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Ricci's Skram Subwoofer & Files

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This is what I have come up with so far. Was thinking of fixing the mesh at the end of the mesh that's easy to get at with screws and washers and the other end will kind of grip into the wood by the sharp ends it has from being snipped to size. The mesh is about 1.6mm strand size, its easy enough to bend by hand but will still have a bid of tension on it from being pulled into position by the screws.

What do you guys think? Could it vibrate perhaps?

 

 

IMG_20191018_095150.jpg

IMG_20191018_095302.jpg

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18 hours ago, jay michael said:

These will make 6 in total

 

Dang...Firepower!

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18 hours ago, menace said:

This is what I have come up with so far. Was thinking of fixing the mesh at the end of the mesh that's easy to get at with screws and washers and the other end will kind of grip into the wood by the sharp ends it has from being snipped to size. The mesh is about 1.6mm strand size, its easy enough to bend by hand but will still have a bid of tension on it from being pulled into position by the screws.

What do you guys think? Could it vibrate perhaps?

 

 

IMG_20191018_095150.jpg

IMG_20191018_095302.jpg

 

This should work to keep critters and junk out. Perhaps add a thin strip of foam under the screen where it contacts the wood to reduce the possibility of any audible vibration. 

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Thinking about the size I thought of changing the dimensions while keeping the specs the same. This would end up being a 60 x 60 x 120cm cabin. Would this work? 

Also, could I make the horn part straight (so S1=S2=S3) as long as it models correctly? 

 

 

IMG_20191018_194922.jpg

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I don't see why it wouldn't work. 

The reason I always try to fold the horn/slot section is to avoid firing the driver directly into an outer panel. There is usually room for only minimal bracing in front of the cone, no damping material and depending on how the driver is accessed it may even be a hatch. There is a lot of acoustic power concentrated into a small area with only a single panel of wood separating it from the outside world. It can result in audible radiation/vibration from the panel at war volume. It's a larger problem with bigger diameter drivers like 21's due to the larger panel area. Just something to consider. 

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I've been to an indoor show tonight with 16 KS28 and while the system was horribly calibrated (if it was, even), the bass was just ridiculous. I've never been to a show this size (2000 people) where the bass drops were so loud that they would make your clothes vibrate below 40Hz. I wonder how the SKHorn compares to the KS28. The Skram vs SB18 should be a clear win for the Skram since the SB18's F10 is 32Hz, but the KS28 has an F10 of 25Hz, similar to the SKHorn.

I'd love to have the KS28 tested on data-bass, but it's unlikely that somebody lends you a cab worth 10 grand, which might break during testing.

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Hi Everyone,

Really enjoying the Skhorn and Skram project's!

I'm really itching to have a go at making sawdust, but don't have funds to splash out on new drivers so was hoping to use stuff I have lying around.

I have a couple of these pr21250 drivers (specs in this link https://www.thespeakershop.ca/store/p94/MAX_Professional.html )

If someone with a better idea than myself of ts spec's could have a look and give me some feedback, it would be greatly welcomed!

Many Thanks.

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Could this same principle be applied to a port? No idea how this would effect tuning though.

 

In trying to see the influence a sloped port has on the velocity I modelled it as a rear loaded horn. Doing it like this requires some parameter juggling etc, but I did manage to create the almost exact same response for the 'rear loaded horn' as for the port in my Skram version. Modelled velocities only differed slightly.

Parameters

1471129250_velocity750750parameters.PNG.a87ab32f6c6233150f7fcea028c2ac8a.PNG

(my version has a 183L chamber, aka 183000 Vtc. Modelling done with the 21" LaVoce). 

Velocities

116792906_velocity(750x85)regularmodel.PNG.0bf26bc27d72ddb445bd113ca2999cec.PNGport modelled as port541104353_velocity(750x85)backloadedmodel.PNG.ef986e4bfd57e524e68a2f87ad644ce7.PNGport modelled as rear loaded horn

 

To try see if a sloped approach could work I tried out a few different parameters. Goals:

1. Keep the port output the same

2. keep the port volume the same

3. use the bends to make up the horn (in my case, 35cm and 50cm)

 

Then we get the following input:

1664824451_velocity5007501000parameters.PNG.bf09404897c9784022f7817f46ef1e62.PNG

Which results in the following velocity at the mouth:

651624660_velocity(500-750x35750-100x50)velocity.PNG.34ada5ce65045c9510324e51e729bdec.PNG

This is significantly lower than the normal unsloped port, from 33m/sec to 25m/sec. Granted, the velocity at S2 and port mouth is enormously high, but S2 is 50cm away from the mouth and and the horn throat 85cm, which might limit the audibility of the port?

