Do the sim and see. Anything over 10 m/s is bad. Over 17 m/s or so there's going to be measurable problems. Maybe not chuffing per se, but problems. Over around 27 m/s or so you are into audible chuffing territory.
This is very vague rules of thumb, of course, different pipe materials and shapes will have different Reynolds numbers and will behave differently.
For a quick sanity check you can check Flare It for acceptable velocities for any vent size, but that's based on round plastic vents so that must be taken into consideration too. And I'm guessing OP won't be flaring the port ends.
Less velocity is ALWAYS better.
A 18 inch driver with 22 or so mm published xmax (or even dual moderate xmax 15s) is going to produce a lot of wind and there's only one place for it to go.
This is very vague rules of thumb, of course, different pipe materials and shapes will have different Reynolds numbers and will behave differently.
For a quick sanity check you can check Flare It for acceptable velocities for any vent size, but that's based on round plastic vents so that must be taken into consideration too. And I'm guessing OP won't be flaring the port ends.
Less velocity is ALWAYS better.
A 18 inch driver with 22 or so mm published xmax (or even dual moderate xmax 15s) is going to produce a lot of wind and there's only one place for it to go.
Basically you want an LLT subwoofer...
I pasted in below a web location for a post and associated threads about transforming an infinite baffle subwoofer into an LLT (base reflex) suboofer designed to reproduce the lowest octave. It seems to be a relevant discussion to this post.
IB makeover.......... - Home Theater Forum and Systems - HomeTheaterShack.com
Retsel
I pasted in below a web location for a post and associated threads about transforming an infinite baffle subwoofer into an LLT (base reflex) suboofer designed to reproduce the lowest octave. It seems to be a relevant discussion to this post.
IB makeover.......... - Home Theater Forum and Systems - HomeTheaterShack.com
Retsel
The numbers help, however I'm really only thinking about this as simply as possible. In my actual experience, considering I currently have two nearly identical cabs (granted slot ported, 2x18) with quite high x-max drivers - and a penchant for flicking the clip indicators on a 4x 1500 w amplifier with any low frequency program material I can lay my grubby mits on, if I don't get audible chuffing 2 feet away from the cabinet, I wouldn't expect chuffing to be an issue when we're talking about people sitting tens of feet away, with half the amplifier power.
I mean, I can simulate it, and so can anyone, but I think things like this need to pass basic quick mental scrutiny before we get too carried away and chase yet another rabbit down yet another hole for yet another 20 pages of yet more bickering.
With my sub (described above) the flawless sim says below 10 m/s at full output (that's 115 dB) at all frequencies above 20 Hz, and at or below 18 m/s at 16.35Hz (bottom C). Not an issue. Ever.
For the driver and box (and organ) being discussed it's also not an issue . . . with a 6" port. Wouldn't be with a second driver in the box either . . .
Of course with 3" pipes it would be an issue . . . (and a dumb design).
It's more like 10% vent mach [~34.4 m/s] than 10 m/s. 5% [~17.2 m/s] satisfies most everyone and Small's ~4.5% has proven good enough for the few super high Xmax driver apps I've calculated for others, so the one to use IME: https://www.trueaudio.com/st_ventq.htm
Note that if the vent has no large round over, then make it at least ~2x whatever dia./area it calculates, i.e. 1.414x greater dimensions and of course longer.
Of course if driver specs, cab alignment allows bigger vents without excessive length, then might as well take advantage of it.
GM
Dunno, use Small's math, etc., that I just posted to calculate the minimum required based on Sd, Xmax.
GM
Did you do high quality measurements to find out how much you are losing to port compression? No, didn't think so. Are you using a high pass filter? If so you probably did not include that in the sim unless you simulated in Akabak, that will change things and the sim remains flawless (within it's limitations of course) but the user didn't understand the simulator assumptions and used the simulator wrong.
As I stated almost a dozen times, four 3 inch ports are identical cross sectional area as a single 6 inch port. The four 3 inch ports will have slightly higher Reynolds number but they are functionally the same thing. If it's not an issue with a single 6 inch port, it's not an issue with four 3 inch ports. Cross sectional area is really simple math. Reynolds number calculations are a bit more in depth but not required it's functionally the same thing.
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Technically losses start as soon as velocity exceeds 0 m/s. Ports have boundary friction issues defined as Reynolds number. The higher velocity gets the worse the problem gets.
MJK's recommendation, and the default in all his worksheets is 10 m/s. Flare It specifically says this:
Just because chuffing is not an audible problem does NOT mean you are not taking on measurable losses due to friction and turbulence.
