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the "swissroll" compact 8" subwoofer
the "swissroll" compact 8" subwoofer
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Old 11th August 2017, 02:56 PM   #41
Hemisphere is offline Hemisphere
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The room gain Dave is referring to can be quite serious. If you really want a flat frequency response you'll certainly end up having to cut some peaks with your current design. There's basically zero chance that it will be flat in your room.

That's why I suggested running some tests before building with an existing sub (borrow one if necessary), you don't need a precise measurement microphone or expensive software to get a rough idea of how your room will impact the frequency response. It could be substantial, like 12dB peaks. As mentioned a sealed box will be easier to EQ than the vented box, but I still prefer the vented. I would try and exploit the room gain to cut the cabinet size down even further and increase max SPL (less excursion for a higher tuning and smaller cabinet)

Last edited by Hemisphere; 11th August 2017 at 02:58 PM.
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Old 11th August 2017, 03:02 PM   #42
layertone is offline layertone  United States
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Quote:
Originally Posted by robinlawrie View Post
out of interest i modelled a box version with standard construction techniques and 18mm mdf.

considering the box form is more space efficient ( for stacking maybe)

its impressive how much bigger it looks that the other two, with the cylinder appearing the smallest. if you overlay them you can see the box is smaller along its 90 degree axes than the cylinder, but the corners fill visual space much more.
Prefer the radius squared one.
This design brings back alot of memories! I started about 13 years ago making multi-layered horns out of 3/4" MDF, never going back to using that material again! I highly recommend using Baltic Birch (13-ply).

What kind of CNC machine do you have & what tooling / feed rates / software are you using?
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Old 11th August 2017, 04:46 PM   #43
robinlawrie is offline robinlawrie  United Kingdom
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ive not looked into the cnc aspect deeply yet. i have a friend in london who runs a cnc shop and owes me a favour, so i am hoping he will cut the stuff out for me.

ill be providing autocad dwg's of the shapes laid out, and whoever does the job will have to work out the cut offsets and correct tooling/feed rates etc.

i will also ask around locally, as being in southern Italy the shipping of the cut parts might be quite expensive!

if i have to actually pay for the cnc work (i.e. do it locally) ive no idea of the costs involved yet. hopefully not too prohibitive.
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Old 11th August 2017, 05:13 PM   #44
layertone is offline layertone  United States
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Ah, I see! You might also want to include 1/4" dowel alignment holes in each layer. Top & bottom layers would be half-way through. I'm estimating under 60 minutes of machining for each enclosure
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Old 11th August 2017, 05:18 PM   #45
robinlawrie is offline robinlawrie  United Kingdom
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Yes i was intending to add alignment holes when the design was finalised. Do you think dowels are preferable? I was imagining threaded metal rods with countersunk nuts on the ends to place the whole thing in compression.
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Old 11th August 2017, 05:41 PM   #46
layertone is offline layertone  United States
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My first ever multi-layered sub I built back in 2001, external source for CNC work & using 1/4-20 threaded rods. I prefer using dowel rods & Harbor Freight clamps.
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Old 12th August 2017, 05:22 AM   #47
planet10 is offline planet10  Canada
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the "swissroll" compact 8" subwoofer
Quote:
Originally Posted by robinlawrie View Post
i was referring to your suggestions of a 17 litre sealed enclosure and a vented box tuned to 20 hz.?
Right, forgot about the sealed box, i thot you were talking about the alignment w 2 different vents. IITC, F10 sealed was near 30 and the vented box just under 20 Hz.

Quote:
Is there any logic to the idea of seeking the flattest response possible to allow more flexibility in eq?
As little EQ as possible is desired, and i donít know that it would counter the large group delay.

Quote:
Its much easier on the driver to subtract power at lower frequencies that try to bump it up.
True

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also the extra extension provided by your proposals is all under 20 hz, where i would need a highpass anyway.
Why? You only need a filter below the tuning of the box to avoid driver unloading (not an issue with sealed)

Quote:
ive already got big bookshelves, a giant sofa in the middle of the room, and pictures on most free wall surfaces. only zones available for any kind of treatment would be the curved cieling or the tiled floor. got some rugs down but carpeting would be a tragedy ( patterned vintage italian tiles)
I have hardwood floors, no rugs, so you are ahead in that respect.

Quote:
ive spent weeks going through dozens of drivers to find one that can provide a flat response as low as possible...
Based on the T/S this driver does go very low.

dave
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Old 12th August 2017, 11:09 PM   #48
robinlawrie is offline robinlawrie  United Kingdom
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ok i am trying to produce a tweaked design closer to the ones suggested by Planet.

frustratingly reducing the box volume and increasing the port length leads to a larger overall enclosure. im trying to find a balance between all the measurements which gives the right results in a small footprint.

at the moment im quite close, but the port airspeed is nudging 26m/s

i understand that the slot has higher resistance which will affect this, and other parameters. maybe this will allow me to use a slightly shorter port...?

i read somebody suggesting that the port length calculator here:

Port Length Calculator

can properly model a slot port.

i input the information and it told me i should be using a 40cm port length!

bearing in winisd it tells me the port should be 1.2 metres long. this is quite wierd.

i thought maybe the difference really is that large, but if i toggle the "slot port" button on that page on and off, it only changes the suggested length slightly.
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Old 12th August 2017, 11:30 PM   #49
planet10 is offline planet10  Canada
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Quote:
Originally Posted by robinlawrie View Post
maybe this will allow me to use a slightly shorter port...?
A shorter vent also needs to have a smaller cross-section, but yes.

dave
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Old 12th August 2017, 11:48 PM   #50
robinlawrie is offline robinlawrie  United Kingdom
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ok so ignoring that wierd website, and following the winisd guidance, here is the latest iteration.. its 17.4 litres internal, with 1.5x9cm slot ports, 1.2m long.

box is tuned to 23.5 hz.

The shorter one on the left is the new one.. its mainly shorter because ive switched to a realistic 18mm material thickness rather than the 20mm previously.

ive also increased the inner wall thickness to `15mm, and added some corner bracing.

the rounded rectangle design was chosen because a) i like it and b) it allows for longer grain lengths in the plywood, hence more rigidity (i think?)

ive also attached the response curve, orange is my original, green was the suggestion by Planet, and the red is my new one which tries to keep cabinet size and port velocity down whilst approaching Planet's advised response curve.

one question.. how much space should i be leaving under the downfiring driver? its got an xmax of 11mm iirc..
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