MomoTon

Sorry for bringing up another sub into this discussion, but I just found this one randomly on youtube and it´s really interesting, yet pretty old:
https://www.youtube.com/watch?v=E7Bkrypxzs4
http://www.eminent-tech.com/main.html

https://www.youtube.com/watch?v=uQfDazQ9Rkg&t=52s


Completely "new" (seems to be already 15 years old now) design compared to any cone woofer, looks and is actualy just a fan looking through a hole into a big closed(?) box to compress the air inside. The complete rotor assembly makes the classic back and forth movements to produce the sound waves.
The motor/ airblade sound is annoying for sure, but why have I never heard about (or felt) something like this?
Anyone here who know´s why this type of sub isn´t already in every cinema? If it´s as good as they claim?

edit:

found a good video about those subs, but I still don't know why the arent use in cinemas or big clubs, at least I have never heard of those ratary subs before

Also I learned that not the rotor assembly itself are moving but just the blades, similar to those of RC-helicopters I gues

Outer dimensions are 43x58x65,5cm so ~164L.

I think I could have lowered the net volume (~90-95L, B&C suggest 80L), also at the cornes I loose a little volume, but as mentioned, the limiting factor was the port lenght and to fit it inside the cab without to bend it while letting the front big enough for driver and vent flares.

Some other facts about the cab:
The "main" front is 30mm, plus 15mm added under the driver cutout to have enough depth for the 40mm Flare, and another 15mm at the backside of the front, as reinforcement for the driver (30mm-20mm would have left just 10mm for screws) and as pipe-holder. (this one was meant to be 12mm, I just didn´t think about it and used 15mm instead).
So at the thickest part, the finished front is 60mm or 2,36" thick, yet it wasn´t that heavy, obiusely because the front is mainly just three holes rather than remained material.
All the other parts were made of 15mm MPX, but as you can see, i didn´t skimp on brazings, so the cab should still be pretty stiff....
I hadn´t time to put it on a scale yet, and this one isn´t a lightweight for sure, but at least, a single person is able to lift it up and put it on a wheel board.

I will post measurements as soon as i have time to make some. The ones I did to test tuning frequency were really dirty (no fix mesaurement distances and even bader, a back panel that was hold together with clamps, as I had to be able to remove the pipes through it to make them shorter).
Actually, I always made a ~2m ground plane measurement and another one at the outlet of the port. With the second one it was pretty easy to determine a tuning frequency, although I have no idea how close this one comes to an impedance measurement.
Honestly, I don´t even know why it´s important to know the exact tuning, or how I can find the optimum, but the measurements showed some dB more around 40Hz with the shorter port, still there was enough low end in the listening test.
The great thing is that the ports are so big, that it should be ok to close one of them for small gigs or a home theatre show at home, tuning drops down to ~25Hz with one port closed.

Yes I did, its more a less the same as on the front without the cutout for the driber but with less space at the cornes, as they are used for the handles

I've finally built my first self- designed subwoofer. 

I decided to go for the 15DS115, ~90L net Volume and big ports with 3D adapted flares. 

The difficult part was actually to make the front small enough that the ports fit within the depth of the speaker, while trying to get as much flare area as possible. A simple 40mm flare radius would have made the front much larger. 

Tuning was more or less a blind shot. The vents were removable and I tryed 3 different lenghts before they and the back panel were glued. As I ran out of time,  I measured just some short sweeps in REW, trying to tell the tuning frequency from the SPL and phase graph and then cut some cm off the vent pipe

Tuning should be somewhere at 38 Hz now, but that's just what I can tell from the Rew measurements.

I put more than 90 hours into the design, but therefore it is a n enjoyment to put all the parts together and see how perfectly all of it fits. 

I havn't really tested them to their limit, they are way to overpowered as monitor subwoofer, but I'm looking forward to hear and mwasure what they are capable of. 

https://www.dropbox.com/sh/frit8jmum8ptds3/AADfJ97ODnKO5Y4Gz1iBtawva?dl=0

There is a new design suggestion from B&C, using one 21DS115 and two 18DS115 in an isobaric arrangement...