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Basic mental scrutiny vs a technical scientific approach is EXACTLY the problem with this thread. Generally it's not a bad thing but when taken too far the science gets lost.
Just for reference, in the sim I showed a few times now velocity was approaching 17 m/s at xmax. That's too high IMO as it is (although most people would probably find it acceptable) and the system will probably pushed well past xmax. Adding another driver or a more capable driver is going to push that number up through the roof.
NOW it has to be stated that if a high pass filter is used that will drop the velocity down quite a bit, but as far as I know OP doesn't have one that is adjustable under 20 hz, so a high pass filter may not be used.
To get an accurate velocity figure WITH high pass filter in place the design must be simulated in Akabak. Easy enough to do. I'm not doing it.
I'm not a fan of losses when all it takes is 5 seconds to sim a better option and put in a bigger port. Others may not agree.
NOW it has to be stated that if a high pass filter is used that will drop the velocity down quite a bit, but as far as I know OP doesn't have one that is adjustable under 20 hz, so a high pass filter may not be used.
To get an accurate velocity figure WITH high pass filter in place the design must be simulated in Akabak. Easy enough to do. I'm not doing it.
I'm not a fan of losses when all it takes is 5 seconds to sim a better option and put in a bigger port. Others may not agree.
I believe that was patented and all kinds of claims were made, but was it ever proven by a 3rd party to be more effective than a simple flared port? I honestly don't know.
I measure and listen to pretty much everything I build.
No significant "port compression" . . .
No, you didn't think . . . you just made one more (more than one more) stupid assumption(s) . . .
False, and ignorant. A high pass filter will have no effect on port velocity at any given frequency and output level.
In the OP's case the signal is completely defined . . . the lowest note to ever be sounded is 16.35 Hz. (the lowest note played will usually be higher) and the output is fixed in level.
What, you mean there's no cannon blast rank? Come on, what kinda toy organ is this, anyway? 😛😛😛😛😛😛😛😛😛😛
Just guessing, mind you, but "the earth will shake and tremble before him" will probably be done with bass shakers in the pews and keyed not from the console but from the pulpit . . .
Port compression isn't something you can hear. How do you hear a loss?
Clearly you don't understand. If you look through measurements of ported boxes at increasing power, as power increases there are losses. This is not something that's audible. (Increasing distortion at increasing power IS audible but that's something completely different.)
Some of these losses are due to power compression, some is due to port compression, some are due to other distant third factors. But port compression is not audible until it's a full blown chuffing sound and at that point there's already significant losses in place, or until the frequency response curve is drastically altered, at which point there's already significant losses.
Wow, this is so far off base I don't even know how to respond. But I'll try.
A high pass filter ABSOLUTELY will reduce velocity down by the tuning frequency. This is really basic stuff.
Here's a random sim of a random ported box. Similar tuning to OP's box with no high pass filter. Shown are frequency response, excursion and velocity at the port mouth.
An externally hosted image should be here but it was not working when we last tested it.
Now let's add a high pass filter to protect the driver below tuning. IIRC it's a 3rd order BW, it's at 19 hz, where it needs to be to prevent the huge excursion spike below tuning that you see above in the excursion graph. The ONLY thing that's different in these two sets of graphs is the hpf, the applied power and everything else is exactly the same.
An externally hosted image should be here but it was not working when we last tested it.
As you can see, a proper high pass filter changes EVERYTHING, it obviously changes the excursion level, that's why we need it. As a result it changes the frequency response, and even this fairly steep filter changes the velocity peak level.
Velocity is reduced by about HALF when using a proper high pass filter in underdamped designs like this one and the OP's.
So ... wrong again Deward. Keep calling me ignorant, I love it. For now I still have time to keep proving you wrong, every time.
You can probably even use WinISD to see these basic effects if you don't know how to use Akabak.
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Seriously? You think there's a few thousand dollars in the budget for bass shakers?
Between you and Ben your recommendations have supassed the budget by several thousand dollars and neither of you have proposed any solutions to do ANYTHING about the original 16 hz too quiet problem.
If you can't hear it it doesn't matter, now does it. But you can hear it, and it's easy to measure . . . just do response curves at varying levels and see if they converge.
Did it really take all those charts to convince you that if you lower the volume you lower excursion and chuffing? Most even modestly clever people had probably figured that out already . . .
Now turn the level up so you don't reduce the desired 16.35 Hz signal and see what happens. Oh, look . . . same excursion, same chuffing . . . the filter benefited nothing. It reduces what the excursion and chuffing might have been if there were signal below 16.35 Hz, but there isn't any anyway. So it doesn't change anything that matters . . .
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