Dave,
I purposely chose the particular panel that I did (and orientation) because directivity tests with a single exciter showed that the coincidence frequency was over at least 15 kHz and likely over 20 kHz. I was hoping to avoid confusion between the two if there was overlap.
This part I can clearly see in your plots, but don't comprehend why it would be so. Any thoughts? It seems to suggest that with the right "interface" (as Christian is calling it) we could possibly avoid this effect, which would be nice.
Nice! Looking forward to it.
Eric
Oops - it was because I wrote a new "point source" function the other day and accidentally swapped the X&Y locations, so in those first graphs the point sources were actually on the vertical midpoint axis of the plate rather than the horizontal midpoint axis. The little red dot indicating the location is too small for me to see now when it's a point source, so I didn't notice! Here are the updated graphs (the small & large exciter data didn't change).
Hi Dave, @EarthTonesElectronics
Did you end up reaching out to Ben Zenker at Xcite about getting some exciters to test? Seems like he would be interested in PETTaLS as well, I'm not sure if he is on this forum or reads it. I guess they may need a commercial license rather than the light version that would work for most of us here.
It would be great to include Xcite exciters sometime but don't want to ask anything more as you are already doing many changes and modifications. Looking forward to having a go with the program when it's available!
Did you end up reaching out to Ben Zenker at Xcite about getting some exciters to test? Seems like he would be interested in PETTaLS as well, I'm not sure if he is on this forum or reads it. I guess they may need a commercial license rather than the light version that would work for most of us here.
It would be great to include Xcite exciters sometime but don't want to ask anything more as you are already doing many changes and modifications. Looking forward to having a go with the program when it's available!
Eric.
For me, the term DML simply means a speaker using bending waves (i.e. a bending wave speaker) designed using the NXT principles. So DML action and bending wave action have no real distinction in my mind.
Are you saying, that a bending wave panel or driver is the same as a dml panel ?
To me these are two very different speakers.
And must be treated very differently, with different goals.
Steve.
For me, the term DML simply means a speaker using bending waves (i.e. a bending wave speaker) designed using the NXT principles. So DML action and bending wave action have no real distinction in my mind.
Are you saying, that a bending wave panel or driver is the same as a dml panel ?
To me these are two very different speakers.
And must be treated very differently, with different goals.
Steve.
I would assume that is true, but only as long as you place them far enough apart for them to act as separate sources. If using a close cluster of four exciters they pretty much combine into a single source. I remember seeing some interview with Tectonic on their youtube channel where they explained something along those lines as the idea behind their designs.
Same principle as with regular speakers. If you place a couple of subs next to each other they will couple their power, but add a bit of distance and they will start cancelling each other at certain frequencies. With exciters on a plate you do not only have the acoustic cancellation to deal with though, but the physical coupling of the plate, so I would expect the effect to be even stronger, but also lessen with reduced distance since bending the plate in a separate direction becomes exponentially harder the closer the exciters are to each other.
Hello Rui,
Thank you for the links.
The first one shows the performance of perforated materials. Could you detail a bit more how you use it and what it looks like? How is the perforated layer?
The third one enters quite easily in my knowledge field. I recommend to read it. It is really interesting the idea the brain can build a representation of the sound of a room.
For the reverberation as a tool, I have no experience...
For the next measurements, I will have a look to them with this possibility of reverberation in mind. For now, I don't see in the spectrograms an energy that will stay long enough in the panel for that... To be continued.
The third document opens to an other hypothesis which is by its wide radiation, the reverberation field of a DML playing in a room is close enough of the representation build by the brain with other sounds that the DML disapears. In the study of this paper, they had the problem of the directivity of the loudspeaker (which was not an omni?) used to measure the IR of each locations.
Christian
I think it is most accurate to say that DMLs are a subset of bending wave speakers. A bending wave speaker is one in which bending waves, rather than pistonic motion, create the bulk of the sound that it radiates. Every DML radiates sound via bending waves, hence every DML is a bending wave speaker. For me, that much is very clear.
But less clear to me is this: what are the boundaries within which a bending wave speaker is a DML, and beyond which a bending wave speaker is no longer a DML? I don't think there is a definitive answer to that question. Certainly the words "distributed mode" implies that there must be multiple modes, but how many? Is two enough? Or 20? or 200? I don't know. How would one even count them? And frankly, I don't really think the answer matters.
From your posts I infer that your view is that speakers with modal behavior are "DML" and with "pebble-in-pond" (PIP) behavior are "bending wave." (sorry if that is incorrect). But I would suggest that virtually everything we discuss and build on this forum exhibits both of those behaviors: modal at low frequency and PIP at high frequency. So what I see are not two distinct classes of bending wave speakers, but rather a continuous range of bending wave speakers with varying degrees of modal and PIP behavior.
