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-   -   Help with vented box alignment? (http://www.diyaudio.com/forums/subwoofers/169271-help-vented-box-alignment.html)

clm811 25th June 2010 12:57 AM

Help with vented box alignment?
 
1 Attachment(s)
Need some help.

I have a generic 12" woofer (someone's prototype driver that I picked up cheap), and have the parameters, but need help with a vented alignment.

The parameters are:
Fs: 23.55 Hz
SPL: 89.89 dB 2.83 V/1m, 90.5 dB 1W/1m
Vas: 6.551 cu. ft.
Cms:0.0901 in/lb
Mms: 3.132 oz
Rms: 7.358 lb/s
Re: 9.207 ohms
Le: 2.836 mH
Z=11.05 ohms
BL: 4.231 lb/A
Pe: 150 Watts
No: 0684%
Qms: 3.936
Qes: 0.341
Qts: 0.314
Xmax: 0.42 in (11mm)
Xmech: 0.63 in (16mm)
P-Dia: 9.972 in
Sd: 78.1 sq.in
P-Vd: 0.019 cu. ft

The box I have is a 15" cube. It has 0.75" walls and a 1"x2" cross brace through the center. I calculate the gross volume at 1.4 cu. ft., but the woofer itself takes up some volume, as it juts 4" into the box, and has a 7" dia. magnet(see pic), so the net(actual) volume is is probably closer to 1.25 cu.ft.

Anybody here to give a recommendation for box frequencies and vent length (I do have some 2" i.d. PVC pipe on-hand) ?

Conrad Hoffman 25th June 2010 04:14 AM

My quick calcs say you need a 2.9 ft^3 box tuned to 30 Hz to give flat response with an f3 of 33 Hz. IMO, anything you build in a 1.25 ft^3 box will be a mess.

clm811 25th June 2010 08:14 AM

Thanks for the advice;

However, I have a plate amp that permits setting frequency/level of bass boost.

If you please, what tuning frequency in existing the 1.4 cu. ftbox?

I can always extend f3 with the bass boost, right?

-Chas

chris661 25th June 2010 08:21 AM

Wrong.

Ported boxes roll off at 24dB per octave. Not only does this take a huge amount of power to compensate, but, when the driver has rolled off, it is not loaded by the box, so you get lots of excursion but relatively little sound output. Most people put a high pass filter on them to keep excursion to sane levels, but you're going to boost it instead?

If you kept the box sealed, you could play around with bass boost, because the cabinet loads the driver all the way down to 0Hz, so it prevents the huge excursion.

Chris

iand 25th June 2010 01:45 PM

Quote:

Originally Posted by chris661 (Post 2226524)
Wrong.

Ported boxes roll off at 24dB per octave. Not only does this take a huge amount of power to compensate, but, when the driver has rolled off, it is not loaded by the box, so you get lots of excursion but relatively little sound output. Most people put a high pass filter on them to keep excursion to sane levels, but you're going to boost it instead?

If you kept the box sealed, you could play around with bass boost, because the cabinet loads the driver all the way down to 0Hz, so it prevents the huge excursion.

Chris

If the 1.25ft3 reflex box is tuned to ~30Hz then it'll need about 7dB of bass boost to flatten the response, then a fast rolloff (HPF) below this to prevent high excursion when unloaded -- you can probably do both with an underdamped 2nd order HPF, but to get a really flat response you will probably need more complex parametric equaliser (as well as a HPF).

The Xmax-limited maximum output of a reflex box is about 8dB higher than an Xmax-limited closed box (or 2/3 octave more extension for the same SPL), and the efficiency near cutoff is usually higher as well.

CharlieLaub 25th June 2010 05:54 PM

Hey Chas!

Looks like you are finally getting around to putting those goods to use!

Here is my take on a vented sub with the driver:
1. A 2" port is too small for a 12" driver
2. The volume of the enclosure is too small. The response will not be any better than what you can achieve if you use it sealed.

So - I recommend that you use it sealed. You would get the following response (unstuffed):
Q = 0.75
Fb = 59Hz
F6 = 45Hz
Adding some stuffing will reduce Q a little and will help to reduce internal reflections, etc.

Still not great as Fb is rather high. In that case, one of those plate amps that has a 30Hz boost of +6dB would be a great match for this speaker + sealed box combo. You could then get pretty flat response down to around 30Hz, where the room boost will kick in.

I think that this amp will pretty much drop in to the cutout in the back:
MnK Amp
It would be a good match to the driver and cabinet.

If you want to chat about it a little, send me a mail here or to the DIY group web site.

-Charlie

clm811 25th June 2010 10:33 PM

1 Attachment(s)
CharlieLaub wrote:
Quote:

I think that this amp will pretty much drop in to the cutout in the back:
MnK Amp
would be a good match to the driver and cabinet.

Thanks, Charlie!

SteveO was kind enough to take time out of his busy schedule to measure a woofer for me, but that sub box wound up being smaller than 1.6 cu.ft, so the vented alignment he gave for that volume won't be right.

The plate amp I have is from Parts Express(Pic attached), which I remember was specified with +6dB boost at 30Hz. I believe this is the same unit as the M&K unit (which was no doubt rebranded for them).

I'll send you mail, once I check out the amp.

-Chas

bjorno 26th June 2010 12:25 AM

1 Attachment(s)
Quote:

Originally Posted by Conrad Hoffman (Post 2226419)
My quick calcs say you need a 2.9 ft^3 box tuned to 30 Hz to give flat response with an f3 of 33 Hz. IMO, anything you build in a 1.25 ft^3 box will be a mess.

Hi Conrad, Agree 100%, As Vas for this driver is about 185 L and a 1.25 cu.ft. is only ~35 L the closed volume Qt seen by the driver is above 0.77: The result if trying to add ports would turn this speaker at most, into a very mediocre mid bass with in band rippled FR.

See a sub design using this driver that IMO would work well as a sub:

b

Conrad Hoffman 26th June 2010 12:41 AM

IMO, it could be done sealed, but the necessary eq would be more complex than should be attempted without a lot of test equipment.

I'm sure the driver and box are just fine, but they're a poor match and "fixing" it in the electronics won't result in anything good enough to justify the effort. Why not make another box of the correct size?

FWIW, I had this very same type of thing in my system for a long time- 10" drivers in somewhat small sealed enclosures. The enclosures were about the right size for a sealed system, but the drivers, like these, really wanted to be in larger vented enclosures. When the surrounds rotted out I tossed both boxes and drivers and replaced them with some nice 8" drivers in correctly sized sealed enclosures. They're better in every way than the old 10" drivers. You just can't arbitrarily put a given driver in a vented or sealed box and, IMO, the selection of big drivers that can work well in sealed enclosures is surprisingly small.

bjorno 26th June 2010 01:00 AM

1 Attachment(s)
Quote:

Originally Posted by bjorno (Post 2227224)

See a sub design using this driver that IMO would work well as a sub:

b



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