Long vent/port as bracing in Ambiorix?

I'm designing my 'Ambiorix' subwoofer, that uses a Monacor SPH-212 in a vented enclosure of 74 liters, tuned to 28Hz, to get an F3 of 30Hz (maximally flat). Minimal stuffing, base is 300mm*300mm, so *total* height is 1,2 meter. I plan on using a downfiring port ant put the sub about 100mm from the floor, so I can put the amplifier module underneath it (near port as cooling air).
It should downfill my CesT's in which I use a Monacor SPH-60x.

I'm a little bit unsure on this: I searched around on the forum about which width of MDF to use. In the most results that came up, people used 3/4" (18mm) MDF, but I plan on using 1/2" (12mm) MDF that is available at my local DIYmarket. 3/4" just seems too heavy, and since I use a pillar-like (floorspace saving :D) design, I think there will be enough bracing from the other panels to prevent flexing.
Just in case, I was thinking about using the downfiring port as internal bracing. So width of 276mm, and by increasing the height till 70mm, the length becomes 927mm. Almost the entire length of the cabineth and still leaving enough space at the end for the air to pass.
Did someone already try bracing like this and what were the results?
Any tips about things that I overlooked?

Also, for stability I will put the driver in the bottom of the front baffle (front mount, not flush), but if I look at the path the backwave will make inside the cabinet, I can't help thinking about a Transmission Line. Unfortunately I don't have the tools or skills to compare my design to a similar (size!) TL line and see if there are any advantages, so if anyone wants to give it a shot ... ;)

TIA,

Cordraconis
 
With a tall design, you need to pay attention to damping because resonances are a problem.

Your 1200mm high cab will have a res of about 146hz.

If your driver is 900mm down from the top, you will get a driver-to-top-wall res of 96hz. Moving the driver up to 700mm from the top will change this figure to 125hz

I would suggest a shelf brace across the cabinet just above the driver, with 2 layers of acoustic wool stapled to the brace.
Put some foam rubber on the top and bottom faces. The other walls don't need damping.

This treatment will cut these resonances by about 6dB

regards
Collo
 
Collo said:
With a tall design, you need to pay attention to damping because resonances are a problem.

Your 1200mm high cab will have a res of about 146hz.

If your driver is 900mm down from the top, you will get a driver-to-top-wall res of 96hz. Moving the driver up to 700mm from the top will change this figure to 125hz

:confused:
ermmm ... now you got me confused. I skimmed briefly trough Serway's "Physics for scientists and Engineers" from my uni days, and for the first constructive interference between 2 immobile boundaries, the distance must be one wavelength. If I take the airspeed as 344m/s, and the inner walls are 1,1 meter apart (remember the 100mm floor clearance for the port), the res freq would be 312,7 Hz.
The same way, a distance of 0,9m gives a res freq of 382Hz.

I don't understand where you get the 96Hz and 125Hz figures ... did I miss some kind of driver-top-bottom-driver reflection? :confused:

Anyway, the res I calculated are far enough above 160Hz, which is one octave above 80Hz = the upper freq I plan on using the sub. I guesstimated a very small amount of stuffing around the driver. My CesT's higher freq range benefited a lot from that, and if there is any high freq spill or res in the sub, the driver stuffing should take care about that.

simon5 said:
12mm is not alot.

If you have some money left, you could get 18mm birch plywood.
About the same weight as 12mm MDF, but way way better.
It's even better than 18mm MDF.

I think that thinner walls but better braced, gives better results than thicker walls with no extra bracing, and is lighter too. (I'm planning of moving out of my parents house by the end of the year.) I remember some discussions about this, and it's also in the B&W Nautilus whitepaper. --> Matrix enclosures!
I was thinking about using some leftovers from cutting as vertical bracing around the port, but I'll see what Collo replies and maybe put them horizontal and hope it doesn't restrict airflow too much.

Thanks for the tip dough, I'll see if they also sell the flywood there. I'll let the price decide.
 
The resonance between 2 walls is half wavelength because there is a phase reversal upon reflection. :bigeyes:

From the perspective of the air....

The walls are each zero velocity / maximum pressure nodes.
Pressure node to pressure node equals half wavelength

A driver is the oposite - maximum velocity / zero pressure
Velocity node to pressure node equals quarter wavelength

So the resonance between the driver is quarter wave.


This is why a driver that is exactly half way down a box excites three resonances that are the same value

Box top to bottom - half wave in distance x gives frequency f
Driver-to-top wall .....1/4 wave in distance x/2 gives freq f
ditto for driver to bottom wall


For a driver near the top or bottom of an enclosure, the standing wave from the driver to the wall that is furthest away, will be of a lower frequency than the box-top-to-bottom res.

This is where the 96hz and 125hz figures come from.


I forgot to factor in the 100mm clearance for the port so my figures were based on a box 1200mm high not 1100mm.

