Advice on building some SPL sub boxes

[acarmody]

Ok so my local WOW is having a comp in a month and I want to build some SPL boxes for the comp. I have two 12" Kicker L7s, and they are getting 700wrms each. I was thinking about 3 cubic foot for each sub (including port) using 25mm MDF.

Slot ports are easy enough to build but what are the advantages/disadvantages to a round PVC port? My dad will be constructing the boxes and hes quite good with woodwork.

[acarmody]

I should also add that I have 100cm wide, 40cm height and 65cm depth, in the boot to play with. Also I would prefer two single boxes as a single dual box would be way too heavy to get out to get to spare tyre.

EDIT: Ok I have worked out that a box 480mm X 400mm X 615mm = 3.003 cubic feet, close enough. And that's outside dimensions using 25mm MDF.

[sweefu]

Is that 3cu.ft net or external volume? Might be worth shooting holdenboy a pm mate he's an expert with everything SPL

Though from a glance, and my short time with spl i would say that those boxes will be a bit big for the 12" L7's

And i know you will want alot of good woodglue/fibreglass resin to seal up the box well.

[acarmody]

3cu.ft is internal volume, not taking the port into consideration. As for the size, Kicker has a 4.2cu.ft box as there maximum size for these subs. And I'll make sure I get a sealant around all joints.

Also I was thinking would using dowels to hold the port wall in work? This way I could just take the top of the box off and swap the port size for everyday use.

[holdenboy]

For your VX i take it?

If so, 3cu.ft. per sub before displacement should be fine.

You wont be chasing every last 0.1db, so a slot port per box should be fine. Try to get a decent sized port in (atleast 5 x 40cm anyway). Im not too sure on how you mean "using dowels to hold the port wall in"?
If you think it'll be strong enough to not leak air and be easy to swap ports etc, then no problem. Just for insurance, put a strip of that foam weather tape etc onto the two pieces.....SPL is all about over engineering

[acarmody]

Yep definitely intending for that foam stuff that usually comes with subs.

I think I will ditch the dowel idea, because each hole would have to be absolutely perfect, screws are easier, they make there own holes.

As for the port it will be 35cm high, outside box dimension is 40cm high and I'm using 25mm MDF. So 35cm high, and is there any particular rules to width and depth or......? Since it will be taking away from the volume of the 'enclosure'.

[holdenboy]

Have a look at this calculator,
You may have to register to use it though?

It'll tell you if your port dimensions are bad

I'd probably tune the boxes to 30-35hz daily, then shorten them up to around 40hz for SPL.

[acarmody]

OK, I'm getting there. If I build a box that has outside dimensions of 480mm x 400mm x 620mm, using 25mm MDF, then I get a box that has a volume of 3.03cu.ft.

So my box volume is 2.5cu.ft, my port displacement (including wall) is 0.45cu.ft, my woofer displacement is 0.087cu.ft, and the two 3"x3" brace triangles equal, 0.005cu.ft, so in total that equals 3.037cu.ft, close enough.

Now my port will be tuned for 40hz, so the port is going to be 2.5" (64mm) wide by 13.78" (350mm) high, by 17.12" (43mm) long. This is for the 2.5cu.ft box volume.

Can't make them any bigger or I wouldn't be able to get the damn things in the boot. Man these will weigh a ton.

[acarmody]

[ToMMMaY]

dont port it

Ports sound better in the car but unported kills it outside

[acarmody]

dont port it

Ports sound better in the car but unported kills it outside

????? I don't understand what you are trying to say. These are definitely going in my car. And sealed boxes aren't going to do to well in SPL comps.

[holdenboy]

When i get time tonight i'll run those measurements through the BassBox Pro calculator and give you some advice.

[acarmody]

Thanks Holdenboy. I'm entering the 'Street B' category, thats 3x10" or 2x12" or 1x15" with 750-1500WRMS, I have 1400wrms. I'm really going to go see the Rep and Pro cars but a trophy would sure make the day awesome .

I'm going to see if I can somehow shoehorn some 3cu.ft + port boxes in the boot, by my preliminary numbers say no. My ol' man can't start work on the boxes until next weekend anyway so I got time.

Also would 45hz be better for burping tones? And would flaring a slot port help any? http://www.subwoofer-builder.com/flares-rectangular.htm

[holdenboy]

Smaller boxes are generally louder than larger ones. This is for a number of reasons, but the main ones are:

* Smaller hatches have a high peak frequency (54+hz) and this suits the smaller boxes which are easy to make peak in the mid 50hz region.
* Specs / Evidence suggests smaller boxes are "peakier", which means they've got a larger peak in frequency response when compared to larger boxes.
* Smaller boxes are easier to position in small hatches, especially using large external ports etc.

Anyway, none of that really applies to you as your using a large sedan (will peak 45-50hz). So instead of going with a 2.8-3.0 cube box for both (the norm in hatches with 3+kw), 4-5 cubes after displacement is better for a lower peak.

A smaller box peaks 7-10db higher than what its tuned at, larger boxes tend to peak closer to the tuning frequency (probably 2-4db). So if you were to tune the box/s to 40hz it would peak around 42-44hz mark in car. This is about right for some Commodore sedans, but by tuning there you can just cut down the port length a few cm at a time to try and raise the tuning freq. Thats just for your SPL port, i'd suggest adding a sheet of 16-18mm MDF to the port wall to reduce the port area, should lower the tuning nicely to suit music.


Im rambling, so i'll stop it there and make up a few box designs

[holdenboy]

Alrighty, heres one design for you......

350mm (H) x 950mm (W) x 450mm (D)

Equals 5.28 cu.ft. before sub and port displacement. The triangle braces arent required. So take the sub and port disp. out the equation and you'll still be over 5 cubes (so 2.5 each).

Port would be around 14.5-15" high, and 3" wide. Depth for 40hz tuning is approx. 8", but i'd probably start with a longer port, 10-12".

Subs / port go on the same side.

i know its a single box but it wont be overly heavy, you can split them into two, just make them 450mm wide, add on the 2 x 25mm sides and you get 500mm, which side by side will just fit into your boot

If you wanna split the boxes then make the ports 15" x 2", 14" deep to get 40hz tuning.

[acarmody]

OK so those are inside dimensions. If I divide the Width by two wouldn't I get 475mm? Add the 50mm of wall and they will be too big for the boot.

EDIT:: I just thought well if I swap the width dimension with the depth dimension its exactly the same box and it fits fine. Let me get my Paint skill ready.

[acarmody]

Ah screw it I'll go with the single box, can't be any heavier than the kegs I sometime have to lift into the dumb waiter.

I was just in my boot with the measuring tape, if I remove the plastic resting cover on the boot (the one that has the hook in the middle that the latch catches) I can just jimmy it in.

[acarmody]

Only problem I can see is that I can't go below 33hz for the port for everyday use.

[acarmody]

Do you maybe think I should use a round port? I know Bunning sell some friggin big and hard stuff, I could try and flare it at home?

[holdenboy]

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.

[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?