That is a parametric speaker which is made to project a extremely loud beam of high frequency sound. So loud (120-140dB) that the air will demodulate it into the audible spectrum. At 120dB you get around 60-70dB demodulated signal: https://www.aapt.org/Contests/upload/Parametric_Speaker_Description.pdf
Fascinating, but that demodulation will not happen for a normal speaker going to 20kHz. Which is a good thing because it would sound like a mess.
Fascinating, but that demodulation will not happen for a normal speaker going to 20kHz. Which is a good thing because it would sound like a mess.
FYI
The Russian company Tefra have now launched a DML loudspeaker range that looks (and probably is) professional - their earlier attempts looked a bit home made. The new models look like serie produced, made of aluminium/metal, membrane probably composite, and the drivers are attached in a spider die-cut out of the back plate. Clever design. I think they have been/are? affiliated with diyaudio member Alvipet.
I hope they will succeed in getting their product sold and used.
Worth a look for inspiration :
https://tefra-audio.ru/
My browser auto translates the russian language.
The Russian company Tefra have now launched a DML loudspeaker range that looks (and probably is) professional - their earlier attempts looked a bit home made. The new models look like serie produced, made of aluminium/metal, membrane probably composite, and the drivers are attached in a spider die-cut out of the back plate. Clever design. I think they have been/are? affiliated with diyaudio member Alvipet.
I hope they will succeed in getting their product sold and used.
Worth a look for inspiration :
https://tefra-audio.ru/
My browser auto translates the russian language.
Interesting. "Demon" seems like it could be a decent PA speaker with 140w rating that they claim to cover 50-18k and have 96dB sensitivity and not that much bigger than mine at 490 х 700mm. The "Nevis" has the same frequency range at only 565х320mm! Very impressive if true if they manage to get a plate that size go so low.
I'm pretty disappointed with this new panel I built, I thought it would be really good but HF really took a hit. I was hoping that the light EPS would extend the HF but it turned out to be even worse that other XPS panels.
This is ~12kg/m3 EPS with printer paper PVA'd onto both sides. It was really tricky to get the paper skins on without air bubbles and wrinkles. I soaked the paper with 50/50 PVA to water on a glass plate with a paint brush, let it sit long enough to fully wet, then very very careful pulled it off the plate and placed onto the pre-wetted panel. Then I used the brush with heavy strokes to work out all the air and excess PVA and press the paper as tightly to the panel as possible. It took ~2.3 A4 sheets per side.
The panel is very rigid with these skins, even more so than I expected. Other tests I've done recently have suggested that PVA damps quite well, seemingly better than epoxy and hide glue and shellac. This is pretty durable too because the PVA fully saturated the paper.
Orange line is this panel, blue line is XPS with hide glue and shellac:
This papered EPS a bit heavier than my XPS+hide+shellac and has very slightly worse impulse response. Distortion is slightly better for everything under 3k.
I might try this again but use less PVA, and maybe different paper. I thought this wobbly light EPS would need lots of damping but I guess not - a little goes a long way it seems.
This is ~12kg/m3 EPS with printer paper PVA'd onto both sides. It was really tricky to get the paper skins on without air bubbles and wrinkles. I soaked the paper with 50/50 PVA to water on a glass plate with a paint brush, let it sit long enough to fully wet, then very very careful pulled it off the plate and placed onto the pre-wetted panel. Then I used the brush with heavy strokes to work out all the air and excess PVA and press the paper as tightly to the panel as possible. It took ~2.3 A4 sheets per side.
The panel is very rigid with these skins, even more so than I expected. Other tests I've done recently have suggested that PVA damps quite well, seemingly better than epoxy and hide glue and shellac. This is pretty durable too because the PVA fully saturated the paper.
Orange line is this panel, blue line is XPS with hide glue and shellac:
This papered EPS a bit heavier than my XPS+hide+shellac and has very slightly worse impulse response. Distortion is slightly better for everything under 3k.
I might try this again but use less PVA, and maybe different paper. I thought this wobbly light EPS would need lots of damping but I guess not - a little goes a long way it seems.
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I tested the paper on EPS samples before doing the panel and I found that when dry it does pull quite hard. I had to do both sides of the panel at the same time to avoid bowing, and that was tricky. After doing this first side I used a hair dryer on it to get a bit of a skin formed so I could flip it over and do the other side. Drying was also tricky, I used a hair dryer again to get some skin on it and then I set it carefully on one edge leaning against a wall with just one corner.
Maybe they only measured the first mode of the panel.
My 60x40 panel played a quite low freq, then not much up to about 90 Hz, then another hole and from about 130 Hz up more evenly. There was "deep bass"but not much usable above that. ?
Or they have found something new. In one of their patents (about a DML subwoofer) they mention wrinkling the paper before glueing it on the honeycomb to make the panel less stiff to get better low end response.
Idk about this "less stiff = more LF" idea. I have a panel that is about 10"x21" that started as 7mm thick EPS of ~12kg/m3. I put e-glass on both sides of it with through-panel acrylic tube supports and a generous amount of epoxy that covers the edges. This panel is super stiff. By far the stiffest I've tested. It is probably as stiff as 5mm plywood though I don't have any on hand to check. This panel weighs 185g.
Here's a graph showing this panel mounted in the same frame I've been talking about recently. The blue line is the same XPS+hide+shellac I showed above, the green line is the e-glass'd EPS panel. Both are mounted the same way in the same frame in the same position.
We can see that almost the entire FR is shifted down about 10dB because of lower sensitivity, likely from weighing 2x as much as the XPS here. There isn't a disproportionate loss of LF though despite the e-glass'd EPS being probably 3x stiffer.
When testing these panels in this frame I kept the short sides (top and bottom) firmly clamped and the long sides (left and right) free. The panel's opening in the frame is just 9"x19".
