Advice on building some SPL sub boxes

[acarmody]

No those dimensions were outside

Yeah youre better off with a single box, means less work for you dad

Your better off going with the port on one of the sides, its easier to make and wont make a big difference to SPL. Also you wont need as much port depth as the side of the box is one wall of the port if you get what i mean.

So yeah, that box is 5.28 cu.ft. before sub / port displacement (wood thickness already taken out to get that 5.28). Take out probably no more than 0.1cu.ft and thats what your left with. Using a slot port to get 33hz tuning (with that port width) you'll need a port depth of 36.5cm, take into account the baffle thickness and you get a port wall of 34cm. Now, your box is 450mm deep (internally), which means you can go to a maximum depth of 40-42cm max. 42cm only leaves a 5.5cm gap between the rear wall and the port end, but as its only for daily use it wont be noticeable (probably 0.3-0.4db difference).

If your dad cant round over the mouth of the port and the end of the port wall really well (like in that link from subwoofer builder), then i'd go with 6" PVC pipe. Just flare the ends like this.

If you do go with a round port, my suggestion would be go 2 x 6" ports, one above the other in the centre of the box. It will give you more port area than the 3" x 15" slot, which is good for SPL purposes. To lower the frequency for music just plug one of the ports with a towel etc. That'll lower the tune to 28hz, haha.....thats a nice low tune anyway, but you can shorten the depth and up the tune if you want.

Cheers Holdenboy. But are you sure that the 350mm (H) x 950mm (W) x 450mm (D) dimensions are outside? Because I have used two different online calculators and if I enter 37.4" x 13.78" x 17.717" with 1" wood thickness, I only get a 3.793cu.ft enclosure, but if I add 2" to every dimension or take the wood thickness out of the equation, I get the 5.28cu.ft.

Also I'll see what my dad says about using PVC pipe, Bunnings sell 150mm PVC for $32/m and the pipe looks strong as. But how do you generally mount them to the box?

 

[holdenboy]

Hmm, ok. Bassbox is giving me figures without taking into consideration wood thickness. So yeah, add 50mm to each dimension. 400mm (H) x 1000 (W) x 500 (D).

Yeah, personally i'd go with round ports. I mounted my 10" port using electrical tape around the outside to act like a gasket, sealing as you push it into the hole (mine was that light i needed to "force" the port in with a piece of 2" x 4" haha). So yeah, it made things easier when i had to change ports. When i went testing i used to take 3-4 different lengths with me, all within a few cms of each other to test the best exact port length.

You can either do what i did (i can supply pics if you want), or just use a sealant (like silicon). The only real problem i can see with using round ports is having enough room to fit two flared 6" ports one above the other......seeing as the box is going to be 40cm high it should be fine, but the ends of the flares its gonna be close, depends on how much you flare them with the bowl / heatgun method.

Its better to flare both ends of the port, just cut each port in two and use two of those pipe joiners.

Sorry about those measurements, although that 3.8 cube one looks like a nice hatch setup for 2 x 12s, maybe even 2 x 15s.....hmmm.

 

[acarmody]

Hmmm I think I'm going to need a hell of a heat gun for these 150mm pipes, they are about 4-5mm thick, solid if nothing else.

 

[acarmody]

OK, assuming you use the 'One flush end' option, if I have two 150mm ports, and I want a Vb of ~5cu.ft, then I will need to make the box (A) 400mm x (B) 1000mm x (C) 515mm , and the two ports 328mm long. So basically if I'm using two massive ports I will need a 15mm deeper box. Does that sound about right?

 

[holdenboy]

The only reasons i can think of for going to a deeper box is more volume and ability to run a deeper port/s.

In your case, if you flare both ends of the port just insert the ports like this:

I havent flush mounted the port into the baffle, so this allows easier port changing and longer port depths. The only reason why i didnt have 2 x 6" ports is because of the windowline rule (couldnt go high enough for two).
Anyway, doing that will allow you to fit a longer port depth without changing the box volume. By all means if you want it for like 50/50 music/spl then go as large as you can and use 2 x 6" ports, it'll play music so much better, but probably cost you a db, maybe 2. So yeah, its all about sacrifices.

If its more for SPL you want to start with 5 cubes before taking out sub / port displacement. So in your case, make the 2 x 6" port depths 40cm (40hz tuning). This means the port will be 10cm from the rear wall of the box, which is fine....usually you want to be atleast the port diameter aways from the wall.....but theres not much to be gained by this IMO.


So... box is 25mm. 400mm (H) x 1000mm (W) x 500mm (D). This equals around 5.2 cu.ft. Add 2 x 6" ports (flared) and make them 40cm deep. This should leave you with about 4.6 cubes after sub / port displacement.

You can use the el cheapo heat guns at bunnings (XU1 or whatever), it'll probably take 4-5 minutes per flare to get the right amount of heat in though, maybe longer. My port was 6mm thick and took over 15 min of heating to flare, man thats boring

Now, can you get access to a termlab meter before the comp by any chance?

 

[acarmody]

Ok sweet it looks like we have nailed the box and port dimensions.

Unfortunately I don't have access to a Termlab. I'll see what the audio shop would charge me for use.

Also it is very hard to find rubber couplings for 150mm PVC. But thats the price we pay for noise. LOL.

 

[holdenboy]

Yeah, you can gain anything from 0.5db - 3db by testing on the termlab. I wouldnt imagine they'd charge more than $50.00 for an hour.

If you can find joiners, just flare the inside of the port and flush mount the other end to the baffle.

 

[acarmody]

Well Bunning sell The plastic joiners but they are a very tight fit, basically have to hit them with a mallet. Only problem is when I want to change the ports to everyday use ones. On Wednesday on my day off, I'll go around to various plumbers joints like Tradelink and ask them, I'm sure they will be able to order them in.

Also I was thinking of that advice from yesterday, about bigger boxes for everyday music. Well I thought about it and if I just get a threaded cap and block one of the ports on the outside, wouldn't the air inside the port actually add to the Vb? I could gain as much as .25cu.ft by doing that.

 

[acarmody]

Also would you recommend Poly-fill for SPL use?

 

[holdenboy]

Nope, it dosent help......i never actually tested it but im yet to see a SPL system that uses polyfill.