Sub Box Too Small?

[CrBoy]

I have a Dx-12 in a .85 cu ft box with a 250W amp... it sounds OK, but when I put the Dx on my brother's sub box (about 1.5 cu ft) the sub hit deeper and harder!!!

The Dx sounded just as loud as my brother's MTX Blue Thunders (2 12" rated as 500W each) and I was using only 250W from one of his amps...!

Does this make any sense? Should I build a bigger box since I'm underpowering the sub? If so what dimensions?

(please excuse my bad english :D )

I need your advice Polk Gurus!

[MacLeod]

Subs really arent my specialty, thats where Neo comes in but Ill take a stab.

A bigger box is more effecient and so it takes less power to fully extend a sub. Since youre underpowering your sub when you put it in the bigger box it had less restriction and was able to work closer to its max potential.

You could build a slightly bigger box to accomodate the smaller power and it would work fine or you could do what I would do and that would be to get an amp that matches the power handling of the sub.

If youre going to build a bigger box, Id give Polk Customer Service or at least email them and get their recommendations on box size taking into consideration your amps power.

[CrBoy]

thanks for the input MacLeod!

So something like the RF X5.C will do the trick, right?

[MacLeod]

Well thats a little overkill. A very expensive amp first of all and second, its a 700 watt amp so youll be paying for 700 and only using 400.

This would be a better choice. Bridge it to that DX and youll get the 400 you need.

[CrBoy]

You're right...

What do you guys think about the Dx subs...?

[MacLeod]

Ive never heard one but I think they look pretty solid. Ive always regarded Polk subs among the best on the market so Im sure this would be no exception.

[spwuinmk67]

I have one, very nice sub. not as loud as some others, but sounds better. It's no slacker.

[CrBoy]

Can I build a ported box for this sub... is it recommendable?

[neomagus00]

for the dx? it's built for small sealed enclosures, it's doubtful that ported would work very well. if you want more output than a sealed and you have the space and the craftsmanship skills, you can try a 4th-order bandpass box. if you do it right, it can actually provide pretty good results.

and mac was right, up above; you can put that sub and amp in a larger box and get potentially deeper and louder bass at the potential expense of transient response.

[CrBoy]

If I go with a bigger sealed box (I don't know what size), will this damage the sub?

The 4th-order bandpass box looks VERY complicated to build!!!

Damn I should just get the right amp and stop making stupid questions... :D

[exalted512]

If its too big, yes, you can damage the sub. But there are advantages to using a bigger box and less power. Well, theres one that i can think of. Thats youll have much less of a chance of thermally overloading the sub.
But it has a trade off because you have more of a chance of bottoming it out and over excursion.
But its easier to prevent that than thermal in my mind because you can hear when your pushing your sub to its limits.

And unless youre an expert craftsmen, i wouldnt recommend attempting a 4th order bandpass. Ive heard boxes that were a 1/4" off in measurement and threw everything out of whack. And even still, if you dont have plans of the DX sub specifically, itd be all trial and error. A lot of trials and a lot of errors...

and yes, the DX is an extremely good sub.
-Cody

[neomagus00]

agreed, bandpass requires excellent woodworking skills. and math, that's good too. the thermal point is good too - i wouldn't have thought of that.

a too-small box won't allow your sub to perform to its best, no matter how much power you throw behind it. a larger box will allow you to get more excursion - and thus sound - with less power, but you'll end up with a smaller maximum SPL by a couple dBs.

[CrBoy]

I decided that I want to build a new box for the sub (bigger, sealed) instead of getting a new amp... but I don't know what size should be?

Can you guys give me an idea...?

[neomagus00]

for a sealed box, 1.05 ft^3 is what i'd reccommend with that amp. if you want to be adventurous and attempt a vented box, a 1.5 ft^3 with a ~32.5 inch slot-port will tune to 25 Hz; that means that you'd have +4 dB at 25 Hz over the sealed, rising to a peak SPL of 113 dB at 53 Hz, which is +1.5 dB over sealed, and at 10 Hz less to boot, then the two curves become even at 80 Hz, which is where you'd cross over.

if you want a design for either, give me space requirements and i'll give you a box design on monday.

P.S. if you want a higher tune on the vented box, say so and i'll give you a couple more options.

[CrBoy]

Can you provide me the design for the Sealed Box...? I have a whole trunk to work with (Geo Metro Sedan 95). I'm more into SQ than SPL...

I'm affraid the vented box will eventually kill my sub... ("optimized for small sealed enclosures")

Thanks for the offering, much appreciated.

[neomagus00]

armed with the actual measurements of the trunk, i can certainly design an appropriate sealed box. FWIW, as far as SQ goes, that vented box will be damn near the sealed one; a very low vented tuning - such as 25 Hz - often approximates the transient response of a sealed box whilst offering much of the benefit of a standard ported box.

i've included the step responses of both boxes for reference; if you don't know, they represent what happens to the cone when you give it a single pulse to its maximum excursion... flatter is better, and as you can see, the curves are very close. for comparision, a Q=.707 sealed box will have that first peak, a dip, then flatness; a bandpass box will be waving up and down to 0.5 even at the end of the graph. you may wish to consider it, if you have the space, but keep in mind that a) the vented box will be slightly more difficult to build and b) it will probably take up nearly twice the room of the sealed box.

your call, either way, all i need are the dimensions of the trunk.