 1072752236_velocity(500-750x35750-100x50)S2.PNG.42373a4efa4bdb5da1904e3e9cefecff.PNG

1707181031_velocity(500-750x35750-100x50)hornthroat.PNG.a7866f72ecfeec686de91d6b22c27d50.PNG

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The problem with too much vent velocity is not just chuffing but also power compression, and substantial compression can set in well before the chuffing becomes audible.

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4 hours ago, SME said:

The problem with too much vent velocity is not just chuffing but also power compression, and substantial compression can set in well before the chuffing becomes audible.

You mean port compression I guess? I couldn't see how power compression relates to wind speeds

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5 hours ago, SME said:

The problem with too much vent velocity is not just chuffing but also power compression, and substantial compression can set in well before the chuffing becomes audible.

Didnt think about that, thats true. Could you theoreticaly model this by the 'pressure' function? In any case, bit of a derailment here, so I'm just gonna start a side topic to explore this further. What I noticed was that 

the peak sound pressure in the mouth was around the same in at mouth (around 9-10 pascals, and a little bit lower for the sloped port) and exactly the same at the throat. So going from this I dont see a disadvantage of doing it like this. Lowering the velocity by at least 25%, without increassing pressure (if this is related to power compression ofcourse).

 

 

 

 

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8 hours ago, peniku8 said:

You mean port compression I guess? I couldn't see how power compression relates to wind speeds

Air flow is very non-linear versus velocity, and losses increase  very quickly at high velocities.  A rough rule of thumb is that you compress 1 dB for every 10 m/s (also depending somewhat on the flow area and overall shape of the passage) , but there is also a saturation point, maybe in the 30-50 m/s range (again depending on details), where output stops increasing altogether.  Often by the time you hear chuffing, you're already at that point.

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2 hours ago, SME said:

Air flow is very non-linear versus velocity, and losses increase  very quickly at high velocities.  A rough rule of thumb is that you compress 1 dB for every 10 m/s (also depending somewhat on the flow area and overall shape of the passage) , but there is also a saturation point, maybe in the 30-50 m/s range (again depending on details), where output stops increasing altogether.  Often by the time you hear chuffing, you're already at that point.

I think you misread my comment or quoted the wrong post. Power compression is the compression introduced through heat (electrical), which I was refering to.

And thanks to @radulescu_paul_mircea I know now that the drivers in the KS28 seem to be the 18SW115. That and some measurements I've found on the internet (especially voltage sensitivity and distortion measurements) were very valuable information for me. Now I feel less intimidated by the huge numbers, which marketing departments come up with.

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Heard a bunch of those KS28's at a Rammstein concert earlier this year, and I must say the sheer wallop and physical presence they delivered was rather astounding. In a some 50,000 ppl stadium we sat about the furthest away from the stage we could, in the upper balcony, and yet the bass pounded in a manner almost beyond belief. It mayn't have been great bass per se, but it sure as hell was physical, dense and loud!

Btw, I thought the KS28's housed BMS drivers?  

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8 hours ago, peniku8 said:

@radulescu_paul_mirceaI think you misread my comment or quoted the wrong post. Power compression is the compression introduced through heat (electrical), which I was refering to.

Oh, I see.  I guess I've always interpreted "power compression" to refer to *any* kind of compression that occurs in the speaker.   Maybe that's not really the correct usage of the term.  What I meant was output compression in general.

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16 hours ago, m_ms said:

Heard a bunch of those KS28's at a Rammstein concert earlier this year, and I must say the sheer wallop and physical presence they delivered was rather astounding. In a some 50,000 ppl stadium we sat about the furthest away from the stage we could, in the upper balcony, and yet the bass pounded in a manner almost beyond belief. It mayn't have been great bass per se, but it sure as hell was physical, dense and loud!