Eric
Hey Leob,
Maybe four is different from two, or it depends on the panel and mounting, but I also tested two DAEXVT25-4 exciters placed as closely as I could (about 2 inches), and I still saw the comb filtering effect. There where fewer minima (as you would expect with the exciters closer), but I still saw minima in the SPL almost exactly where I calculated they should be based on the path length difference at various angles (see below). The results were slightly less "perfect" than in the earlier example where the exciters were 7" apart. For example there are some extra dips in both the 40 and 70 degree curves that can't be explained by comb filtering, and the dip marked "2" in the 40 degree curve should be at over 15k, but overall it looks like pretty much the same thing is happening even when the exciters are as close as I could get them. At least on this particular panel.
Eric
Thanks, that is very interesting! I'm curious how much difference using four exciters makes, but I am surprised how strong the effect is with two exciters that close.
My initial impression of using four exciters was that it sounds really good, and makes transients very clear. But I never did measurements comparing different configurations, and do see quite a difference in off axis response, so I definitely have to do more tests with single exciters and compare with clusters. There are just so many variables to test :\
It does seem like there is nothing clearly indicating that the exciters interfere at the panel level though, since you based your calculations on the wavelength in air? Considering the wavelength is a lot longer in the plate than in air, that would mean that any interference would be above 20kHz?
My initial impression of using four exciters was that it sounds really good, and makes transients very clear. But I never did measurements comparing different configurations, and do see quite a difference in off axis response, so I definitely have to do more tests with single exciters and compare with clusters. There are just so many variables to test :\
It does seem like there is nothing clearly indicating that the exciters interfere at the panel level though, since you based your calculations on the wavelength in air? Considering the wavelength is a lot longer in the plate than in air, that would mean that any interference would be above 20kHz?
This is the layout for the composite panel I am planning to build. The honeycomb core is from black stained 3.2mm lightweight plywood (laser cut) and measures 600x367mm. The top layer will be from 0.03mm thin fiberglass fabric on both sides which is adhered with photo ink stained water based PU. The edges of the panel are straight to allow easy trimming of the excess fiberglass fabric
The exciter placement is app. in the 2/5 3/5 position and in direct contact with a solid core area for better energy transfer. The panel is freely suspended and mounted with soft rubber grommets inside a wooden frame. The wooden frame has embedded warm white led strip lights on the inside which (hopefully) illuminate the panel and highlight the honeycomb pattern. The wooden frame is also closed from the back and has damping material to avoid unwanted reflections since I plan to mount it onto a wall. Suggestions for improvement are welcome, cheers shido
Our hearing system is probably very tolerant or maybe sensitive to more parameters than the "simple" FR. Measurements are a great tools to understand what happens but it is much more tricky to say what is important for the listening pleasure.
For sure!
Keep in mind the results from the coincidence frequency evaluation tests. The coincidence frequency of the panels tested are above 5kHz and in my opinion for the most appreciated panels above 10kHz. At this frequency, the speed of the waves in the panel is equal to the speed of the sound in the air.
It is only above, that what happens in the panel is faster than in the air.
Below, the waves in the panel are slower in the root square of the ratio of the frequency to the coincidence frequency : v(f) = cair * sqrt(f/fc)
So at fc/100, 10 times slower (this is in the 100Hz range for fc=10kHz), at fc/10, about 3 times slower (1kHz range).
In the frequency range of the interference Eric shows, the speeds in the air and in the panel are similar. A deeper look to the mechanism might be needed to understand why the path in the air seems to be dominant... or maybe as Eric suggested, the fc of the panel is just high enough (15k or more) to make what happens in the air dominant?
This topic of what happens in a exciter cluster has been opened for a long time with 2 "opinions" : everything is blended in the plate or there are interferences. I don't remember for which reason I made the proposal of a shunt capacitor in the case of a 2 exciter configurations... I remember that at this time, I made the choice for my panels that don't see a high electrical power to stay on the conservative way of one exciter.
For high power applications, it is maybe not a so big problem to have interference in HF for a compact cluster of exciters.
Christian
I agree with most of what you are saying, but not all.
The way I have described how a dml panel produces sound is as you say, a pebble in a pond ,pip, and then dml action.
But also there is the pistonic action of the exciter (pebble) (ideal point source) however you wish to call it.
ALL three actions happen in varying levels depending on panel materials and treatment of the panel movement.
You can change the panel properties by varying the amount of all three action on the panel.
I have described this in the past as pushing the panel more into one of these three modes than the others.
Which sound you prefer is up to the designer.
I prefer all three in large quantities without damping.
Although I do use minor tweaking.
Steve.
The first words that come to me are : "Whaou! Audacious".
My main advice would be : proceed by steps. If you can, test the panel in open back conditions to evaluate the properties of your composite. If you have the possibility, a directivity measurement is a great tool to evaluate the coincidence frequency and the panel self damping. From them depends how the response of axis is . My (current!) opinion is to have the coincidence frequency as high as possible, if possible out of the audio range. A 2mm plywood is a bit to thick for that. There is a risk with a light and stiff composite to have the coincidence frequency too low. I am not able to predict something about the damping...
With the back side load, the response in the mids might change. Maybe some EQ will be needed.
