Someone gave me a free 12" sub enclosure. It's not the wisest design, but instead of throwing it away I figured I'd throw some drivers in it for use at the theater (I work as a Tech Director for a live performance theater - plays/musicals)
It's particle board. Appears to be built for a Ford Ranger application. Dual cutouts for 12" drivers, shared volume, single 4" x 4" port. 3.007 cuft (we'll call it 3)
First, how do I find the tuning of the box? Can I adjust port length to drop it a tad lower? Add baffles to go third order? Seal up one of the cutouts and put a high-X 12" in the other hole? I looked at a few calculators and they want parameters that I don't know like air compliance. Second, what driver(s) might you throw in it for a large 50' x 60' theater. The goal is not necessarily SPL, but I'm hoping for low. I have a couple 300w Yorkville 10" active subs that round things out nicely for music and instruments, but they drop off around 35-40 hz. I'm hoping to fill in the octave below it for things like SFX - thunder, explosions, etc. I have more wattage than a deity, but don't need to use it all.
It's particle board. Appears to be built for a Ford Ranger application. Dual cutouts for 12" drivers, shared volume, single 4" x 4" port. 3.007 cuft (we'll call it 3)
First, how do I find the tuning of the box? Can I adjust port length to drop it a tad lower? Add baffles to go third order? Seal up one of the cutouts and put a high-X 12" in the other hole? I looked at a few calculators and they want parameters that I don't know like air compliance. Second, what driver(s) might you throw in it for a large 50' x 60' theater. The goal is not necessarily SPL, but I'm hoping for low. I have a couple 300w Yorkville 10" active subs that round things out nicely for music and instruments, but they drop off around 35-40 hz. I'm hoping to fill in the octave below it for things like SFX - thunder, explosions, etc. I have more wattage than a deity, but don't need to use it all.
The volume of the port and drivers needs to be backed out of the box size. Various online calculators can predict the Fb (box tuning) which would be about 40Hz for 3 cubic feet with a 4" diameter 4" long port.
Yes, but the port is already undersized for a high excursion 12".
The Fb must be as low as you want the speaker's output to extend to, and a steep high pass (low cut) filter used just below Fb to keep excursion under control.
A ported box is considered "fourth order", bandpass boxes are higher orders.
A 6th order bandpass box could increase output over a limited bandwidth.
The lower the frequency, the more SPL required to sound as loud.
Dropping an octave from 40 Hz to 20 Hz might require another 10 dB output to sound as loud as your present subs.
At 20Hz, about four times the displacement would be required just to equal the same SPL at 40Hz.
Each doubling of displacement requires +6dB (four times) the power.
If your deity has over 4800 watts, you may not use it all ;^)
That said, really high displacement 12" with small VAS to work in a small cabinet aren't cheap,
you might suggest a budget cap .
Thanks for the detailed reply.
When I say "deity" I mean I have about 3000wrms to play with. I wouldn't call it super clean (some older, donated Yamaha amps from a local proscenium theater that closed about 10 years ago) but enough to move some air.
I was actually wondering how this box would have prevented any driver from over-excursion unless it was used with small wattage. It appears to be commercially made which doesn't imply quality, but it's my guess that someone crunched some numbers before sending it out into the wild.
I'm sorta aware of the physics of going higher numerical order being a narrower freq range. Can I get an octave from a 6th order within -3db on both ends?
I have no problem ditching this box and building something else, but this is more of an exercise in "can I make it work." Budget is zero pennies. This isn't something I'm making a purchase order for drivers. This would be "cool, I have this free box, some scrap lumber to modify it if needed, and Jim found these JBLs with rotted surrounds in a trash pile." I'm not going for awards or audiophile quality. I'm hoping to make patrons feel a little more in their butts. I'm sure I can find a little money in the budget for incidentals, or if I can't find something that works, I can pony up for purchasing drivers.
This is more grassroots, let's twist some wires together kinda thing.
When I say "deity" I mean I have about 3000wrms to play with. I wouldn't call it super clean (some older, donated Yamaha amps from a local proscenium theater that closed about 10 years ago) but enough to move some air.
I was actually wondering how this box would have prevented any driver from over-excursion unless it was used with small wattage. It appears to be commercially made which doesn't imply quality, but it's my guess that someone crunched some numbers before sending it out into the wild.
I'm sorta aware of the physics of going higher numerical order being a narrower freq range. Can I get an octave from a 6th order within -3db on both ends?
I have no problem ditching this box and building something else, but this is more of an exercise in "can I make it work." Budget is zero pennies. This isn't something I'm making a purchase order for drivers. This would be "cool, I have this free box, some scrap lumber to modify it if needed, and Jim found these JBLs with rotted surrounds in a trash pile." I'm not going for awards or audiophile quality. I'm hoping to make patrons feel a little more in their butts. I'm sure I can find a little money in the budget for incidentals, or if I can't find something that works, I can pony up for purchasing drivers.
This is more grassroots, let's twist some wires together kinda thing.
No bass reflex box has over-excursion protection, that requires processing. To integrate a sub-sub will require processing, especially if you want to use the power available.
It's possible, but the alignment is much more tricky than bass reflex, and tends to shift more between low and high level signal.
Probably the cheapest source for high excursion drivers would be automotive subs from someone upgrading, or needing trunk space. Check their functionality before dropping any pennies.
Currently in the pile I have two CR-Vs with partially un-glued surrounds. I can glue those back on and test. My guess is that they separated because someone abused them. I have two GAD monitors with 12" lows and blown horns, but I don't expect much from paper-cone stage wedges designed for full range coverage.
Forgive my salivation at a new chance to fire up the soldering iron. I love making junk from other junk
Forgive my salivation at a new chance to fire up the soldering iron. I love making junk from other junk
85L (3cuft) should be fit to fit 2x B&C 12BG100, but that port will be too small for a good tuning. The tuning of that will be arround 40Hz, but the air velocity will be way to high to play it loud. In general, it's hard to find a good cone driver to fit an existing build as it will always a bit off when the cabinet is not build for the driver. If you want it lower, it's probally a better id to give up this box and build a dedicated box for a high excurision sub that can do that.
And 35-40Hz for a theater is what most have and need. I don't think theaters try to go lower on subs, because for theaters that do plays and concerts, there is no need for it. It's only movie theaters that need lower.
And 35-40Hz for a theater is what most have and need. I don't think theaters try to go lower on subs, because for theaters that do plays and concerts, there is no need for it. It's only movie theaters that need lower.
