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Is it possible to model port compression?

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Has any work been done on controlling the boundary layer in ports to delay separation? (e.g. targeted roughness, turbulators, etc.)  I know you need to have a precise idea of the Re range, and any given solution will be very specific to that port geometry.  Also, I imagine everything may be different in an acoustic resonator; not only is flow not fully developed, it's constantly changing direction.  Still, it sounds like an interesting area of study.

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Calculating this kind of thing is a job for a FEA program.  

 

One thing I noticed in the comments above by SME.  You are applying fluid flow calculations to air flow calculations.  There are some similarities between the two mediums.  Where the differences begin is that a fluid is pretty much non-compressible.  Whereas a gas is.  And that is where we run into problems when we want to do some useful calculations in the junctions of the interior of a container with a port and the the end of the port.  Much has to be taken into account in order to make any useful calculations.  And that makes the calculations a real pain in the but.

 

From lurking around on some of the threads I see that some of you guys are pretty handy with Hornresp.  A few years back I asked David McBean to include particle velocity measurements in Hornresp to use in some very high SPL type of cabinets.  It is not a full around every corner, nook and so on calculation.  But it does get you somewhere in a hurry.

 

The keeping it below 10 meters/second rule of thumb goes a long way into making a serviceable port.  

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Calculating this kind of thing is a job for a FEA program.  

 

One thing I noticed in the comments above by SME.  You are applying fluid flow calculations to air flow calculations.  There are some similarities between the two mediums.  Where the differences begin is that a fluid is pretty much non-compressible.  Whereas a gas is.  And that is where we run into problems when we want to do some useful calculations in the junctions of the interior of a container with a port and the the end of the port.  Much has to be taken into account in order to make any useful calculations.  And that makes the calculations a real pain in the but.

 

From lurking around on some of the threads I see that some of you guys are pretty handy with Hornresp.  A few years back I asked David McBean to include particle velocity measurements in Hornresp to use in some very high SPL type of cabinets.  It is not a full around every corner, nook and so on calculation.  But it does get you somewhere in a hurry.

 

The keeping it below 10 meters/second rule of thumb goes a long way into making a serviceable port.  

 

 

Hey Mark,

 

 Welcome.

 

I'd agree with your last sentence but the caveat is that it is basically impossible with any type of serviceable design using modern high power drivers. By the time the vents are made that large, the overall size is completely out of control, or the pipe resonances are bad, or you are faced with a large system with little useable output compared with the size of the device, or a combination of all of the above.

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Hey Mark,

 

 Welcome.

 

I'd agree with your last sentence but the caveat is that it is basically impossible with any type of serviceable design using modern high power drivers. By the time the vents are made that large, the overall size is completely out of control, or the pipe resonances are bad, or you are faced with a large system with little useable output compared with the size of the device, or a combination of all of the above.

 

Thanks Josh.

 

I found this forum by accident.  And most of the posts I have read are by very thoughtful people.  I'll hang around from time to time and see what's interesting.

 

You are very right in that statement.  And I have no answer other than design to the goals that you are seeking.  A clean sounding bass from say a properly damped sealed enclosure with a low distortion woofer, or a well designed front loaded horn set the standards as to what is clean sounding.  The job of a proper design is to get you there or as close as you can.  Designing a port has always been a set of compromises.  And there are ways to get a decent sound out of a vented enclosure that have not been discussed such as the power port type of vented enclosure.  It has pretty much the lowest turbulence for a given diameter of enclosure.

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One could write a phd thesis on modeling flow of a fluid -- this simply comes down to FEA, and any other modeling is just an approximation. But lets face it, this is not rocket science, its just a speaker :)

 

I will say there was only one subwoofer I have seen that did not have port issues and it was a twin 18" box with a single 18" port - no curves. Very low velocity, extremely capable. When you see ports with high air velocity then that's normally an indication of being undersized. There are reasons to do this (extension, size etc) but it does mean the port it exhibiting its limits.

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One could write a phd thesis on modeling flow of a fluid -- this simply comes down to FEA, and any other modeling is just an approximation. But lets face it, this is not rocket science, its just a speaker :)
 
I will say there was only one subwoofer I have seen that did not have port issues and it was a twin 18" box with a single 18" port - no curves. Very low velocity, extremely capable. When you see ports with high air velocity then that's normally an indication of being undersized. There are reasons to do this (extension, size etc) but it does mean the port it exhibiting its limits.

 

Hello Kyle.  I have heard about you from Josh but never had the pleasure of talking with you.  I guess this is a close second.

 

I think quite a few people have written their PHD on that subject!

 

Just a speaker   :o

 

Don't get me started on that one.  The truth of the matter is at very few times are we going to tax the port compression on a properly designed subwoofer.  And the times when that does happen we usually have program material that masks the effect in the first place.

 

And the difference between no problems from a port at any time is the difference between a realistic enclosure size and a not so realistic enclosure size.

 

Twin 18 with an 18 inch diameter port.  Wow.  A genuine phallic idol to the vent gods if there ever was one!

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Hello Kyle.  I have heard about you from Josh but never had the pleasure of talking with you.  I guess this is a close second.

 

I think quite a few people have written their PHD on that subject!

 

Just a speaker   :o

 

Don't get me started on that one.  The truth of the matter is at very few times are we going to tax the port compression on a properly designed subwoofer.  And the times when that does happen we usually have program material that masks the effect in the first place.

 

And the difference between no problems from a port at any time is the difference between a realistic enclosure size and a not so realistic enclosure size.

