Sub Box Shape

[gti-hysteria 0]

basically im building a sub box to fit in my dead space in my 306 cabriolet boot, and wat i wanted to know is, do most sub boxes have one edge slanted for acoustic reasons or is it just so it fits flush to the seats???

Edited July 10, 2007 by gti-hysteria

[Paintguy 0]

There's no acoustic advantage to having one side of a sub box slanted. My own has a slanted back, but that's just so it fits up to the back seats nicely

 

 

 

You might hear some people saying that it helps to cut down reflections or standing waves inside the box, but due to the frequencies a sub operates at, the wavelengths are several feet long, so it makes no difference at all.

 

Midrange speakers are a different story though....

[gti-hysteria 0]

thats what i wanted to hear makes it a bit easier for me now, thanks

[tidypug 5 2 Cars]

will it make any difference with the sub firing across the boot?

[Paintguy 0]

It's the general opinion that the way the sub fires in your boot can make a difference, although it's not something I've experimented with myself.

[gti-hysteria 0]

mine will be facing upwards toward the place that the roof sits in when folded down. in previous cars iv had subs facing up and outwards and cant say iv noticed the difference

[DrSarty 90 1 Cars]

Paintguy knows his stuff - as ever .

 

I've built many sub boxes and read a great deal on loud speaker enclosure design and principles (sad git eh ) and have also had an Escort cabriolet ( ) that I built an enclosure for containing the 2 x 15" subs that are currently in my Mi205.

 

To answer your points: the direction the sub faces and its/their distance from a surface WILL have an effect on bass performance, primarily boosting it by up to 3dB, which is the same effect as doubling the power into the sub. This distance must be roughly half a cone diameter speaker to surface (wall of boot). It also lends a hand in helping protecting the cone itself.

 

A sub will still work, even if pressed right up against the back of a seat, but that will have the opposite effect, just like putting your hand over your mouth. There is a 'bubble' around speaker cone (the half diameter I mentioned above) which really shouldn't be encroached into.

 

My cabriolet subs faced backwards, so effectively reflected off the inner boot surface and then the bass travelled back through to the car's interior by working around the box, through the firewall/hole in the back of the seats and the seats themselves. I also had the added advantage of it being a ported box, where the ports fired forward through a neat slot I had cut and upholstered through the rear seat upright, a bit like an arm rest hatch. The latter will really help you out BTW.

 

Your idea of firing the subs up through where the hood folds WILL work, but the principle behind sub bass in a car is about pumping up the air in a small space, so once the roofs down it WILL diminish. Your ways around this are: high power amps and subs to really give it some welly to overcome the above issue (which don't forget implies cash and a decent power supply) - the venting of a ported box into the car itself or even firing the subs on the front facing side of the box (slanted or not) towards the seat backs, and helping them out with a upholstered fold down arm rest which will vent the subs and/or ports into the car interior.

 

The good news is, if you've managed to create decent mid-bass with your other speakers, the subs should only need to play pretty low bass, which, as the sub box is practically fixed to the car/chassis, will create the desired effect and feel by vibrating the car itself.

 

Interested to see what you do and sorry to bore you rigid. Or maybe it helps?

 

[Super Josh 4]

and have also had an Escort cabriolet ( )

 

 

 

Blimey Rich, you kept that one quite And I thought you Army chaps were supposed to be tough Don't let the Boys in the barracks know about that, you'll never hear the end of it.......

 

 

 

 

 

 

Josh

[gti-hysteria 0]

that helps alot thanks, the only reason im having the subs facing up is because they just wont fit any other way,m at the mo i have 2 12" fli subs in there fli trap boxes but these are it the main part of the boot and take up all the room, but they sound fine. i will keep posted how it goes and try and add sum photos.

[gti-hysteria 0]

also the volume of the box is going to proberly smaller than it is ment to be, i wasnt going to port the box, but as its going to be small should i?? or wont htis make ant difference?

 

oh and dont get Alf at dartford B&Q to cut your wood, he cut mine a tad wrong so had to get the jigsaw out once i got home

[DrSarty 90 1 Cars]

GTI HYSTERIA - avoiding boring you, when you reduce the size of a box (sub enclosue), porting it IS NOT the fix. I would be tempted to leave it sealed with the two subs, or (and I do mean this), consider removing one sub and making it a single sub, ported pox, which if done properly will out-perform the twin sub box. Believe it.

 

And yet another alternative, is to make it into a ported box and mount one of the subs face-to-face (aka isobarically) and wired in reverse. This is what I have with my twin 15 inch subs. They have a 2.5 cuft box, but as they are 'isobarically paired', it's the same as one sub having a 5cu ft box. This box is ported, and ported boxes have a 3dB higher output generally across the range as the air in the port is behaving like the second driver. So all is not lost when you do this. You get deeper bass from a smaller box, and high output to boot!

 

As a general rule of thumb, 8" subs SHOULD have a 1cuft optimum box - 10" 1.5cuft - 12" 2cuft & 15" around 4cuft plus.

 

Not knowing the size of your sub box OR subs (was it 12") I can't comment on what you're working with. If you want to check what I've tried to explain, visit JLaudio.com and read their tutorials; they're great. If my figures don't match above, its' only due to market pressure to make subs appear that they need smaller boxes to appeal to buyers. They do this by stiffening up the supsension, which has the added effect of them being less efficient, which means they need more power to drive them (more cost etc as I mentioned before), which surprise surprise means they have huge power handling figures. It's a vicious circle.

 

In short, for less effort, leave the box sealed. For an adventure (possibly download WinISD or Blaubox software to experiment with your subs in different enclosures) consider either removing one sub and going ported, or go ported and mount them as an isobaric pair.

 

Rich

 

P.S. If leaving it sealed, get some wadding from John Lewis or a haberdashery shop to stuff the box perhaps 70% full with. This may have the effect of making up the volume you've just lost/cut out.

Edited July 16, 2007 by DrSarty

[gti-hysteria 0]

thanks rich, you know your stuff, all should be completed by this weekend, im just waiting for the subs to get dilivered, there jbl gt4 12 or something like that got 2 brand new for 45quid the pair off of our favourite auction site.

[DrSarty 90 1 Cars]

Firstly: one thing I didn't really mention in my above waffle is that typically a ported box for a sub is larger than the sealed box would be.

 

And your 'new' JBL subs, if packaged, should have a recommended enclosure design (or 2) with them, so you can compare that with what space your box provides. It's always easier to port a sealed box rather than seal up a ported one.

 

Good luck