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Author Topic: Vented Sub Tuning....  (Read 9163 times)

Nick Enright

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Vented Sub Tuning....
« on: January 20, 2012, 06:56:40 pm »

I've built a 312.5L box for the JBL 2240H driver. Using different port lengths tuned to 28Hz, 32Hz, 36Hz, 40Hz I can get really nice pink noise results. (using Smaart for analysis) They follow almost exactly the results predicted by WinISD.

However:

When I playback program material with significant low bass at a decent volume it seems the cone is unloading and doing the vvvmmmppoobrap sound.

I've adjusted the high pass at various frequencies, both 48db/oct and 24db/oct (L-R). Either I have to set the high pass 10-20Hz above the tuning, and even then the driver will still "brappp" with sufficient program. Or limit the drive to the cabinet to well under RMS power. (300w sine)

I've free air swept the driver and aside from the tinsel rubbing, slightly at around 42-44Hz it sweeps clean. I bent the lead off the terminal and while the tinsel still oscillates at particular resonances, it doesn't contact the cone.

Of the four drivers I have this one is the best, the others either have ripped surrounds or spiders. No air is leaking past the driver frame, or at the surround, and the cabinet is well sealed.

Any Ideas? (Quite possibly another bad driver?)

Note: These are all going to be re-coned to 2245's, hopefully taking advantage of the large cabinet volume and allow a lower tuning with less distortion than the 2240. I'm just worried that I'll encounter this same problem even with the extra Xmax.
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Nick Enright
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Lyve Productions
Detroit, MI

Jeff Bankston

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Re: Vented Sub Tuning....
« Reply #1 on: January 20, 2012, 10:43:41 pm »

if you dont mind buying new drivers check out this one > FaitalPro 18XL1600 with an xmax of 13mm. max excursion -/+28mm. 1600 watts rms/aes power handling. 4 or 8 ohm. BASSMAXX uses the older 18XL1500 version.
« Last Edit: January 20, 2012, 10:45:28 pm by Jeff Harrell »
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Iain.Macdonald

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Re: Vented Sub Tuning....
« Reply #2 on: January 21, 2012, 03:19:50 pm »

I've built a 312.5L box for the JBL 2240H driver. Using different port lengths tuned to 28Hz, 32Hz, 36Hz, 40Hz I can get really nice pink noise results. (using Smaart for analysis) They follow almost exactly the results predicted by WinISD.

However:

When I playback program material with significant low bass at a decent volume it seems the cone is unloading and doing the vvvmmmppoobrap sound.

I've adjusted the high pass at various frequencies, both 48db/oct and 24db/oct (L-R). Either I have to set the high pass 10-20Hz above the tuning, and even then the driver will still "brappp" with sufficient program. Or limit the drive to the cabinet to well under RMS power. (300w sine)

I've free air swept the driver and aside from the tinsel rubbing, slightly at around 42-44Hz it sweeps clean. I bent the lead off the terminal and while the tinsel still oscillates at particular resonances, it doesn't contact the cone.

Of the four drivers I have this one is the best, the others either have ripped surrounds or spiders. No air is leaking past the driver frame, or at the surround, and the cabinet is well sealed.

Any Ideas? (Quite possibly another bad driver?)

Note: These are all going to be re-coned to 2245's, hopefully taking advantage of the large cabinet volume and allow a lower tuning with less distortion than the 2240. I'm just worried that I'll encounter this same problem even with the extra Xmax.

What port dimensions are you using?
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Nick Enright

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Re: Vented Sub Tuning....
« Reply #3 on: January 22, 2012, 01:13:54 am »

What port dimensions are you using?

Dimensions:
Box: 20"t x 24"w x 60"d

Vent: 3"t x 22.5"w x (var)"d

@ 40Hz the vent is 19mm (3/4") deep
@ 36Hz the vent is 79mm (3.1")
@ 32Hz the vent is 163mm (6.4")
@ 28Hz the vent is 286mm (11.25")

I've used each vent board to do both impedance (current & voltage measured True RMS) and pink noise testing. There is a definite minimum in impedance between 28-30Hz, this measurement is almost completely independent of vent length.

These are a (hopefully) creative solution to an install problem. At right around $120 (aftermarket) for 2245 recones into these baskets there is not at all a budget for driver replacement.

To fit the 18" driver into the total cabinet ht of 20" I've tilted the baffle board to approximately 60deg. attachment below is a solidworks screen shot of the design, with the 28Hz vent installed.


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Nick Enright
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Lyve Productions
Detroit, MI

Paul G. OBrien

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Re: Vented Sub Tuning....
« Reply #4 on: January 22, 2012, 10:57:37 pm »

I've built a 312.5L box for the JBL 2240H driver.

