A Study of DMLs as a Full Range Speaker

Food for thought.. air spring panel stiffness:

https://www.mdpi.com/2076-3417/10/2 4/8926
Has anyone experimented with this airspring closed back mounting? From my understanding they found a 120mm radius cutout for the exciter and 12mm for the gap was best. I couldn't see how much further back the closed back was but it looks like the whole enclosure is approximately 100mm deep in total. I may give it a try when I get a chance as it looks interesting and could be benificial.
 
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Yes, I noticed there was some audible rattle there and I'm gonna investigate what it can be. A bit unusual at that frequency. If something is rattling, typically that happens in lower frequencies.
I found on my plates if I didn't support the short sides there was a buzz/distortion from them freely vibrating at higher volume, sounded like a lot lower frequency than 4.2khz though.
 
@JoskaNZ very nice results, thank you for sharing. Your distortion values are very low compared to what mine have been and the FR is pretty well controlled compared to many of my tests. I'd love to see some pics of your frame and mounts.

You said you sanded the panel and that extended LF. The sanding could well be the cause, but I've also found with all my testing that remounting a panel can make a pretty big change too. These panels are very sensitive to the way they are mounted, and the driver too if it is braced, and remounting it can make small changes to how pressure is applied and this can have a large effect. One of my latest panel experiments is extremely rigid fiberglassed EPS and it gives the deepest LF of all my tests. I think rigidity is required to control the power at LF.

Your peak around 18khz is interesting. I suspect it might come from your driver, and the driver could be limiting the LF a bit too. I've only got the DAEX32EP-4 and I believe it is known to give stronger LF but weaker HF. I don't know much about voice coil engineering but I've been wondering if there's a better way to make a full range exciter since it seems like there's a lot of compromises made here. I think the Xcite driver is better than these Dayton exciters, based on results others have shared.

I had an opportunity to quickly do a loud test today with one of my panels and after just 1 minute of run time at ~95dB at 100hz my driver started smelling and feeling quite warm :unsure: I also had a lot of noise from my duct tape surround flapping...
Here are some photos of the panel.

I'm not sure what the 18khz peak is from, my measurements were pretty quick and sloppy as I was just curious on how they were working out with the new coating and the epoxy to attach rather than the superglue which was weakening the EPS. I was happy to see they improved in the low end after sanding as I had been disappointed in the 300Hz up previous measurement. Otherwise the mounting etc was unchanged.

I burnt out one of my 25fh exciters doing a high spl test recently too, I had turned down the bass on the amp but forgot there was no crossover to limit LF, short term around 100db at 1m, it ripped off the eps and the voice coil melted through too whoops.

Another mistake was when I didn't have any epoxy so tried to attach the exciter with JBweld, I hadn't considered that it had steel filings in it and when I tried it the next day some had been attracted to the magnet and into the voice coil gap.
 

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Ah, I see, you are clamping the panel with blocks. Doesn't it wiggle free after a while and contact the frame? I suppose one could cut notches in the panel for the blocks to fit in to prevent that, and maybe clamp over that too.

The airspring idea is pretty interesting, I've been wondering about something like that for a while. I think that would lose the open-baffle character though?
 
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Yes, I noticed there was some audible rattle there and I'm gonna investigate what it can be. A bit unusual at that frequency. If something is rattling, typically that happens in lower frequencies.
Hi Leob,
As a follow up in the "methodology", if it can help : The testing panel I am currently playing with has even worth distortion figures than yours. So to investigate that, I pointed in the distortion plot one frequency with a bad behavior and played a 1/3 of octave pink noise near this frequency. It helps to find the source of buzzing/rattling.
In my case, the main source were the kitchen strings I used as suspension for testing. Not a good idea! Even if it is much better than tape. That's terrible how those strings or the tape are noisy! Steve warned about that but I didn't imagine at this level.
Christian
 
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Ah, I see, you are clamping the panel with blocks. Doesn't it wiggle free after a while and contact the frame? I suppose one could cut notches in the panel for the blocks to fit in to prevent that, and maybe clamp over that too.

