Eric:
I tried other angles too, just mentioned 90 degrees for brevity. I guess you would see coincidence if you had a directional mic, and averaged 3 readings at a distance just clear of the panel, moving the mic above and below centre in a circle around the panel centre whilst maintaining the angle from the normal. Then repeat at several frequencies, you should see a peak moving.
On response regions/regimes. Yes would be wonderful to have such tools but expense is prohibitive.
It looks like I have assumed at some point that coincidence was indeed at lower f than it really is, even for non-exotic materials. I read that Zenker paper (and thought it was a good one), looks like the penny didn't drop, guess I assumed his material choice put fc up high. I remember reading fc values for aluminium panel at several thousand Hz, and had assumed for foam/ply it would be much lower. Plus, some papers stress how, below coincidence, radiation is very inefficient with only edge and corner modes, and I made the connection to the low frequency rolloff. If fc is typically much higher, looks like panels get along with edge modes just fine! If that is the case, then fc may be of much less consequence for practical designs in foam or ply.
I tried other angles too, just mentioned 90 degrees for brevity. I guess you would see coincidence if you had a directional mic, and averaged 3 readings at a distance just clear of the panel, moving the mic above and below centre in a circle around the panel centre whilst maintaining the angle from the normal. Then repeat at several frequencies, you should see a peak moving.
On response regions/regimes. Yes would be wonderful to have such tools but expense is prohibitive.
It looks like I have assumed at some point that coincidence was indeed at lower f than it really is, even for non-exotic materials. I read that Zenker paper (and thought it was a good one), looks like the penny didn't drop, guess I assumed his material choice put fc up high. I remember reading fc values for aluminium panel at several thousand Hz, and had assumed for foam/ply it would be much lower. Plus, some papers stress how, below coincidence, radiation is very inefficient with only edge and corner modes, and I made the connection to the low frequency rolloff. If fc is typically much higher, looks like panels get along with edge modes just fine! If that is the case, then fc may be of much less consequence for practical designs in foam or ply.
I've estimated fc for many typical panels, and for the most part they come in between about 4 kHz and 10 kHz for the things most people are using.
That 5mm carbon/nomex panel you were looking at a while ago was among the lowest, at about 1.2 kHz. And there are two old patents (Heron, Warnaka) that talk about 40 mm thick honeycomb panels with steel or aluminum skins with fc down to almost 200 Hz! So it can be that low, but you have to be really trying hard!
Eric
I have only Dayton Audio DAEX25FHE-4 as exciters and for those measurements (I mean what I have been posting for some weeks before now) I use one pair. The measurements in post 5334 were done with the same exciter of this pair. A possible source of difference is in the double side tape and in the gluing preparation... The piece of tape comes each time from the same roll. I don't have one tape as good as the original 3M VHB and not a process "fully transparent proof". By the way since I have proceeded like that, the repeatability of measurement seems good.
I also went back yesterday evening in the thread to old posts. It seems there is a common shape in HF with a decrease in level after 10k. Exception to that are some reports of panels (ie on youtube) with a rising level to HF with acrylic which I have not tested
That's certainly what's happening with the Dayton thrusters Im using on XPS. A hump followed by a cliff! I'd always assumed that this was due to shortcomings of the exciter. Now evidence it ain't necessarily so. Maybe it's coincidence frequency
I think investigations of this sort makes you feel like the old alchemists must have done. When I was a schoolboy, there was a program on TV called 'Catweasel' which had an alchemist/sorcerer transported to the modern age. It was very good, as I remember it. Every time he turned on an electric light, he made the incantation 'shine, tiny sun!'. At this point, I can relate.
That's certainly what's happening with the Dayton thrusters Im using on XPS. A hump followed by a cliff! I'd always assumed that this was due to shortcomings of the exciter. Now evidence it ain't necessarily so. Maybe it's coincidence frequency
I think investigations of this sort makes you feel like the old alchemists must have done. When I was a schoolboy, there was a program on TV called 'Catweasel' which had an alchemist/sorcerer transported to the modern age. It was very good, as I remember it. Every time he turned on an electric light, he made the incantation 'shine, tiny sun!'. At this point, I can relate.
I must say that DML does seem to attract eccentrics. Being from Sweden, I only know Catweasel from by British friends who claim that I remind them of him
I also doubt it is exciter variance. I have 8 DAEX25FHE-4 so can do some checking when I remove them from the plates.
Ha! As it happens I'd had a couple of glasses of wine and looked it up. I watched a couple of episodes. I still feel it was a wonderful children's show - witty, charming and thought-provoking. And I would walk 5 miles at least for an explanation of that hump. A compression-wave reflection from the opposite side? Dunno.
Christian
If the hump is not seen with acrylic, then I guess that rules Helmholtz resonance. Seems to rule out breakup too because while light foam may be soft enough for breakup not to occur, with acrylic you would expect it to have occurred at a lower frequency, and the response to fall off a cliff at all higher frequencies.