(https://www.bcspeakers.com/media/W1siZiIsIjIwMjEvMDgvMDYvMTZfMDlfMjRfMTQ5X1MyMUlCLnppcCJdXQ)

I'm not sure how to compare the outputs from the BC design to a skram or skhorn.

From a first look, it seems that this design could have the same output as a Skhorn, (driven with Sp6000 and Ipals, so not full power?) in an enclosure, smaller than a skram, with just little higher driver costs...

Anyone here who can say something about this design or isobarics in general?

All I read in a short overview is that an IB arrangement is used for compact, low end and low distortion output designs, for the tradeoff of higher driver costs... But how would the max SPL of an IB with two 18s or 21 look like, in comparison to two BR enclosures with the same drivers?
I didn´t got time to try to make a sim about that cab, but I´m curious about what else could one do with tha tkind of alignment

(I just found this during my search about Isobaric subs: https://vue-audiotechnik.com/active-compliance-management-acm/)

S21IB Suggested Design(1)_komprimiert (1).pdf

We finally "finished" our two skrams, at least so far, that we could fire them up for the first time, last weekend.

Together with our self designd Top (AMT + double 12" in  CB, also just "finished") this was one of the most stunningly setups I´ve ever heard.

The skrams made the perfect underline for the Tops, wich were designed to produce the cleanest sound possible.

It´s absolutely crazy what output just 2 Skeams have, while the Ipals arent´t even moving.
Looking forward to build 2 more of them (but the next will be made of the good old BB and be Paintet with duratex. No fancy carbon enhanced popplar plates, epoxy logos or acryll glass front)

-Josh, you are my bass-hero

kipman, we bought the epoxy and carbon fibre on a german website, "HP-Textiles". 

Important is to not use the classic fibric mesh for the good look, but the one where the carbon fibres are just laid and needled together.

We tried different epoxy systems, but both, the one especially for laminating, but also the universal did a good job, if you know how to do the job haha

Unfortunately, we still didn't finish our subs, so we dont't even know if all the work brought also some improvements except the weight savings, hopefully we know it in some weeks...

Through recommendations on other forums, I came to the conclusio to try a closed box first, if the output doesn´t hit my expectations, I will change to a BR design

External dim. should stay less than 150L
if going for sealed, I would make one duoal opposed box, a second would be added later
in a BR design, I think 2 drivers should already hit my expectet SPL so this one would be cheaper obviusely
Drivers that came in my mind:

15DS100 (~250€)   120L Vb recommendet for 40Hz tuning
15DS115 (~300€)   80L Vb recommendet for 40Hz tuning

18DS100 (~264€)   rec. encl. size 200L 😕 (for 35Hz Tuning) In 150 L ext., I don´t think it will get much more than the half of it...
18DS115 (~322€)   100L Vb recommendet for 40Hz tuning

So although I would prefer the 18DS100 for it´s price/ output, I think the 50€ upward for the DS115 would pay off for the smaller enclosure size?

Any other driver suggestions/ good enclosure recommendetions for those drivers?

Thank you two for your comments! 

I will look for different drivers in BR designs and also 4th Order BP. 

I did only a little research on them, but wasn't able to get a 12" in a 4th o. near to the response of the 6th o. design. So I tried to use a 15" instead, and with a little more modeling here,  in the same nett volume, a B&C15Sw100 or 15ds100/115 sems to be the better option over the 12", especially as the price is the same as for the beyma 12p1000

hope to have some time at the weekend to model some 4th o. and compare them to the BR design

On my way through the forum I came across a sketch Ricci put in another thread which looks pretty similar to my sketches, except the front chamber being a horn like in the skram design (and its for a 21" chassi).

What are the advantages/ differences using a horn instead of a volume + vent? I would tip on more output due the compression, but because it´s physics, there must be some tradeoffs?
Can anyone help me understand how to splice this design into sections to put it into hornresp, and with what kind of compression ratio I should start messing around with?

Alternatively, are there good papers/ explanations on this subject to find?

