Long vent/port as bracing in Ambiorix?

[Cordraconis]

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 ) 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

[Collo]

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

[simon5]

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.

[Cordraconis]

Originally posted by Collo
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

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?

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.

Originally posted by simon5
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.

[Collo]

The resonance between 2 walls is half wavelength because there is a phase reversal upon reflection.

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]

Originally posted by Collo
So the resonance between the driver is quarter wave.

That should read:
So the resonance between the driver and any wall is quarter wave.

For estimating the driver-to-rear-wall resonance, consider the acoustic centre of the driver to be at half the depth of the cone.

Collo

[paulspencer]

I'd consider 18mm to be minimum for anything using MDF. If you want to go thinner, then you have to be tricky, do things like use curves, very efficient bracing and sandwich construction. If going to the effort of DIY you might as well do what commercial speakers can't do with their affordable speakers.

[Cordraconis]

Originally posted by Collo
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.
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?

Originally posted by paulspencer
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 ...

[RobWells]

"" 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......

[Collo]

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