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.
I am planning on building the box that Adire suggests; 3cu ft., tuned to 20Hz.
Thanks for any suggestions.
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
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
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...
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...
>Hi GM, thanks for the reply!
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You're welcome!
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>As for the application; I plan on using this in my home theater ......., but I am open to any other suggestions...
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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 $$ by massing up the cab to damp the vibrations and only use one PR with ~304g. total.
GM
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You're welcome!
====
>As for the application; I plan on using this in my home theater ......., but I am open to any other suggestions...
====
Well, normally I recommend a ML-TL, but Vas is way too low, so I guess it's a 3ft cube tuned with dual PRs, unless of course you want to build a 9" dia. x 90" long vent, then play with stuffing to smooth it out.
WRT PR requirements, all the Stryke's have too much mass, the new PE's don't have enough travel, so I guess it's Adire's 15" with only 285g.. Since ~709g is required, you'll have to add the difference. If the $180/pr is too much, then you can save a few $$ by massing up the cab to damp the vibrations and only use one PR with ~304g. total.
GM
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
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
>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...
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So will the Brahma, just add more mass to the PR(s). For organ/HT, I tune to 14-16Hz.
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>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?
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Correct.
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>Does it matter where the PR's are placed on the box?
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They need to be on opposite sides so they will cancel out their vibration, the point of using two (in most cases).
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>Is there a performance trade-off if I only use one PR and a sturdy cabinet?
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An inaudible amount of SPL near/at Fb.
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>Also, are the advantages/disadvantages of PR about the same as with ports?
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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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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
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.
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.
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.
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...
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...
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
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
So, if I use dual 4" vents, the length of each port should be:
11.3" long for a 21Hz,
12.6" long for a 20Hz tune,
14.2" long for a 19Hz tune,
16.0" long for a 18Hz tune.
Does this sound about right?
Also, is there a problem with tuning the box below Adire's suggested 20Hz, or is the 20Hz figure designed to keep port lengths under control?
Dickason points out that vented enclosures increase the slope of rolloff. Does this only occur at the low end rolloff?
11.3" long for a 21Hz,
12.6" long for a 20Hz tune,
14.2" long for a 19Hz tune,
16.0" long for a 18Hz tune.
Does this sound about right?
Also, is there a problem with tuning the box below Adire's suggested 20Hz, or is the 20Hz figure designed to keep port lengths under control?
Dickason points out that vented enclosures increase the slope of rolloff. Does this only occur at the low end rolloff?
Raoul,
You're using WinISD, right?
Here's what you should get for lengths with dual 4" long vents in a 3 cubic foot box:
21 Hz: 45.5"
20 Hz: 50.5"
19 Hz: 56.375"
18 Hz: 63.125"
As you can see, WinISD is a bit "off". These come straight from LspCAD Pro, and it's proven to be extremely accurate.
Dan Wiggins
Adire Audio
You're using WinISD, right?
Here's what you should get for lengths with dual 4" long vents in a 3 cubic foot box:
21 Hz: 45.5"
20 Hz: 50.5"
19 Hz: 56.375"
18 Hz: 63.125"
As you can see, WinISD is a bit "off". These come straight from LspCAD Pro, and it's proven to be extremely accurate.
Dan Wiggins
Adire Audio
Actually I used a formula I found in Dickason's Loudspeaker Cookbook. I guess I didn't apply the formula correctly. I calculated the lengths, then divided them in half. Looks like it didn't work out very well. It did seem like I was getting off with some pretty short ports.
Are those the lengths for each port? Jeez, that's going to be pretty hard to stuff into a three cubic foot box. Maybe I am back to looking at PRs and sealed enclosures. Incidentally, I found the err of my ways with regard to the PR and Qts. If the PR compliance exceeds the driver then the Qts is not necessarily a concern.
Are those the lengths for each port? Jeez, that's going to be pretty hard to stuff into a three cubic foot box. Maybe I am back to looking at PRs and sealed enclosures. Incidentally, I found the err of my ways with regard to the PR and Qts. If the PR compliance exceeds the driver then the Qts is not necessarily a concern.
A good alternative to WinISD is Subwoofer simulator from the FRD consortium. It's freely available, and the results are similar to LspCAD, from what I can tell, more accurate than WinISD.
Also I find that a single larger vent is easier to fit in a box than two smaller ones given the same cross sectional area. If you want to do a quick test, you can buy some pipe, try different diameter pipes as well, and just stick it on the outside of your box. That way you don't have to
Also, make sure you try flared vents before you make a decision. Using a heat gun you can make flares by bending the pipe. Or you can turn something up on the lathe if you have access to one. Keep in mind that talking about vent diameter is pointless if you don't flare both ends - your vents will have to be unnecessarily large.
Also I find that a single larger vent is easier to fit in a box than two smaller ones given the same cross sectional area. If you want to do a quick test, you can buy some pipe, try different diameter pipes as well, and just stick it on the outside of your box. That way you don't have to
Also, make sure you try flared vents before you make a decision. Using a heat gun you can make flares by bending the pipe. Or you can turn something up on the lathe if you have access to one. Keep in mind that talking about vent diameter is pointless if you don't flare both ends - your vents will have to be unnecessarily large.
Raoul said:
Raoul,
When you double the vent area, you need to double the vent length, not halve it. So for your calculated lengths, you want to quadruple what you posted above, which would be really close to the lengths I posted.
And yes, that length is for each port!
Dan Wiggins
Adire Audio
Hi Dan,
I noticed the half vs. double port length relationship after I looked at your numbers for a while. I think I punched in the radius for one pipe instead of both, then halved it or something. Ooops.
I think I might be able to fit the port (20Hz) into the box with a 180 degree turn, but I am kind of worried about the penalties for such a long pipe. I'll have to do some reading this weekend.