I´m working with two friends on a high end sound reprudaction system, and although this is our first project, we are about to aim for the highest.
The main Speakers use an Mundorf Pro AMT with two Beyma 12MC700ND in closed boxes, in MTM config., driven by a FP10k. Under the mains, we use two skrams, loaded with the 21ipal + Mod, another two are hopefully coming soon. 

Now we are designing a smaller Setup for monitoring or to use in smaller rooms. For the heights we use again a Mundorf AMT (for half the price as seen in the link, but still crazy expensive, but they are worth all the Money), and one 10MC700ND in a closed Box, which we choosed for it´s extremely low distortion. Hopefully a Hypex Fusion FA503 Amp will provide enough pover to come down to a crossover point at 100Hz, maybe 80Hz, when not driven too hard. 

So there are missing one or two small, compact subs, and as we couldn´t find a design which was promising enough, we desided to design our own 6th order bandpass. We found design recommendations on B&C, (S12BP) and 18sound (12in BP subwoofer), but both of them just doesn´t seem right.

So I started to design my first 6th order BP from there, calculated the volumes and port dim. of the two designs, threw it into Hornresp, looked at the response and was not really satisfied. So I searched for other similar drivers and found the 12P1000ND but also the SICA  12 S 4 PL, both for around 320€. I threw them both into the sim. and they looked pretty good, much better than the orignial drivers. As the new drivers have much more powerhandling than the originals, I would tend to use the B&C design, as it uses 220cm² Ports (rect.), instead of 157cm² (round), also the tuning of those seem to make more sense.

A first big question is about the different alignmet of driver and the vents between the two designs as seen below. Intuitively I would say the 18sound has an advantage here, as the B&C let one part of the driver look directly into the vent, which I would guess makes it easier to hear distortion? It seems they did this as an tradeoff for the longer rear Chamber vent.

I would try to do a mix of these designes, mainly like the 18s., but with bigger, rect. Vents like the B&C.

Which of those (four) designes would you recommend? Or is there another smart possibility? I would go for my left one, if I can fit the vent lenght inside, alternatively for the right one.

My other quastions are basicly about how to use hornresp:

-LC1/LC2: is here the longest path for a standing wave requested, or the nearest parallel wall of the driver?

-should I always click on the "Lossy Le" button in the Wizard schematic window?

-I tried to hold the Port velocity at ~ the rated RMS Power under 25m/s, is that ok or still too high? the tradeoff for bigger Vents are it´s lenght and port resonance, right? How much over the crossover of ~100Hz I should hold that one?

-Does it even make sense to go for a 100L cab, or would it then be better to use something like a 14in or 15in driver?

-maybe one could look at my hornresp input to see if it makes any sense at all?

-any other advice/ design suggestions are very welcomed, as this is my first try on designing a 6th o. BP, I would appreciate any comment/ suggestion/ idea or other help.

On 2/25/2021 at 12:10 PM, bee_dub said:

While I'm at it, what are the important considerations for the type of plywood?  Is it just stiffness?  I used some lightweight plywood a little while ago, and from memory it was something like 40% lighter than the usual stuff.  The seller claimed it was as strong as a standard type of plywood, but I don't know by what metric.

I don't know if this is a foolish aim, but I'd love to be able to throw one of these around by myself if necessary..

I don't know if you saw my skram built on page 26 or so, I built 2 of them, made of popplar plywood with a carbon-epoxy layer around to make it stiffer. Then we made the front out of 25mm transparent acryl glass, just to loose the weight savings due to the 10kg (just a wild gues, I forgott the real numbers) of those fronts haha. We didn't put them on a weighting scale yet, but without the heavy front, they would be ridiculously light! So light that we will make a base filled with sand to make sure they dont move more than the drivers do.

We also didn't fire them up, but that will happen soon, hopefully... so I cannot say anything about the stiffness and if there is anything hearable compared to our baltic birch skram, but I will male a post as soon as that happened. 

