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Old 23rd May 2004, 02:41 AM   #1
Raoul is offline Raoul  United States
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Default Brahma vent dimensions

I was planning to build a box for my Adire Brahma mk I this weekend, but I was having trouble determining vent size. I have been unable to successfully download any of Adire's software, and the only book I have found locally suggests a vent of 2" diameter and a 5 5/8" length. The diameter alone seems pretty tight to me, though. Does anyone have any suggestions, or could pass on a formula for determining vent diameter and length?

I am planning on building the box that Adire suggests; 3cu ft., tuned to 20Hz.

Thanks for any suggestions.
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Old 23rd May 2004, 03:32 AM   #2
GM is offline GM  United States
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Assuming you mean the 12" version, this size vent will have a ~0.5 mach speed at 1kW (ideally it should be <0.05 mach), which is about the minimum power requirement to get decent output for organ music/HT app. At its rated 4.8kW it's supersonic! Regardless, this is a driver that requires dual PRs in a bipole layout if it's going to be used as intended. Since the vent/PR requirements are based on available power and app, I need this info to calculate what's required.

Sounds like this book assumes only a few watts input, which is worthless unless the speakers are 95-115dB efficient, depending on the app.

GM
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Old 23rd May 2004, 05:45 PM   #3
Raoul is offline Raoul  United States
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Hi GM, thanks for the reply!

The book is old, and probably does assume a much low power situation. Anyway, I was planning on driving the Brahma (yes, 12") temporarily with a bridged Hafler 9300, which claims 450W in 8ohm; not sure what it puts into 4ohm. Eventually, I plan on trying the "OMP 1000 1000 Watt RMS mosfet mono Amplifier" available from http://www.bkelec.com/ This amp will probably be the permanent power source, as long as it doesn't turn out to be a dog...

As for the application; I plan on using this in my home theater set-up with the D'Appolito designed Audax system that Madisound sells. The 3cu. ft. vented enclosure was what Adire recommended for home use, but I am open to any other suggestions...
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Old 24th May 2004, 06:20 AM   #4
GM is offline GM  United States
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>Hi GM, thanks for the reply!

====

You're welcome!

====

>As for the application; I plan on using this in my home theater ......., but I am open to any other suggestions...

====

Well, normally I recommend a ML-TL, but Vas is way too low, so I guess it's a 3ft cube tuned with dual PRs, unless of course you want to build a 9" dia. x 90" long vent, then play with stuffing to smooth it out.

WRT PR requirements, all the Stryke's have too much mass, the new PE's don't have enough travel, so I guess it's Adire's 15" with only 285g.. Since ~709g is required, you'll have to add the difference. If the $180/pr is too much, then you can save a few $$ by massing up the cab to damp the vibrations and only use one PR with ~304g. total.

GM
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Old 24th May 2004, 08:22 PM   #5
Raoul is offline Raoul  United States
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Thanks, GM. This is really helpful. It seems like it may be cheaper to go with a Adire DPL12 and PR15. It looks like that combo might reach down lower, somewhere below 20Hz...but I have the Brahma, so I guess I might as well try this setup first.

A few more questions...

The above model is for a box with 2 PR and an internal volume of 3 cu. ft. without the driver displacement (driver + 3 cu ft), right?
Does it matter where the PR's are placed on the box?
Is there a performance trade-off if I only use one PR and a sturdy cabinet?

Also, are the advantages/disadvantages of PR about the same as with ports?

Thanks
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Old 25th May 2004, 04:06 AM   #6
GM is offline GM  United States
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>Thanks, GM. This is really helpful. It seems like it may be cheaper to go with a Adire DPL12 and PR15. It looks like that combo might reach down lower, somewhere below 20Hz...

====

So will the Brahma, just add more mass to the PR(s). For organ/HT, I tune to 14-16Hz.

====

>The above model is for a box with 2 PR and an internal volume of 3 cu. ft. without the driver displacement (driver + 3 cu ft), right?

====

Correct.

====

>Does it matter where the PR's are placed on the box?

====

They need to be on opposite sides so they will cancel out their vibration, the point of using two (in most cases).

====

>Is there a performance trade-off if I only use one PR and a sturdy cabinet?

====

An inaudible amount of SPL near/at Fb.

====

>Also, are the advantages/disadvantages of PR about the same as with ports?

====

No, the only downside to PRs is the parts cost. The upside is more output, no organ pipe resonances, and takes up less space.

GM
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Old 25th May 2004, 02:34 PM   #7
RHosch is offline RHosch  United States
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With the displacement available from a Brahma, you could try a sealed enclousre with Linkwitz transform to flatten the rolloff. Not as much output as a PR'd design, but would still be capable of very solid bass, and at very high fidelity, throughout the audible frequency range. A lot cheaper than PR's as well... something you might want to try before going to more expensive routes, or before getting a new sub.
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Old 26th May 2004, 12:35 AM   #8
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I'm not sure what troubles you are having downloading software...
But for subwoofer design, I most often use WinISD, which is free, and very good... you might want to download the standard version (not the pro version), as it's easier to reach port size suitability determinations... there's a mach number given right there as you play with port size, and it turns red when it 'thinks' you are going to encounter wind noise.
www.linearteam.dk

I've also got considerable experience with Brahmas, as I have a fairly close working relationship with Adire.
I can tell you from this, at least, that a 9" port is completely unnecessary (of course, if you have the space, "overkill" is always nice ).

You could do dual 3" ports, 24" long each (bending them with PVC elbows as needed), which would be whistle-free. That would tune you actually just a hair over 21hz... that's not much of a compromise (20hz is actually dual 3's, 29" long each).

You probably should download WinISD, as that'll also allow you to see the frequency response you are building.
This is essentially one heck of an extended-shelf response, just flat all the way down to 20hz, essentially.... at least, the one tuned just a hair over 21hz is.
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Old 26th May 2004, 03:17 AM   #9
Raoul is offline Raoul  United States
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I think I might try the dual pvc port set up this weekend, if I can find the time. I should have some 3" pipe around here somewhere, so I won't have to wait for parts to arrive. I can experiment with other options later, but I am itchin' to see this guy in action.

I picked up a copy of Dickason's Loudspeaker Design Cookbook yesterday, and really it has created more questions than solutions so far. The book suggests that the Qts of the Brahma (0.45) might result in too low of an alpha figure to be used effectively with PRs. I am also trying to wrap my head around all of the different alignments available for vented enclosures, and what they all mean. It seems like the alignments are just box size and port combinations, so maybe WinISD will speed up that process...
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Old 26th May 2004, 02:44 PM   #10
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Raoul,

For the box and power you're talking about, you'll want a pair of 4" diameter vents, at a minimum. You need around 25 square inches of port area to lower the port air velocity sufficiently.

Note too that the port velocity is a function of SPL, not power. A highly efficient driver in a large box will still require a very large port, even with extremely little power applied.

Dan Wiggins
Adire Audio
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