Re-doing for 1100mm high (external) with 12mm walls you get:

Box top to bottom - 159hz
Driver just below centre of box, (650 down)
Driver to top resonance- 134hz

If you leave the driver near the bottom of the box (800 down)
you get Driver to top resonance- 109hz

which is near enough to 80hz to be a problem...

regards
Collo
 
Collo said:
The resonance between 2 walls is half wavelength because there is a phase reversal upon reflection.
(...)
Box top to bottom - 159hz
Driver just below centre of box, (650 down)
Driver to top resonance- 134hz
Woops! After taking a good night's rest, I discovered chapter 16 was followed by chapters 17 and 18. :eek:
For air, a boundary indeed gives 180° phase reversal since there is no air motion. Woops, guess my memory starts to dent after 2 years of working life!

Well, Internal length is thus 1213mm (after some reiterations for a 18mm thick port.)
Mounting the center of the driver at 707mm from the top, (so the top of the cone is at cab center and I can still keep the driver in the lower half of the cabinet for stability), gives a resonance of 120Hz, right?
That's half an octave above 80Hz, and with some stuffing on top and bottom of the cab I think I should get away with it. No? :apathic:


paulspencer said:
If going to the effort of DIY you might as well do what commercial speakers can't do with their affordable speakers.
You have a good point there.
I checked the difference in size between 18 and 12 mm.
With 18mm MDF (or plywood for that matter), the total height is about 100mm higher than with 12mm MDF. Something I could live with. And with plywood I might as well sand an varnish the whole thing ... Hmmm ... :)
 

SpeakerBob

Member
2002-08-15 6:21 pm
UK
"" I checked the difference in size between 18 and 12 mm.
With 18mm MDF (or plywood for that matter), the total height is about 100mm higher than with 12mm MDF. Something I could live with. And with plywood I might as well sand an varnish the whole thing ... Hmmm ... ""



Going from 12mm to 18mm should only increase the outside dimension by 12mm on each dimension. Ie: 12mm taller, 12mm deeper, 12mm wider.

Cheers,

Rob.

ps: you could use the 12mm and skin the finished box with 6mm ply......
 
Now you're getting the hang of it!
Your calcs on the resonances are fine. As you said, some stuffing top and bottom will be fine.

Here is a little more fine tuning to consider.
Where possible, I like to keep an unimpeded path from the driver to the port intake. After all, at low frequencies much of the energy will be coming from the port.

If you put the driver a little above the box centre, rather than below it, you will get the lowest frequency resonance between the driver and BOTTOM of the box rather than the top.

This allows you to put all the damping treatment at the bottom of the box - dealing with both the driver-to-box-bottom res and the box-top-to-bottom res in one go. You can then leave the area at the top of the box free from wool which could lessen the effectiveness of the ports.

The downside is that you would need to add some mass to the bottom of the box. The upside is that the extra mass will couple the sub to the floor more effectively, which is good (unless you live in an upstairs flat)


Next question. What diameter port are you using and what is the peak airspeed going to be?.

regards
Collo
 
Don't have much time, work is lurking in less than 20 minutes. :)

RobWells said:

Going from 12mm to 18mm should only increase the outside dimension by 12mm on each dimension. Ie: 12mm taller, 12mm deeper, 12mm wider.

But not if I took a fixed floorspace, so to adjust for the increasing port size (I'll also take 18mm for that), the height gets a little bigger.

Collo said:

Next question. What diameter port are you using and what is the peak airspeed going to be?.
300mm internal width, makes the port also that wide if it is to be used as bracing. By increasing the height till the length of the port is 1015mm (about 200mm clearing from the top), the height became 70mm.

So port is 70mm*300mm, and 1015mm long. I think that is *more* than wide enough to prevent shuffing.

Form follows function (= bracing) to the extreme, and better sound as a bonus. :cool:
 
Now I understand! You are using a rectangular port. I can see now that the bracing would work very well.

The 300 * 70 mm port is about 210 cm^2 in area which is about the same as a 160mm dia round port. I agree that this should be large enough to avoid chuffing.

Dont forget to take some photos along the way to show us. I think it will look great!

Collo
 
Collo said:

Dont forget to take some photos along the way to show us. I think it will look great!

First I'll need to invest in a digicam, but I'll see if i can speed that up. :xeye:


a little more than a week till D-day, so here is my last proposal:
I increased the width of the base till 310mm internal (so 346mm external), so the resulting height for the enclosure will be 1070mm internal. I did this since the SPH-212 is a 210mm driver (8.4" ???), and maybe later I could fit a 300mm (12") driver and I want the baffle to be just big enough for the cutout hole.

To fit the port inside with about 200mm clearance from the top, I resized it till 310mm(off course!)*60mm. Height became then 897,5mm.

Total cabinet size is now about 6 liters smaller due to the smaller port. Whiiiiiiiiii! :D

If it all checks out ok, then I'll start drawing everything on wednesday.
 