LspCAD is pretty nice, by the way. Funny, I modeled some really nice curves, and then I found the Le simulator button. The curves weren't so nice anymore...
Just out of curiosity, how did you intend for the Brahma box the be vented (PR, ports, slot ports)?
I noticed the half vs. double port length relationship after I looked at your numbers for a while. I think I punched in the radius for one pipe instead of both, then halved it or something. Ooops.
I think I might be able to fit the port (20Hz) into the box with a 180 degree turn, but I am kind of worried about the penalties for such a long pipe. I'll have to do some reading this weekend.
LspCAD is pretty nice, by the way. Funny, I modeled some really nice curves, and then I found the Le simulator button. The curves weren't so nice anymore...
Just out of curiosity, how did you intend for the Brahma box the be vented (PR, ports, slot ports)?
geolemon:
>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).
====
I guess I should have pointed out that this is based on the 4800W peak power rating, I didn't have time to calc it for his future 1kW rating. Even then, 9" is a quite a bit smaller than R. Small's 13.19" ideal recommendation. Not that it changed much, dropping to 'only' 8.5827".
This puts the vent mach way too high per R. Small and more importantly to me, my ears, and why I didn't/don't/won't recommend them.
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DanWiggins:
>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.
>Here's what you should get for lengths with dual 4" long vents in a 3 cubic foot box:
20 Hz: 50.5"
>As you can see, WinISD is a bit "off". These come straight from LspCAD Pro, and it's proven to be extremely accurate.
====
I assume you mean if they're flared since the vent mach is otherwise high enough to be audible to me in a reflective room.
Have you actually measured these results? While I know that LCP is pretty accurate when the cab is a golden or acoustic ratio, MJK's worksheets are ~spot on (so far anyway) for when the cab shape changes and/or the vent lengths start getting ridiculous (>24"). Using the 1st gen of LCP I get ~46.5" for (2) 4" in 3ft^3/20Hz and 44" with MJK's WS (the only one I had time to sim), so I'm curious about later versions of LCP seemingly going in the wrong direction.
Again, WRT actual measurements Vs sims, with MJK's WS I historically have gotten as good, or better, looking/accuracy plots as LCP, but in the Brahma's case LCP shows a nice EBS while the MJK plot shows an unacceptably high ~5dB of Fb peaking using the specs in the original Brahma Owner's Guide. LCP's impedance peaks are a bit lower also, so I'm not sure what's going on here.
As you can guess, the predicted impulse response looks more like a gong's decay plot than a well damped EBS alignment's.
TIA,
GM
>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).
====
I guess I should have pointed out that this is based on the 4800W peak power rating, I didn't have time to calc it for his future 1kW rating. Even then, 9" is a quite a bit smaller than R. Small's 13.19" ideal recommendation. Not that it changed much, dropping to 'only' 8.5827".
This puts the vent mach way too high per R. Small and more importantly to me, my ears, and why I didn't/don't/won't recommend them.
====
DanWiggins:
>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.
>Here's what you should get for lengths with dual 4" long vents in a 3 cubic foot box:
20 Hz: 50.5"
>As you can see, WinISD is a bit "off". These come straight from LspCAD Pro, and it's proven to be extremely accurate.
====
I assume you mean if they're flared since the vent mach is otherwise high enough to be audible to me in a reflective room.
Have you actually measured these results? While I know that LCP is pretty accurate when the cab is a golden or acoustic ratio, MJK's worksheets are ~spot on (so far anyway) for when the cab shape changes and/or the vent lengths start getting ridiculous (>24"). Using the 1st gen of LCP I get ~46.5" for (2) 4" in 3ft^3/20Hz and 44" with MJK's WS (the only one I had time to sim), so I'm curious about later versions of LCP seemingly going in the wrong direction.
Again, WRT actual measurements Vs sims, with MJK's WS I historically have gotten as good, or better, looking/accuracy plots as LCP, but in the Brahma's case LCP shows a nice EBS while the MJK plot shows an unacceptably high ~5dB of Fb peaking using the specs in the original Brahma Owner's Guide. LCP's impedance peaks are a bit lower also, so I'm not sure what's going on here.
As you can guess, the predicted impulse response looks more like a gong's decay plot than a well damped EBS alignment's.
TIA,
GM
DanWiggins said:
Dan,
It seems to me that slot ports need to be much larger than flared circular ports as they are more difficult to flare. This would then mean that they are much much longer.
GM said:
Seems to me it would make more sense to look at the amount of power that is needed to reach xmax in the box in question, even 1kw seems inflated to me.
Another question that seems relevant to which I don't have the answer, is at which point does a vent become too long? I have a 4" vent that is 1m long and it tends to ring - it would be better as a digeridoo than a vent!!!
I had the same question about the vent length. I am trying to find out how to predict where the resonances will occur. There are a few threads here that discuss it, just a matter of wading through it.
I think 1kw is excessive. I also don't think the amp can handle a 1 ohm load (Brahma wired in parallel), so the driver will probably see something in the region of 700w (probably still excessive, depending on the box configuration). I am not sure if wiring the driver in series or with seperate sources will alter the parameters. I think I have an audioXpress article on wiring DVC's.
I can see the attraction of PR's, but I haven't come up with a nice model on LspCAD yet. I am just going to keep juggling GM's figures till I find something tasty...
I think 1kw is excessive. I also don't think the amp can handle a 1 ohm load (Brahma wired in parallel), so the driver will probably see something in the region of 700w (probably still excessive, depending on the box configuration). I am not sure if wiring the driver in series or with seperate sources will alter the parameters. I think I have an audioXpress article on wiring DVC's.
I can see the attraction of PR's, but I haven't come up with a nice model on LspCAD yet. I am just going to keep juggling GM's figures till I find something tasty...
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