Well unfortunately.. that was your first mistake. I don't know what your goals are for this project but what you have are drivers that aren't really "subwoofers" by modern standards and for some reason you have built a cabinet that is about 2 times too big! This configuration puts the driver into overexcursion with as little as 150w at the 30hz tuning. What you need to do is download WinISD Pro to get it's excursion prediction capability(you need full driver specs for this not just the basic TS parameters.. though that's not a problem with JBL), and then you'll see what you can get from a particular driver before building anything. Quick note.. the only way a 2240 with handle 600w is in a 125l cab tuned to about 55hz, and as stated above that's not much of a sub by todays standards. The 2245 does much better it'll take full power in a 186l cab tuned to 32hz, that makes it a good pro audio sub but still not a great HT sub.
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Nick Enright

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Re: Vented Sub Tuning....
« Reply #5 on: January 24, 2012, 03:39:57 am »

Well unfortunately.. that was your first mistake. I don't know what your goals are for this project but what you have are drivers that aren't really "subwoofers" by modern standards and for some reason you have built a cabinet that is about 2 times too big! This configuration puts the driver into overexcursion with as little as 150w at the 30hz tuning. What you need to do is download WinISD Pro to get it's excursion prediction capability(you need full driver specs for this not just the basic TS parameters.. though that's not a problem with JBL), and then you'll see what you can get from a particular driver before building anything. Quick note.. the only way a 2240 with handle 600w is in a 125l cab tuned to about 55hz, and as stated above that's not much of a sub by todays standards. The 2245 does much better it'll take full power in a 186l cab tuned to 32hz, that makes it a good pro audio sub but still not a great HT sub.

Thank You!

I had figured I had way too large of a cabinet, but I've only built up one, and its unfinished, so I can make it smaller very easily.

Why did I build the prototype so big:

1) JBL Data comes from a 10ft^3 cabinet, I wanted to assure I was around that size even after the massive amount of bracing needed for the long panels. (JBL recommends cabinet volume btw: 225-450L for the 2245, and 140-340L for the 2240)

2) I did understand that power handling due to excursion drops off as cabinet size increases, however it's always easier to make the cabinet smaller. "Add it on" saws are still vaporware, AFAIK.

I've downloaded WinISD Pro... Way better than the beta version, but the driver data had to be keyed in and the software didn't like all the JBL spec parameters. I've entered enough to get it to calculate, and the results match your statements.

Application: Pro-sound duty, Install system, 160-200ppl, bands/DJ's

Under stage in fully double walled insulated chamber, 4pcs center clustered.

By shrinking the depth of the cabinet to 45" (front to back) the new estimated (solidworks) volume (after subtracting bracing and driver) works out to 174L.

I really like the new dimensions, I'll post another screen shot after I've rebuilt the prototype. I add the bracing through testing, allowing me to damp resonances in the cabinet walls very precisely, then re-build the model in CAD.

An interesting result of the original design is a large increase in output btw 50-80Hz... exactly the 1/4 wavelengths btw the angled baffle and the back panel. (not surprising, just interesting.)

I'll collect all the prototyping data and post it up to this thread, I'm hoping to be able to eventually simulate the behavior of this driver/cabinet using both FEA and CFD. Having empirical data is essential to determine the accuracy of the simulation.

Thanks Again!

Nick

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Nick Enright
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Lyve Productions
Detroit, MI

Paul G. OBrien

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Re: Vented Sub Tuning....
« Reply #6 on: January 25, 2012, 12:40:15 am »

I've downloaded WinISD Pro... Way better than the beta version, but the driver data had to be keyed in and the software didn't like all the JBL spec parameters.

Yeah that's common. If you look through the notes for the software it'll explain that it's best to enter the parameters in a certain order and let the software auto-calculate the rest, the numbers it generates are ususally pretty close to the published driver specs.
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Nick Enright

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Re: Vented Sub Tuning....
« Reply #7 on: January 25, 2012, 11:41:31 am »

Yeah that's common. If you look through the notes for the software it'll explain that it's best to enter the parameters in a certain order and let the software auto-calculate the rest, the numbers it generates are ususally pretty close to the published driver specs.

Essentially thats what I did, the calculated numbers are quite close to the JBL data, and honestly if the design is close at this point it'll be just fine. Next week the first re-cone will be done and I'll have a 2245 with new moving parts to test with.

Here's the new screen shot.
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Nick Enright
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Lyve Productions
Detroit, MI

ProSoundWeb Community

Re: Vented Sub Tuning....
« Reply #7 on: January 25, 2012, 11:41:31 am »


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