Btw, I thought the KS28's housed BMS drivers?  

afaik the SB18 and SB28 come with the BMS drivers, while the KS28 uses the B&C.

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On 10/20/2019 at 7:11 AM, Droogne said:

Didnt think about that, thats true. Could you theoreticaly model this by the 'pressure' function? In any case, bit of a derailment here, so I'm just gonna start a side topic to explore this further. What I noticed was that 

the peak sound pressure in the mouth was around the same in at mouth (around 9-10 pascals, and a little bit lower for the sloped port) and exactly the same at the throat. So going from this I dont see a disadvantage of doing it like this. Lowering the velocity by at least 25%, without increassing pressure (if this is related to power compression ofcourse).

 

 

 

 

Don't miss the forest for the trees. It is not only air velocity at the mouth that is of concern, but also at the throat or entry inside of the cabinet. If the port is flared, bowtie or otherwise more complexly shaped the performance at the "choke" points should also be examined. Any of these points can trigger compression. It's more complex than just airspeed at the mouth. You may decrease audibility of air noise but greatly increase output compression. The Bowtie type of slot vent is probably the easiest to incorporate into a design but it increases the total size of your speaker, or you end up with not much in the way of improvement, due to downsizing the minimum area of the duct to keep the same tuning. I decided not to bother with it for this design due to size and packaging constraints. 

More involved programs such as ABEC or Akabak can model, velocities, pressures and accelerations at any point in a pipe, duct or horn. 

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3 minutes ago, Ricci said:

Don't miss the forest for the trees. It is not only air velocity at the mouth that if of concern, but also at the throat or entry inside of the cabinet. If the port is flared, bowtie or otherwise more complexly shaped the performance at the "choke" points should also be examined. Any of these points can trigger compression. It's more complex than just airspeed at the mouth. You may decrease audibility of air noise but greatly increase output compression. 

More involved programs such as ABEC or Akabak can model, velocities, pressures and accelerations at any point in a pipe, duct or horn. 

Yesss! Sorry I hadnt posted my conclussion, but here is what I realised and left in message to peniku: 

'It dawned on me that using sloped ports (so eg 250=>500=>750 instead of the regular straight, eg 500=>500) is wrong for the same reason[creating a bottleneck]. Port compression would occur sooner because you start with a throat that is smaller, so higher velocity at the throat in the sloped design than wherever in the straight design. I will not be going that route afterall for that reason.'

 

So yes, I realised exactly what you just said :) I can post the modelling in hornresp here, but you already understand what I mean. 

Also, wouldnt noise created at the throat (if your  using a horn shaped port) be 'hornloaded'. So the noise would be created deeper in the cabin, but it would be increased. 

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Also @Ricci, what is your opinion on using a rectangular port instead of a squared/rounded one? Design wise it's obvious why you decided on using a rectangular one, but by my calculations (based on info from the Flare-it app (https://www.subwoofer-builder.com/flare-testing.htm) using an elongated port like the one in the Skram or Skhorn decreases the required velocity where port compression starts by almost 50%!

I calculated velocity where compression start for the Skhorn (with IPAL), which resulted in around 40m/sec at 30hz. If youre interested I could PM/post my calculations here. In any case it seems to correlate with what people are noticing in their Skhorns. Using a square port with the same area I got a 80m/sec value. 

As port compression results from core turbulence (something that isnt influenced (much) by flares) due to friction/turbulence with the walls, the bigger (wall-circumference : area ratio) of a rectangular port results in compression at lower velocities. For the Skhorn the ratio is exactly 153% bigger (0,29) than for a square port (0,19) with the same area (418cm²), and increases further to 190% (0,37) when considering the dividers which add a lot of wall surface.  

Going back to the Skram we can ofcourse see that the bigger ports help reducing velocity, but as compression happens at a certain velocity at a specific port area the bigger ports of the Skram can also take on higher velocities before compressing. So both lower velocity, and better toleration of high velocities. Win win. Sadly, we do find the same 'problem' because the Skram is still has a very rectangular port which uses even more port dividers. The ratio is around 195% higher than a square version would, but the circumference/area ratio is still way better than the Skhorn (0,29 for the Skram vs the 0,37 of the Skhorn) so it will be able to take on higher velocities. 