We will make 2 more of them, without acryl glass front, because we have the materials, but I also have to say that it is a hack of work to prepare all the plates, and also, it is really expensive of course, so i dont think that we will do this again after those next two.

In Europe (Austria) , you would pay easy 3-400€ for the BB here. 

The cool thing with the price list is, one can still get a good discount on them, as those are the recommended retail prices. With the discount my salesman told me to expect (also for single units!) i could get all the parts for the EDM sub, so two M-Forces, the Amp, DSP and all the other components, for under 6k before VAT, but in austria, you just open your busines, and don't have to pay VAT.

And then you have a beast that can go 145dB+ and down to under 20Hz (half space) with, as they say, nearly no compression... Damn I want to build one of those so hard! 

actually the price for one Transducer is at ~3000€ before taxes, with the Amps, DSP, Interfaces ect. you should land under 6k, even with the big DSP.... AND it´s much easier to get those than you might think. You don´t even need a VAT number, at least, if you get to the right salesman.

I´m actually thinking to build one of the EDM subs (2x M-Force) and put it under 2-4 Skrams... I think that would be a great match up.

the acrylic is 25mm (and about 10kg haha) and has a tongue on its 3 sides. the side panels and the top plate were slotted, there you can also see the epoxy edges we had to cast, as popplar is really soft. 

to fix the brazings on the front we made external 5mm acrylic tongues and slotted the brazings as well as the acrylic front

after 4.5 months of preperation and four 12h days for assembly, we finished our second skram, the first one world wide with an acryl glass front? 

We didnt measure the weight yet, but although the front has about 12kg, it feels a lot lighter than our prototype made of birch, as we used popplar instead. It also feels rock solid due to the carbon-epox enhancement on all surfaces and edges, but the measurements will show if the light material has any disadvantages...

Wellif we would change tuning and the relationship, I think we will stick to the original design and re-cut all the internals to 574mm...
 

Yesterday we started with the cf layer on the side panel... with a massive cross and a frame on one side (later the inside), and a recessed drive-in nut on the other side, we pull the panel vie a m12 bolt towards the cross. After the cf layer, this nut should be hidden in the panel.

The same way we will also insert more drive-in nuts on the bottom of the side panels, as well as in top and bottom panels... Instead of the cross, we are then easy able to screw Handles in and out, attach stands and even a easy detachable metal frame is planed for transportation. Instead of handles or feet, we are also able to use Pins to stack two skrams precisely, and to fix them together, without lashing straps.

I´m not sure if this level of details is ok in this thread, i don´t want to overload it, but i think this way of making detatchable handles may be interesting for some others too.

I finished my first skram some months ago, which was only a prototype and reference built for us.

The next 2 skrams shall be absolutely high end- not only in terms of it´s fantastic bass, but also in manufacturing. They will not be made of 18mm birch, but poplar. Due to the lower density and stiffness, we will overlay all the panels with carbon fabric and Epoxy. To get the side panels even stiffer, we will pre stress them (seen in the first picture) and overlay the stressed panel with carbon, when we press it flat again, the carbon on the outside of the skram should have enorm pre-tension. Due to this  this pre-stress, we hope to reduce enclosure resonances to a minimum. (our tests look good so far, I will post them, if we have final results if one is interested.)  We should be able to save 15-20kg+ and hopefully still have a resonance-dead Enclosure.

There will be also other crazy features like a transparent 25mm acryl glass front or detachables handles. As the brazings behind the front will be seeable, we want to re-design them, seen in the second picture. We removed material from the corners, as they should be stiff enough through. See you any problems concerning those brazings?

 main question is about the enclosure size. I read somewhere in this Thread that it should be possible to make the skram a slimmer, but this would reduce the Output... Is it also possible to increase the witdh to gain some more output, but to hold all the other dimensions the same? How much gain could we expect (i.e. with +10cm), and would have the same tight bass?

//edit:

I forgott to say hello to all, and to thank you, Josh, for all the work you have put into your designs and the all the work around them...

A friend I'm doing this project together with, posted some months ago ... We still don't know much about sound, but we learned a lot since summer haha