Maybe its a second order system formed from the mass of the exciter and the spring of the foam's compressibility? With softer EPS, it may occur earlier and be lost amongst the other peaks, and for acrylic (which is by comparison essentially incompressible) would not occur within audio range. If that were true, you would expect the hump freq to go down with heavier exciters, or with softer materials.
If the hump is not seen with acrylic, then I guess that rules Helmholtz resonance. Seems to rule out breakup too because while light foam may be soft enough for breakup not to occur, with acrylic you would expect it to have occurred at a lower frequency, and the response to fall off a cliff at all higher frequencies.
Maybe its a second order system formed from the mass of the exciter and the spring of the foam's compressibility? With softer EPS, it may occur earlier and be lost amongst the other peaks, and for acrylic (which is by comparison essentially incompressible) would not occur within audio range. If that were true, you would expect the hump freq to go down with heavier exciters, or with softer materials.
Looking at the graphs from the Dayton data sheets, to 10k hump doesn't seem to be present with the DAEX25FHE-4, and while it appears in some models, it doesn't seem to be a common trend in their tests either. However they only specify that they use "foam core board", and it is hard to know what one can read out from those graphs.
If they do have identical setup apart from the exciters, the difference between different exciters seems quite big. I guess since FR is a result of the interaction of the plate and exciter it is very hard to generalize, and it only makes sense to measure them as one whole system.
If they do have identical setup apart from the exciters, the difference between different exciters seems quite big. I guess since FR is a result of the interaction of the plate and exciter it is very hard to generalize, and it only makes sense to measure them as one whole system.
I saw this inline high pass filter:
https://www.parts-express.com/Speaker-Protector-300-Hz-150-Hz-266-200
https://www.parts-express.com/Speaker-Protector-300-Hz-150-Hz-266-200
Thank you Eric! Your two posted links have done more to further the discussion and real knowledge about this topic, than all the other 268 pages of this thread put together.
Sing little panel! Will it lead us to the philosophal stone?That's certainly what's happening with the Dayton thrusters Im using on XPS. A hump followed by a cliff! I'd always assumed that this was due to shortcomings of the exciter. Now evidence it ain't necessarily so. Maybe it's coincidence frequency
I think investigations of this sort makes you feel like the old alchemists must have done. When I was a schoolboy, there was a program on TV called 'Catweasel' which had an alchemist/sorcerer transported to the modern age. It was very good, as I remember it. Every time he turned on an electric light, he made the incantation 'shine, tiny sun!'. At this point, I can relate.
"Oil can resonance" is the name of the phenomenon. I have in mind 2 countermeasures : a very small mass at the center or a hole. Does this say the root cause is the 1st mode of the disc within the coil area (with a pinch of the air enclosed behind between the panel and the magnet?). With the stiffness of the plywood, the mode is rejected above the panel bandwidth?
Yes, I read yesterday Erics's investigations with tap/impulse testing. That's just the type of approach we need to get a handle on this and go from flailing about towards real design.
The 5 miles was meant to be a reference to the Bay City Rollers that JonnoG mentioned. Realized later I had the right brogue, but the wrong decade!
Yes, I do remember you mentioning oil can resonance previously. I found one mention of the term, and it's been applied to resonance in the dome of tweeters. So a presume that you mean its a drum-like bending wave resonance within the circle of the exciter.
I did some further tests last year to the area opposite the exciter on the same ellipses setup I showed yesterday.
First I drilled a hole through the XPS to test cavity resonance. Purple curve is with hole. So, not cavity resonance.
Then I did a series of tests with blutack balls, radial strips, and discs of different sizes. All of them, where they helped the peak at 10 kHz, also affected the response much lower down. Here are a couple of relevant ones with discs. Brown = no blutack, Green: thin 30mm disc, Blue: thicker 30mm disc.
The peak is moving to a lower f, which does support the oil can resonance theory. But both effect the lower part of the spectrum, and bring on the HF rolloff even sooner than before.
I think the correct approach will be surface treatment or surface layers, to stiffen the surface or increase the bending stiffness to the point where the peak occurs above 20 kHz. Note this XPS had no surface treatment, but did have a circular layer of epoxy below the exciter to get a good base, then it was glued onto that with superglue.
Yes, see my response to Steve. I think increasing surface hardness or bending stiffness with panel surface treatment or additional composite layers respectively is the correct approach for foam, to take the peak/cliff beyond 20 kHz.
Last edited:
Yep. I looked a little bit deeper and it's nothing more than a glorified 270 µF capacitor in series for 4 ohm (or a 135µF will do it for a 8 ohm driver).
Hello Christian,
the lighter colored strips are poplar and the darker and longer ones are red oak.
Poplar is quite soft and bends easier and the red oak is much harder and hopefully prevents the panels from bending long-term.
The hand planer I used is a very small convex japanese style planer that lookes like this one:
The strip have to be glued onto the board so that you can tune the board step by step via tapping and carving.
Sebastian