Peniku did make me aware of the fact that panel vibration could be a problem when using square ports, but its clear that you dont need square ports to take advantage of this. Using less dividers (so less tuning opportunity and structural rigidity) or port which are less rectangular could help reduce compression OR help making ports smaller without changing compression. Pick which one you need ;) 

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On 10/21/2019 at 12:56 AM, peniku8 said:

And thanks to @radulescu_paul_mircea I know now that the drivers in the KS28 seem to be the 18SW115. That and some measurements I've found on the internet (especially voltage sensitivity and distortion measurements) were very valuable information for me. Now I feel less intimidated by the huge numbers, which marketing departments come up with.

It is in fact a modified version of 18SW100. Slight difference, mostly in suspension

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9 hours ago, Droogne said:

Peniku did make me aware of the fact that panel vibration could be a problem when using square ports, but its clear that you dont need square ports to take advantage of this. Using less dividers (so less tuning opportunity and structural rigidity) or port which are less rectangular could help reduce compression OR help making ports smaller without changing compression. Pick which one you need ;) 

You mentioned the port dividers and how they add more wall area to the port, which made it sound like you were thinking about removing them. If you did so, the panel vibrations of the outer (now unbraced) side panel would introduce more distortion that removing the braces would lower it. That's what I was refering to.

I still think vent noise can almost completely be ignored in 25Hz and above PA applications, while port compression needs to be minimized. 

If you think about your wall area and air friction so much, maybe try polishing the inside of the port or apply black paint -> clear coat -> polish. I'd be interested to see if theres any difference :)

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1 hour ago, peniku8 said:

You mentioned the port dividers and how they add more wall area to the port, which made it sound like you were thinking about removing them. If you did so, the panel vibrations of the outer (now unbraced) side panel would introduce more distortion that removing the braces would lower it. That's what I was refering to.

I tend to agree, but if you follow my comments earlier you can see that by using a square/less rectangular port and removing port dividers would allow to use a smaller area because there is less friction.. So yes, no port dividers and a more squarely shape would give less bracing to the outer wall BUT the smaller port would give more bracing (or would take in less wall space, so would allow for more bracing) . I think if you meet it somewhere in the middle there can something he gained. It would also allow for a smaller cabin, which is sturdier in itself. 

Quote

I still think vent noise can almost completely be ignored in 25Hz and above PA applications, while port compression needs to be minimized. 

Also agree. I just want to know it precisely because if you can achieve the same amount of compression with a smaller port you save up space. In other designs it could allow for longer ports (not so relevant in the Skram/SKhorn) 

Quote

If you think about your wall area and air friction so much, maybe try polishing the inside of the port or apply black paint -> clear coat -> polish. I'd be interested to see if theres any difference :)

I was actually planning on maybe using 'concrete plywood' (the dutch word, I don't know the English one.) its basicly plywood with a thin but very smooth layer on it. Specifically made to be smooth (and waterproof) actually. Wouldn't be much harder to work with than with regular ply, so I don't see why not. 

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Droogne I've done square ports before and they work well. The ideal is round and heavily flared or shaped which I've also done where it makes sense. Have you read the available AES and other papers on ports? There are many. Compression and chuffing noise do not necessarily correlate. There are also papers which point to some possible advantages with multiple smaller vents despite the increased skin effect.

In the case of the Skhorn and Skram variable tuning is an integral part of the design that I considered fundamental. That is why they have many smaller vents instead of a larger single. Also it is structurally very strong. When considering them as individual smaller vents they are not too far from square. 

Its all about the tradeoffs. I think you may be overestimating the effect a change to the shape may have on compression. A single square or even round vent the area of one of the Skhorn vents isn't going to gain much. They will still overload and compress. The area is not enough to cope with the demand once the sub is pushed at tuning. This is true of almost every sub tuned under 20Hz. In the Skhorn's native tuning with all vents open with 3X the vent area this isn't really an issue as the testing showed. At the end of the day it's all about vent area in my opinion. 

 

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