Short update on my Ambiorix project:

- Schematic/drawing complete
- Went a few hours ago to the DIY shop to look for suitable 18mm wood, but they are pre-cut and shrink-wrapped (140cm*40cm) and therefore they don't cut them to my desired size. The 18mm MDF (125cm*55cm) is not shrink-wrapped, so the shop will cut them for free. I also did a brief knuckle-tap test, and the MDF is *much* more dead than wood, and it's even cheaper!
--> MDF all the way!
- Tomorrow morning I will go back and have everythink cut.
- I ordered a Sony P200 digital camera in a local shop, but now it is monday here, and they still didn't call me. Hopefully I'll have it tomorrow, just in time. ;)
 
Final stages

OK Collo: as promised:

"I am finishing the last stages of the building process:

Yesterday I carefully glued all the vertical panels to one side panel. The 2 shorter panels in the middle are the bracings/vent.
Notice the L-shaped clamps I used as guiders and to withstand the pressure of the bottom panels in the next stage.
[IMGHTTPDEAD]http://users.telenet.be/Hellhouse/Ambiorix/gelijmdboven.jpg[/IMGHTTPDEAD]

After putting the glue on I measured the panels and noticed there were several millimeters differences in the spacing between top and bottom. I had to throw myself in a sawing and sanding fury to make the wooden pieces while the glue was drying, but it all worked out like a charm. :D
[IMGHTTPDEAD]http://users.telenet.be/Hellhouse/Ambiorix/parallell.jpg[/IMGHTTPDEAD]

This morning I went back to my granddad's garage, removed everything and put the other pieces in place. Then I noticed something odd:
[IMGHTTPDEAD]http://users.telenet.be/Hellhouse/Ambiorix/NIETparallel.jpg[/IMGHTTPDEAD]

:mad: I made sure all the panels were perfectly parallel, but it seems they weren't at 90° with the side (bottom) panel !!!
To make it short: today was full of 4-letter-words (but since I'm not a native english speaker, they were more-letter-words. :mad: )

The L-clamps I used (and hand-sawed and drilled myself from an aluminium L-profile my dad had lurking around), proved well to pull, skew, stretch and swear the panels back to place as well as possible. (skewed 7mm, now bent back to about 1-2mm ... I can live with it, but I really hope the bending pressure is not TOO severe ... 18mm MDF glued together is rather stiff !!!)

The dark patches you see on top of the cab is glue where I put the "mousse" or foam-sheet. It should work out well enough to dampen the res peak from the 1070 mm inside cab dimension (130Hz), and prevent any high freq content to enter the vent. Since it's a sub, there won't be anything above 80Hz, but still ... ;)
[IMGHTTPDEAD]http://users.telenet.be/Hellhouse/Ambiorix/screwher.jpg[/IMGHTTPDEAD]

After that I finally managed to get to the last construction stage and finish the cabinet. Notice the EXTENSIVE clamping and weights I used to keep the cabinet in shape while the glue sets my skewing and screwing attempts.
( my father wants his heavy toolkit back, and my grandfather thinks I'm crazy I used his parasol pedestal :cool: )
[IMGHTTPDEAD]http://users.telenet.be/Hellhouse/Ambiorix/enlijmenzalzeboven.jpg[/IMGHTTPDEAD]

And from another view: (Notice the "band' I used to clamp bottom and top panels. The clamps my granddad had were not long enough ..."
[IMGHTTPDEAD]http://users.telenet.be/Hellhouse/Ambiorix/enlijmenzalzeonder.jpg[/IMGHTTPDEAD]
 
Slot port

Congratulations

its always nice to get to the stage where you can see results. Thanks for posting the pictures. We can all see how the port works now.

I see you have found plenty of weights. In one of my stories I jokingly recommended weighing down with large motorcycle or small car!

We all have a few setbacks - it's part of what is to be a DIY'er. You are fortunate that your language allows more letters for your expletives. Sometimes four letters are not enough....:hot:

I look forward to hearing a report on how it sounds. Not long now.

regards
Collo
 
Re: Slot port

Collo said:
Not long now.

hahah! Well, it looks longer/taller than I expected once it stands here next to my PC. And difficult to grasp (not lift) by myself ... :D
[IMGDEAD]http://users.telenet.be/Hellhouse/Ambiorix/biggrin.jpg[/IMGDEAD]

The sound? Well, i took the old amp of my father (Kenwood KR-5600), and connected one channel to the sub. The input comes from the LFE output from my nVidia SoundStorm. It is set at about 2.84Volts, but i still have to take some testtones/SPLmeter to SPL match them to the B&W's from my father.

First impression was very boxy, but lowering the cutoff freq from 125Hz to 80Hz immediately solved that.
After some testruns i settled for 70Hz cutoff. Averything matches very nicely, aldough i feel the sub should be next to one of the main speakers for better time aligment, which is not possible.

The B&W's sound like they have more *body*, like there feet now reach all the way to the ground. The effect of the sub is very subtle with norah jones and classical music, but with electronic music the LF "hang in the room" very nicely.
The music sounds a bit dryer, probably because of the group delay of the higher tuned B&W's now is eliminated. Room nodes are in both the corners, which I like since my PC stands in one of them. :D Bass-spill is almost nothing in the kitchen and hallway. Very nicely contained in the same room. Can't wait till the neighbours are not at home to try more than 80dB ... ;)

Gotta leave town for a few days, so I'll probably do an update on movies when I return.

Cheers and thanks you all!