Here's a graph showing this panel mounted in the same frame I've been talking about recently. The blue line is the same XPS+hide+shellac I showed above, the green line is the e-glass'd EPS panel. Both are mounted the same way in the same frame in the same position.
We can see that almost the entire FR is shifted down about 10dB because of lower sensitivity, likely from weighing 2x as much as the XPS here. There isn't a disproportionate loss of LF though despite the e-glass'd EPS being probably 3x stiffer.
When testing these panels in this frame I kept the short sides (top and bottom) firmly clamped and the long sides (left and right) free. The panel's opening in the frame is just 9"x19".
I made paper coated eps panels years ago ,and have commented on how this over damps the eps and lowers efficiency.
With poor hf performance.
Not a good combination.
I have even used toilet paper soaked in 50x50 mix pva.
This still over damped the eps.
You do get a smoother sounding panel, but it sounds less interesting.
Steve.
Hello
Thank you for sharing your measurements and your set up. I have added your full top and bottom configuration to my on going tests... and thought to other configurations to add. I will continue those days and we will see what it will come out!... hopefully.
Christian
Haha, I tried toilet paper on small test blocks too. Mine is a bit thick though, with a waffle pattern, so it held a lot more glue than the printer paper.
Your "sounds less interesting" observation is interesting. "Interesting" is a good word for it. Is that what the "DML sound" is?
I've noticed when testing unframed panels that if I let the top be free while holding the bottom they'll sound more "alive" in a way, like maybe higher "dynamics" or something, and certainly more "interesting". Holding the top in addition to the bottom deadens it. This "aliveness" is an appealing sound at first but in my tests with these panels I've prepared it comes with the cost of reverberations at some frequencies and a requirement for heavy EQ'ing.
When measuring with REW I think this "aliveness" correlates to more wild oscillations in FR with 1/24 or less smoothing. Generally I think this result is less accurate sound reproduction but something about it does sound more interesting.
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Here's a measurement of a 10x21x0.75 XPS panel with heavy shellac comparing bottom-only support and top+bottom support.
The orange line is the panel standing tall on carpet supported only by a microfiber blanket folded up behind it. Blanket is not touching the panel itself, it is only very gently touching the wires soldered onto the exciter. The panel is almost straight up so there's very little pressure on the blanket & exciter.
The green line is the same panel in the same place except that time I laid my hand flat across the top edge, pressing down a little so that the skin on my fingers wrapped around the thin edges a little bit. I held it fully upright so that it was not touching blanket at all.
This is just one test but I have tested this many other times and generally adding the hand on top "calms" a panel. Less interesting, but more accurate.
The orange line is the panel standing tall on carpet supported only by a microfiber blanket folded up behind it. Blanket is not touching the panel itself, it is only very gently touching the wires soldered onto the exciter. The panel is almost straight up so there's very little pressure on the blanket & exciter.
The green line is the same panel in the same place except that time I laid my hand flat across the top edge, pressing down a little so that the skin on my fingers wrapped around the thin edges a little bit. I held it fully upright so that it was not touching blanket at all.
This is just one test but I have tested this many other times and generally adding the hand on top "calms" a panel. Less interesting, but more accurate.
Sure, but I shared that FR overlay because we can see that many places where the green line has a peak or a dip the orange line adds a little extra flair to it. I'm thinking that this extra flair, spread out over the panel and distributed through the room and processed by humans, may be that "interesting" "DML sound". A lot of that is lost when I hold the top of the panel or add lots of damping to it.
This is even more apparent with 1/48 smoothing:
Unfortunately I lost the raw data because REW threw it away when I saved it with smoothing applied
This is even more apparent with 1/48 smoothing:
Unfortunately I lost the raw data because REW threw it away when I saved it with smoothing applied
Given the trove of data in this thread, I'm very sceptical.
FWIW, I did stumble upon a research paper studying the effect of a soft RTV gel pad between the driver and panel for a DML woofer The claim is a significant increase in the maximum panel displacement (2 to 3 times) at low frequencies over that of a traditional exciter rigid mounting.
Something else to try ...a decent DML woofer is the holy grail for me ... May be as difficult to find as the original ! 😵💫
Eucy
I believe one basic problem here is that this grade of EPS is simply to soft to transmit HF efficiently. 25 kg/m3 EPS will yield better results and allow a thinner panel (try 10mm). The 80gsm printer paper is also too thick for the purpose. Maybe look back at my posts regarding the use of yellow trace paper.
Eucy
Can you link to the yellow trace paper? I was searching for such a thing but I couldn't confirm if any of them were what you meant.
I went to the craft store two weeks ago and looked closely at several kinds of paper and it seemed like anything lighter than ~70gsm is called "trace paper" and it has a smooth plastic feel. I'm not sure if it will absorb PVA, or if paper should even absorb it or float on top for best results.
I went to the craft store two weeks ago and looked closely at several kinds of paper and it seemed like anything lighter than ~70gsm is called "trace paper" and it has a smooth plastic feel. I'm not sure if it will absorb PVA, or if paper should even absorb it or float on top for best results.
Just google yellow trace paper.it's 27gsm (you can also get it in white).. Links are abundant. Must be cut oversize, soaked both sides and laid over the PVA coated panel. Use a soft brush with more watered down adhesive to work out bubbles and wrinkles. Shrinks to a drum tightness so both sides need to be treated. As a starter, I recommend you try strips rather than a full cover. Much easier and more control over the stiffness. Make the strips about 30mm wide. Start in the centre of the panel and work outwards. Adjust gaps/ number of strips) to get the desired stiffness. That way you can adjust longitudinal and transverse stiffnesses independently
Cheers
Eucy
Cheers
Eucy
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