termination. Is that still impedance?
Thoughts?
Eric
To eliminate a reflection at the intersection of two plates, to eliminate a reflection, one should match impedance. That means, mainly (I think) to match D (bending stiffness and areal density. But can the same kind of "match" actually be made at the termination of a panel (boundary)?
In the first case (panel to panel transition), it seems damping might not be necessary. But in the second, it seems essential.
For panel to panel, you can transfer momentum and energy from one panel to the other. But for panel-to-boundary, is there any way to transfer that momentum and energy, except to absorb it? Somehow it seems like a different thing then, to transfer from a panel to a termination. Is that still impedance?
Thoughts?
Eric
Any Sorbothane sheet material with a high ductility/softness should work Eric. 3M provide a range and this is a source in the USA.
https://www.isolateit.com/products/...with-and-with-out-3m-adhesive-backing-1-sheet
Burnt
The only case of impedance adaptation I can refer for now is electric signal in a cable. The characteristic impedance of a cable is link to the inductance and capacitance by unit of length. The terminations have to match this values being resistance.
In standard mechanics to electricity analogy, inductance and capacitance are mass and spring, the exact correspondence depending of the choice force and velocity to voltage and current. The resistance is a damper.
Does this analogy apply in our case?
Christian
Thanks Burnt,
I did try some sorbothane a few months back. My results were kind of mixed. But due to the cost, I used it pretty sparingly, and my expectations might have been too high considering how sparingly I used it. But I have some left, and I'm playing with a new test method. I'll give it another look!
Eric
Eric, use this sorbothane caculator to match plate impedance with boundary and try again.
Change dimensions and loading and see what percent deflection is best suited to your panel.
Sorbothane is usualy used compressed, so use it on both sides of the panel and clamp it.
But it would be interesting to use it uncompressed aswell and what will messurements show.
Durometer 30 is preffered if used continuously around perimeter.
https://www.sorbothane.com/technical-data/design-guide-calculators/vibration-calculator/
https://www.sorbothane.com/technical-data/design-guide-calculators/load-rating-calculator/
Change dimensions and loading and see what percent deflection is best suited to your panel.
Sorbothane is usualy used compressed, so use it on both sides of the panel and clamp it.
But it would be interesting to use it uncompressed aswell and what will messurements show.
Durometer 30 is preffered if used continuously around perimeter.
https://www.sorbothane.com/technical-data/design-guide-calculators/vibration-calculator/
https://www.sorbothane.com/technical-data/design-guide-calculators/load-rating-calculator/
Thank you for the link Lenta. First time I see in those threads some engineering approach for the suspension! As I am far from those kind of domain and material, do you know some papers or pages as an introduction. To be more precise I am at the level where such material are springs and dampers and probably mass... In the white papers of this sorbothane site, it seems used as representation. I would like to push more this kind of representation. does it make sense?
Christian
First time I heard about cone edge and surround impedance matching was in Bowers&Wilkins developement papers.
I have seen those drivers, they use open cell/close cell memory foam. Very soft to the touch, dense, viscoelastic, slow recovery back to shape.
Sorbothane is similar, but closed cell and we have free access to design tool.
I agree, panel and surround impedance matching is the future for dml and it should be explored.
https://pearl-hifi.com/06_Lit_Archi...ns/05_Bowers_Wilkins/B&W_800D_Development.pdf
I have seen those drivers, they use open cell/close cell memory foam. Very soft to the touch, dense, viscoelastic, slow recovery back to shape.
Sorbothane is similar, but closed cell and we have free access to design tool.
I agree, panel and surround impedance matching is the future for dml and it should be explored.
https://pearl-hifi.com/06_Lit_Archi...ns/05_Bowers_Wilkins/B&W_800D_Development.pdf
Eric,
see appendix II, page 21. It explaines the inner workings of the midrange fst driver that is in fact a bending wave driver!
They are using a pretty firm surround that matches the average impedance of the fst driver. Not much detail though.
But, they explain the problem with reflections from the edge and the inpact on the sound. I’m in complete agreement with them.
’Thomas
see appendix II, page 21. It explaines the inner workings of the midrange fst driver that is in fact a bending wave driver!
They are using a pretty firm surround that matches the average impedance of the fst driver. Not much detail though.
But, they explain the problem with reflections from the edge and the inpact on the sound. I’m in complete agreement with them.
’Thomas
There was some pair of inexpensive concentric coaxials sold on Amzn that was a hit several years back. I found a pair in a thrift. Followed up on the web, hit an article where one fellow did some pretty extensive mods to make them sound better, including changing the surround to "minimize reflections back into the cone". That's where I got the idea of characteristic impedance load by the surround I spewed about in my previous post.
I wish I could be more help. I can only say some experimentation would probably be necessary. Even in the electrical analog, we'd have to build the physical strip line, send a fast edge down it into a 50 Ohm termination and watch what happened with an oscilloscope, to be sure we were building 50 Ohms impedance with the specific PCB etch width FR4 layup.
Perhaps computer based mechanical simulation has got so good these days you dont have to do that. I wouldnt count on it, cept inside the ballpark. You'd still have to build and verify the simulation accuracy.
Thank you @lenta for this article and fo your reading Thomas pointing to the appendix.
In page 22 :
This fits to my understanding of such impedance adaptation. This is also the typical aspect of a DML to show mainly a resistive impedance to the exciter (8.sqrt(B.µ))
So the next question for me is : how to find and charaterized such material?
Having a look to the white papers in the previous link about sorbothane, I was not sure it is apurely resistive material.
Any idea?
Christian