The airspring idea is pretty interesting, I've been wondering about something like that for a while. I think that would lose the open-baffle character though?
The blocks don't seem to move they are closed cell foam so you can cut them to be tight, Leob uses a similar method for his PA panels with 4 exciters running 110Db for hours at a time.

I was also wondering if the closed back would mean you would loose some of the holographic character. Might mean they could be closer to the back wall without detrimental reflections perhaps.
 
I just did an inside & outside test and the results are mostly as I expected except for a few new dips revealed in outside measurements. Idk how to explain the huge dip outside at 400hz. The reduction in the 1k-6k range is just 'meh', but that 400hz dip could be a problem? 2.5khz often shows up in tests I think it might be exciter related.

I setup in my living room for the test, then moved outside by picking up the whole heavy frame and hauling it out. I don't think any component was disturbed during the move. After seeing the 400hz dip I moved the whole setup back inside to retest and the 400hz dip was gone. Here's the results with red being inside and green being outside, mic at 6ft (2m) for both:

1711748864341.png


The inside test waterfall does not show any unusual stored energy around 400hz. I did repeat tests and moved mic a few inches and repeats of that, all show same results. I also measured outside at 3ft (1m) and it too shows the same dip (purple is 3ft, green is 6ft):

1711749023728.png


Note: I did not test multiple panels or mounting adjustments, I just used what I already had on which was a 10"x22"x0.5" XPS panel with thin epoxy on both sides and a full duct tape surround, and a solidly braced driver pressed into the panel enough to tension the duct tape a bit. Not played too loud because turning my AVR's volume from 60 up to 70 would cause a whole lot of noise from duct tape.

The only difference between inside and outside that I can think of was the mic stand. Inside it was around 20" off floor on the front edge of a couch cushion. Outside it was about 26" off the ground on a hollow plastic tee-ball stand. I should have tested different mic elevations outside and a sturdier stand but I didn't think of it at the time and I'm not hauling everything out again to try that :p The yard is about 35ftx45ft with 6ft tall wood panel fence on 3 sides and 2-story house on 4th side, panel was roughly in the middle of yard.
 
I just did an inside & outside test and the results are mostly as I expected except for a few new dips revealed in outside measurements. Idk how to explain the huge dip outside at 400hz.
420Hz wavelength is 809mm (30"). Look for an 809mm reflection somewhere. It could be from the reflective path from the ground to the mic, or from the panel to the ground. How high was your panel above the ground?
It's possible that the reflection gets swamped by other reflections when you're measuring indoors.
 
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I just did an inside & outside test and the results are mostly as I expected except for a few new dips revealed in outside measurements. Idk how to explain the huge dip outside at 400hz. The reduction in the 1k-6k range is just 'meh', but that 400hz dip could be a problem? 2.5khz often shows up in tests I think it might be exciter related.

I setup in my living room for the test, then moved outside by picking up the whole heavy frame and hauling it out. I don't think any component was disturbed during the move. After seeing the 400hz dip I moved the whole setup back inside to retest and the 400hz dip was gone. Here's the results with red being inside and green being outside, mic at 6ft (2m) for both:



The inside test waterfall does not show any unusual stored energy around 400hz. I did repeat tests and moved mic a few inches and repeats of that, all show same results. I also measured outside at 3ft (1m) and it too shows the same dip (purple is 3ft, green is 6ft):



Note: I did not test multiple panels or mounting adjustments, I just used what I already had on which was a 10"x22"x0.5" XPS panel with thin epoxy on both sides and a full duct tape surround, and a solidly braced driver pressed into the panel enough to tension the duct tape a bit. Not played too loud because turning my AVR's volume from 60 up to 70 would cause a whole lot of noise from duct tape.