Keep us informed
Christian
Shido,
This sounds like a neat idea. I like your honeycomb composite panel idea. I considered doing something similar at one time but never actually did it. I found this drawing I made (in January 2020), which looks similar to yours! I can't recall for sure but I think my idea at the time was to use wood veneer for the face, rather than fiberglass, but the concept is the same.
I do agree with Christian and Steve with respect to the basic idea of testing some prototypes before going all-in on a particular design. Start with a single panel in a crude frame and see how that sounds, and then proceed from there.
I also agree with Christian concerning his "coincidence frequency" comments, but to be honest I would not concern myself too much about that if I were you. It is what I consider a second or third order effect, to be concerned about after you have sorted out more basic things like frequency response and efficiency, Maybe worry about that on your tenth design!
Concerning the composite panel itself: Have you made fiberglass composites before? In your description, you refer to "adhering" the fiberglass fabric with PU. What is not clear is if you understand that in order to work properly the fiberglass fabric itself has to be completely saturated with resin (PU or epoxy). I have only experience with epoxy resins, but I understand that there are PU resins that are also made for this. If you are not familiar with composites, there are great videos here:
https://www.easycomposites.co.uk/learning
Also, I would suggest making the the honeycomb openings as small as possible. It looks to me like yours are about 15 mm wide. I fear that is too long a distance to span with a thin fiberglass skin. When the fiberglass is on the compression side of a bending wave, it may buckle, and not provide the stiffening that you want. The openings in aramid honeycombs for composites are typically on the order of 5 mm. You may want to get as close to that as is practical.
Concerning the mounting of your panel to the frame, I would suggest that you plan to try many different configurations, rather than simply planning to mount at the two corners you have chosen. I think more mounting points will work better, and likely the best is attaching the panel to the frame around the entire perimeter using a soft foam or double sided mounting tape. The main benefit of attaching the panel around the entire perimeter is an increase in SPL at lower frequencies. I have done many test which show this, including these two:
But as I said, you should plan to test different configurations and decide for yourself what you like best. Good luck and keep us posted on your progress.
Eric
In my view there is no question that there is indeed "interference" in both the panel and the air. But what is the nature of that interference and what would it sound like?
I think that at least part of the reason that the "in air" effect seems dominant is mainly because "in the air" is where our ears are. That is, our ears are in a particular place in the air at some distance (and more importantly angle) from the panel. And since the peaks and dips due to interference of the waves in air occur at particular, fixed places, we can hear those peaks and dips, and hear them change if we move our ears (or mic) from one place to another.
But the interference that occurs on the plate is of a different nature. Here I think it's useful to consider two cases, low frequency and high frequency, or as Dave would call it, the modal (low frequency) and statistical (high frequency).
At low frequency, I think it's likely there would not be any real interference as long as the exciters are close together and driven in phase. If they are far apart, or driven in opposing phases, then the effect would depend on the position of each exciter relative to the nodes and antinodes of the plate's natural frequencies. And the effect would essentially be to selectively drive different sets of modes at different levels.
But at high frequencies, when the plate is no longer modal, then I suspect the effect on the plate is different. In that case I think the movement of the plate looks something like this: (try the second link if the first doesn't work). Note, the simulation shown is not actually a panel, but rather light wave, but I think it is still applies, at least qualitatively.
https://en.wikipedia.org/wiki/Wave_interference#/media/File:Two_sources_interference.gif
https://en.wikipedia.org/wiki/Wave_...aves overlap,net displacement at these points.
There are certain angles on the panel where there are stationary nodes (no motion, blue-green in color) and other intermediate angles on the panel where there are nodes and antinodes that move radially from the center out. So if our ears were on the panel, we could probably hear the difference between various locations on the panel. But our ears are not on the panel. So the only way we would hear that effect is if that pattern on the plate is somehow translated into some pattern in the air. But is it? And what would it look (sound like)? How would we detect it? I don't know.
In principle, Laser Vibrometer might be able to detect this behavior on a panel, but I don't know if the resolution is good enough. Likewise, the velocity map in Dave's model might also, but again the resolution of the model may not be sufficient. And still there would be the question of whether or not the pattern on the plate is even translated into a pattern in the air that can actually be heard.
Eric
I have in mind my first panel which was a 20mm XPS PVA coated. The directivity measurement shown a coincidence frequency at about 6kHz! To be honest it had in addition the bad idea to give higher SPL out of axis than on axis... In the priority of the different criteria, I prefer for now to get the smoothest directivity than for example the highest efficiency. The good point now is that we have measurement techniques to make choices.
As such composite is out of my possibilities, I have not that much focused on the honeycomb... I have in mind a cell dimension in the 5 or 6mm (???). 15mm is the domain of one wave length of the high frequencies.
Hello Rui,
Have you made tests in this idea? Which kind of sound treatment?
I ask because what you wrote about reverberation opened questions to me.
Christian
And perhaps not even for low power applications. My point was only that they are there. and measurable, not necessarily that that are big problem.
Eric