Box sizes

[Lee]

Any Ice-heads out there? I've got an Alpine 10" Sub..the alpine docs recommend .5 cuFt, I'm running it in a Subworx fibreglass box which apparently is 1.3 cubic ft. Does anyone know how to size a box correctly ? What difference would a box too big make (as I think mine is!)

[Andy Tang]

The best thing to do is get some speaker wadding, and pack out the box. Try halving the size first and keep listening until you get the soud you like.

The box requirements are usually the smallest you can get away with. Try it without the wadding first, if you like the sound, then what the hell!

[This message has been edited by Andy Tang (edited 15-04-99).]

[Mike Tuckwood]

The box needs to be the correct size, packing it out with wadding will not work, the wadding is there to prevent reverberation and other technical acoustic stuff.

Any decent ICE specialist will be able to tell you the optimal size required for the enclosure combined with the best box design to use. It's more complicated than you could imagine.

[Andy Tang]

Although there is no mention of whether it's a ported or a sealed box. We haven't even started to mention about port widths and lengths!

You will find that most subs will have a minimum requirement (most of the newer 10" subs of around 0.3-0.5 cubic foot) although the technical documentation will recommend a larger size for a wider range of bass notes (around 1-1.5 cubic foot in the example before)

I would recommend that you double check the documentation first, most of it is available on the internet!

[Lee]

Thanks guys. To clarify a bit more..
The sub is a 10", the box is sealed.
Alpine recommend measuring from rear left to front right inside the car (approx 8ft in a subaru)..this gives a car size just over their "compact" and just in "mid".

According to alpine's sizing, a vehicle of this size with a sealed box requires:
0.47-0.52 cuft for a tight sound,
0.39-0.45 cuft for a balanced sound,
0.34-0.39 cuft for loud n boomy.

for vented/bandpass boxes the boxsize goes up.

Now my predicament is that I wanted the subworx box since this is moulded to fit the awkward shape in the rear side of the boot..they provide the box (and a hole according to your speaker size!)..they have told me that this is 1.3 cuFt.
I bought the alpine sub since dreaded MaxPower rated it highly.

The Subworx dealer recommended a sealed box for that sub so that's how I've arrived here. They also recommended crossover at around 75-100Hz whereas alpine recommend as low as possible < 65Hz.

The sub wants upto 300RMS, I'm giving it this (from an alpine amp certified with that power output).

2 problems -
a) after 2-3 hours the amp cuts out as it is STINKING hot.
b) the sub isn't making a clean sound, but a raspy flappy sound.

Also I've been told numerous times that a sub wants a simple rectangular shape - a box moulded this way and that is no good.

I'm considering blanking off the "weird end" of the box which will leave me with a smaller, squarer space behind the sub, but I was wondering if my "symptoms" would be cured.
Appreciate the advice.

[RobScreene]

Lee,
It sounds to me that your low-pass sub-bass crossover isn't working.
The 75Hz vs 65Hz would just make the sub slightly more locatable, but both are reasonable crossover points assuming the crossovers have a normal roll-off.

I wouldn't worry about the shape of the enclosure, square boxes are just easier to make resonance free when using wood, etc. As long as the ensclosure is strong and takes a knuckle rap test, it should do an ok job. I can recommend the loudspeaker coockbook and bullock on boxes if you want to find out some background on sub-bass and enclosure design.

If you can post me (ideally by e-mail) your Alpine speaker's thiele-small parameters (from the manufacturer or their web?) I can probably dig out an old bit of shareware to give you an expected response plot for your sealed enclosure.

cheers,
Rob.

[Lee]

ta very much. I've mailed these to you.
Since the box is so huge maybe I should have gone for a bigger sub ?? do people make anything bigger than a 12 ?

[Andy Tang]

15" & 18" are fairly common sizes, but why not try the Clarion 32" sub!!!

[Lee]

That would certainly make the "lowering kit" a bit redundant !

what sort of power would you need to drive that ? frequence response 1Hz - 20Hz !

[Mike Tuckwood]

See.... I told you it was more complicated than you could imagine.

With size, you also need additional structural strength. An acquaintance of mine has a turbocharged vauxhall astra, with a big ICE system (including television with surround sound) the sub takes up his whole boot in a custom built box, you couldn't put a fag paper in there, it has two 18" speakers facing backwards.


The speaker enclosure complete weighed 8 stone.

[This message has been edited by Mike Tuckwood (edited 20-04-99).]

[RobScreene]

Lee,
My conclusion is your closed box looks like it suits the speaker.

A vented added to your enclosure tuned to 25Hz would allow -10dB at 25Hz against -3dB at 50Hz, giving -7dB system response down to 25Hz when used with your Alpine crossover however this is easy to get very wrong and will also give a slightly more loose bass note. I think the ported designs need larger enclosure boxes statement is incorrect.

We are also assuming your main speakers are able to get down flat to about 60Hz. I doubt those in the front of standard Impreza doors could do so there may be a dip in response at 50-80 Hz loosing some definition from mid-bass guitar notes, but you should have a nice, deep, tight bass making drums solid and have life-like slam. The dip will be doubly worse if the phasing is out slightly between the sub and the reset of your speakers try it with the spear wires normal and switched round to see which you prefer.

This is the discrepancy of the suggested crossover points, Subworx are making the whole system it work with flat response in a Scoobie (75Hz low-pass), Alpine are talking about best low frequency response where you cannot hear where the sub is installed (50Hz low-pass), that's where the lower the better comes it. The higher the crossover the the less you can electronically flatten the speaker box response and the more you can hear bass notes coming from a box in your boot. Ideally you cannot tell where sub-bass is coming from and it sounds just like your front speakers go fantastically low.

You might like to ask the installer to measaure the system response at 25Hz, 35Hz, 50Hz, 75Hz, 100Hz and 150Hz. Ideally they may rave a real-time spectum analyser (RTA) to do this and print out or you can do it with test cd and tandy sound pressure meter, lots of patience and some ear defenders.

If I had a 4-door with saloon boot (so the sub is hidden), a spare amp and a few days R&R I'd definitely give things a go.

I've zipped the Word 97 document with about 6 response plot diagrams into a 75KB .zip file at:

<A HREF=http://www.netcomuk.co.uk/~screene/scooby/scoobsub.zip>http://www.netcomuk.co.uk/~screene/scooby/scoobsub.zip</A>

Good luck,
Rob