The only difference between inside and outside that I can think of was the mic stand. Inside it was around 20" off floor on the front edge of a couch cushion. Outside it was about 26" off the ground on a hollow plastic tee-ball stand. I should have tested different mic elevations outside and a sturdier stand but I didn't think of it at the time and I'm not hauling everything out again to try that :p The yard is about 35ftx45ft with 6ft tall wood panel fence on 3 sides and 2-story house on 4th side, panel was roughly in the middle of yard.
Hello,
Long time here without doing outdoor measurements... I have in mind 2 observations from these trials :
  • a difference in the low frequency (better outdoor) as if my panel in the 25m² suffers from some bass crenelation. It was a quite large plywood panel.
  • in one position I had a strong reflection from my house but I was much closer from it than you.
The bass extension of your panel is good (25 x 55cm, 12.5mm thick?), even very good in the "in room" conditions.
About the noise from the duct tape, following my current test, it confirms it is not a good material for suspension (not enough absorbing).

The dip a 400Hz is strange. Have you had a look to the Impulse response to check if there is an unexpected reflection?

Christian
 
The bottom edge of the panel is about 18" off the ground, and the bottom of the baffle below it is about 12" off the ground. So the center of the panel is about 28" off the ground, and the exciter should be around 30" off the ground.

I guess it probably is a ground reflection, thanks for the hint. I don't think it could be anything else. I see the peak of that 420hz dip shifts down to 400hz when the mic moves closer which I think makes sense for ground reflections given the increasing ratio of bounce path length to direct path length.

@homeswinghome - correct about dimensions. 29 kg/m3 XPS sanded with light epoxy coating. Weighs 110g iirc. Full duct tape surround covering the 1/2" gap to the 2"x6" wood frame with the 6" wide faces showing to the mic.
 
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correct about dimensions. 29 kg/m3 XPS sanded with light epoxy coating. Weighs 110g iirc. Full duct tape surround covering the 1/2" gap to the 2"x6" wood frame with the 6" wide faces showing to the mic.
Understood. Your frame is wide compare to what we usually see here : +2x6" in addition to the membrane... It might help to the good bass extension. I think Eric has also large frame but in the other direction.
With such weight and density, the raw XPS panel is 50g (360g/m²) so one coating layer is 30g or 220g/m² which shows again an important part of the weight is in the coating. Your panel areal mass is 800g/m² which means a not too low efficiency.
Interesting. Thank you
 
@Veleric

Hello Eric, it's been a long time!
Just to share observations from last week.
When I commented the measurements from Joskaz, I was surprised by the level of the 1st reflection :
1711818467258.png

I was even more surprised when I made just after some measurements.
Here is my panel with a polypropylene membrane in association with a tweeter (a FT17H, so a bit directive)
1711818596074.png

Almost no reflection... and now an EPS panel at exactly the same position with the same mic position
1711818698959.png

There is a stronger reflection...
Which in my opinion shows the DML throws some high frequency energy at a large angle... which is expected by the scientific papers about DML (important lobbying at high angle at the coincidence frequency)

After that, I updated my REW to last 5.31 version. From 5.30, REW offers more variant of wavelet spectrogram. So I come again to the idea to check the content of the response around 0s but now also at the time of the first reflection and it appears that just after 0s, there is a hole in the HF with a peak (rather flat) at the same frequency at the time of the reflection. Here is the example of my EPS panel.
1711819165019.png

1711819267997.png

To be verified if it is a plausible value...
Even if it is not the coincidence frequency, it is interesting to have a better look to the content of this reflection... I read somewhere that DML are not fully suitable (even if it work not so badly in my opinion) for an accurate stereo imaging because of this particular increase of the energy in HF at high angle.
Christian
 
Hi Christian.
If you can remember, further back in this forum, I had problems with a 1inch eps panel that was reflecting back peaks in the 10k area.
Causing painful noises such as sibilance.
Placing cushions on the wall suppressed this problem preventing the pain.
If I remember correctly turning the panel on its side also reduced this problem.
Steve.
PS.
Sorry, I still have not managed to get into my room 🙄
 