[CrBoy]

You're awesome neomagus00!!!

Measurements are (approximately):
Width: 34 in
Depth: 29 in
Height: 18in

Obviously the trunk is a perfect rectangle... lol

The trunk has a capacity of 10.2 cu. ft...

I would like to try the vented box tuned at 26Hz or so, sounds good...

Thanks for all the help... you rock!

[CrBoy]

What would be the difference if I build the box tuned at 50 Hz instead of 25Hz... which one will hit harder?

n00b question I know...

[neomagus00]

man, work was nuts today! so, i should be able to post your box design tomorrow evening.

a 50 Hz tune has the potential to be exceedingly loud, but whatever you play through it won't actually sound like music :). 50 Hz is an SPL tune, the 25 Hz tune will still sound like music (more so than a badly-constructed sealed box, not quite as smooth as a well-constructed sealed, though), and it will be louder than either kind of sealed box. it will also give you a slight bump in the very very bottom of the bass spectrum, which could be good if you listen to organ music or techno.

ah, yes, i forgot to mention this - with a vented box, it's always reccommended to put on a subsonic filter at the tuning frequency. this is because, below the tune (25 Hz in your case), the subwoofer loses the majority of its suspension, and excursion will jump dramatically, far beyond the limits of the sub. the upshot of all this is that a strong signal below 25 Hz will abuse your subwoofer. you could get away without a subsonic because there is very little music below 25 Hz, but it's a risk. your call. i'll give you both designs, and you can build the one you like better.

[CrBoy]

I like rap (Jay Z), rock (Linkin Park) and techno (Chemical Brothers)... and the like...

I know, work can be a b!tch... mine is not so bad I work in a Travel Agency here in Costa Rica... Jeje!

Cheers! :D

[neomagus00]

rendition one... ba ba bum! failed! that's good for you, though, cause i won't need the whole vertical space. just letting you know inspector neomagus is on the case...

[CrBoy]

I know you're the Man for the job neomagus00... :D

[neomagus00]

crikey, this box is tough... i keep getting volumes that are too small to hold the actual subwoofer! i'll keep at it, though, worry ye not... now it's personal, no sub box has beaten me yet!

[CrBoy]

Jajaja... I know you can pull this one off... I shall not worry!!!

[neomagus00]

done, w00t!

this is the top view of the ported box. it's a slot port, but there's a sneaky bit - it's divided into two separate ports. so the front view will look like the picture in my next post... this part's pretty important, if you forget to put that divider in or it doesn't seal, it will completely wreck the alignment of the box. all of the measurements are in inches, sorry about that if you use centimeters, but if you do feel the need to convert, this is in 1/8 inch resolution, so you'll need to be pretty exact in your conversions... using the inches side of a tape measure would probably be much easier.

um... that should be about it, fire away with any questions you may have...

[CrBoy]

Thanks for the design... you're da man!

I just have one question... if this is a top view of the box... what would be the recommended height? this box is tuned at 25 or so Hz, right?

Other than that everything is very clear...

[neomagus00]

the red piece is the one i'm talking about, it has to run all the way to the back and then around, to the end of the vents (the vents are the green bits).

a couple more random points: every edge of the vent should be gently rounded, this will help prevent the air whistling as it rushes through the vent. it doesn't matter where you put the sub, it should have enough room to fit anywhere... please measure the depth and compare it against that 6.5" i've left it, to make sure it'll fit... the polk site says so, but i wouldn't want you to build a box and then not have the sub fit.

ah, another important bit: in the picture above, the shaded areas are 3/4" MDF (ALL of this wood is 3/4" MDF), and the rest of the measurements you should be able to extrapolate from the ones given. i intentionally did not suggest a cutting pattern because i'm not sure what size MDF board you'll be starting with. i'd suggest making the smaller pieces fitting inside the larger pieces for strength, though; here is an excellent walkthrough, it's good reading even for the experienced builder, with a bunch of little tips hidden throughout... read it carefully before beginning.

a final important bit: the internal height of the box is 16.5 inches, which makes this box as wide and as tall as you allowed, but actually pretty shallow, so you should be able to shove it into the back of the trunk and have a good amount of space left.

[neomagus00]

ah, yes, the tuning comes out to about 24.36 Hz. this box ranges anywhere from +1 to more than +6 dBs over the best sealed box i could design (where best is a good balance of size, transient response, and SPL).

[CrBoy]

I can't thank you enough for all your work... :)

I'm pretty sure I'll build this beauty over the weekend... I'll try to take some pics to post later on...

[neomagus00]

excellent, i can't wait to see how it turns out!

p.s. if it sounds like ****, tell me and i'll find out why :)