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My foil-coated EPS panels > 600mm x 500 mm ( 2' x 1'7.5" ) = 430 grams with exciters attached.
Hello
I haven't found the weight of the EX32EP2-4 to extract your panel areal mass. Do you have it? The spec I have found as the one of the tweeter is weak.
Other question :
  • what leads you to add a tweeter (sorry if it is in one of your previous post)?
  • its cut off frequency is 6.2kHz?
Christian
 
@Veleric

Hello Eric, it's been a long time!
Just to share observations from last week.
When I commented the measurements from Joskaz, I was surprised by the level of the 1st reflection :
View attachment 1292638
I was even more surprised when I made just after some measurements.
Here is my panel with a polypropylene membrane in association with a tweeter (a FT17H, so a bit directive)
View attachment 1292644
Almost no reflection... and now an EPS panel at exactly the same position with the same mic position
View attachment 1292646
There is a stronger reflection...
Which in my opinion shows the DML throws some high frequency energy at a large angle... which is expected by the scientific papers about DML (important lobbying at high angle at the coincidence frequency)

After that, I updated my REW to last 5.31 version. From 5.30, REW offers more variant of wavelet spectrogram. So I come again to the idea to check the content of the response around 0s but now also at the time of the first reflection and it appears that just after 0s, there is a hole in the HF with a peak (rather flat) at the same frequency at the time of the reflection. Here is the example of my EPS panel.
View attachment 1292649
View attachment 1292651
To be verified if it is a plausible value...
Even if it is not the coincidence frequency, it is interesting to have a better look to the content of this reflection... I read somewhere that DML are not fully suitable (even if it work not so badly in my opinion) for an accurate stereo imaging because of this particular increase of the energy in HF at high angle.
Christian
Hi Christian,

My measurements were pretty quick and I didn't pay a lot of attention to the set up, I was pleasantly surprised with the FR so posted. I think the reflection could be because the mic was on the edge of a set of drawers about 1m high, the panel was 1m away, 1m from the floor the wall behind it was probably around 1.2m behind the panel so maybe some of these reflections caused the spike.

The panels sound fantastic with most genres of music but some hiphop sounds a little strange, not sure if it's male vocals getting a little echo/reverb from inside the panel itself. I have them crossed to a front loaded horn subwoofer at 130hz. Anyone else noticed spoken things sounding strange?
 
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Even if it is not the coincidence frequency, it is interesting to have a better look to the content of this reflection... I read somewhere that DML are not fully suitable (even if it work not so badly in my opinion) for an accurate stereo imaging because of this particular increase of the energy in HF at high angle.
Christian
I have also noticed increased HF off axis. For a HiFi application it is of course not ideal with extra HF content to the sides adding reflections. You basically have one listening position and want balanced sound emitted in that direction.

For applications like mine where I want to cover a space with sound, it is still an improvement over regular speakers. Instead of getting a more balanced sound if you are in front of the speaker, you have to be either 45 degrees to the right or to the left, which means more people will be at a sweet spot.
In the setup I use with speaker stacks in each corner it sure gives a lot more even FR over the dancefloor than any regular PA I have heard.

Perhaps it even helps to make imaging better in my case. I'm really amazed that you can stand right in front of a stack and hear detailed stereo information from a stack on the other side of the dancefloor, and it should help a bit that you get more HF from other stacks and less from the stack next to you in that position.
 
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I experimented a bit with angled baffle wings today. My frame has 6" wings parallel to the panel all around. I placed some extra boards left and right of my high aspect ratio panel and I tried different angles, sorta like a horn, facing the listener. I noticed that at any angle, and even when flat because of the extra thickness, the baffles focused the sound some. With no angled baffles everything sounds very open and diffuse and that does sometimes sound a little odd with vocals to me. Adding these baffle wings and angling them has a significant impact. The higher the angle the more focused the sound stage seems to be. I did a few FR measurements but there weren't any huge changes there that I could identify by graph alone because a few dB difference here and there is not very significant on these charts. The difference in soundstage and vocals was audible though.

I don't know how I feel about using angled baffles but I did take them off for a listening session after I was done playing around. This might help you if you think vocals sound weird - try putting some angled baffle wings on and see how it sounds.
 
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