 

Twin 18 with an 18 inch diameter port.  Wow.  A genuine phallic idol to vent gods if there ever was one!

 

Hello! I'm the lurker of this forum, lol.

 

Indeed, very good points. I'm often amazed how bad a subwoofer can sound with a sinwave at full volume (capability) around port tuning, but put music into it and it just seems to work and the distortion seems to subside :)

 

The real question is how many more db can you get out of a driver + amp if you increase the box and or port port and the art of the whole process becomes the design trade offs. The double 18 box I spoke of is very huge and very impracticable. A few dB loss here and there for a subwoofer half the size might add a lot of value to most people.

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Some of the best work done in this field is by people wanting to simulate musical instruments.  And this video is of a mitered organ pipe.  Or a rectangular organ pipe that has a 90 degree bend.  Sometimes they even have a 180 degree bend.  I have a boat load of thesis papers and other studies that get to the number crunching but it's nice to see something rather than read about it.

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Kyle makes a good point. We often simulate airspeeds under a worst case scenario with a full power sine wave at the airspeed maximum, but that's very rare. most of the time we aren't running the speaker wide open for all it's got and the content is almost always much more transient, wide bandwidth and complex. In a way it's sort of similar to the reason that I now prefer very low qts drivers for their higher efficiency. With complex, wide bandwidth material the power requirements are lower and it leaves more effective headroom in the amplifier on those types of signals. Also less thermal demands on the voice coils.

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Kyle makes a good point. We often simulate airspeeds under a worst case scenario with a full power sine wave at the airspeed maximum, but that's very rare. most of the time we aren't running the speaker wide open for all it's got and the content is almost always much more transient, wide bandwidth and complex. In a way it's sort of similar to the reason that I now prefer very low qts drivers for their higher efficiency. With complex, wide bandwidth material the power requirements are lower and it leaves more effective headroom in the amplifier on those types of signals. Also less thermal demands on the voice coils.

Probably the best selling point on a high efficiency driver is the correlation between where they are efficient and where the bulk of what is considered "bass" is centered. 60 hertz is where the money is. And many pro drivers will get you higher spl for a given size versus a conventional subwoofer with much less power input. Last point. Amp power never makes up for the nearly 10 dB of greater efficiency. Do the math and ask a very simple power compression question. You will find the answer!

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is it possible to approach this from a different angle? i.e. assume that port compression exists and model that effect (on driver excursion and output)

 

this would be analogous to the way you can model the effect of power or excursion so you'd set a port velocity limit of, e.g., 10m/s and then see what happens next.

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On 11/2/2016 at 2:16 PM, Ricci said:

That's why you see 15 and 18" drivers with 3 and 4" ports or thin slot ports. PR's have advantages but also some shortcomings themselves. Cost being one and a lot of real estate on the baffle/s for them being another. Also the advantages of running them in opposed pairs make that arrangement almost a necessity. Mechanical displacement limits is another one.

Josh, after reading through this thread, I was hoping you could provide your opinion on a couple of commercial offerings.

You tested the FV18 and found it was limited by vent noise at 10 and 12.5 hz.  Rythmik then updated their design to use 4" ports instead of 3.5" ports which should squeeze a little more out of the design.  I am getting ready to purchase new subs and I am down to the FV18 Rev 2 and the new PSA TV1812.  Both are tuned to 12.5 hz and have similar power and cabinet size.  The biggest difference is that in 12.5 hz mode, the FV18 has 2 4" ports and the PSA sub has a single 4" port which is straight with flares to 6".  I suspect that this port is going to limit output around tune with compression and chuffing setting in much earlier.  I'm guessing the output loss could be as much as 4 db.  Does that sound accurate?

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Old thread!

Yes @Charles the dual 4" ports will have a measurable advantage over the single, even with flaring. PSA subwoofers are interesting to me, it feels like they lean heavy on the driver and its motor and squeeze it in a smallish box (not every sub they sell is small) but many of the B&C drivers they use could gain more from a larger, box, however there is something to be said about how capable they can still be in a reasonably size boxed for home applications. Its kinda cool actually. What is the TV1812? I see V1812 with a slot port? that is ~6.1 cubic feet or so while the Rhythmik is closer to 8.8. If that's is accurate, then there is doubt the larger box and port will have a big advantage down low. But I could be wrong about my numbers ... someone should double check.

 

 

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Thanks @Kyle, the TV1812 is actually a new model for PSA that has a similar volume to the FV18, tuned to 12.5hz with a single down-firing 4” port.  I ended up buying a pair of the Rythmik FV18s and they are very potent.  

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5 hours ago, stoneeh said:

Why guess (a simulation will always be a more or less elaborate guesstimate), when you can measure?

Good guessing can save a lot of time and money that would be spent building physical things that don't work.

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In the end though, you have to build & test to be 100% sure.

There are (also empirical, not theoretical) studies / papers on the topic, from the AES for example, that are freely available; the issue has been figured out for decades. The conclusion overall is that you should always use the maximum port size that you can possibly accomodate in a speaker box, because there will always be some compression. There are other concerns like port resonances (only relevant for full range boxes, not subwoofers), or cooling (only relevant for PA subs).. but generally, the larger the port, the better.

Flared ports reduce air turbulence at the port ends, which results in noise. So flared ports reduce noise. They do however hardly or not at all reduce compression, which is a result of an inadequate inner port cross-section area. Port noise and compression are / can be two different things.

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In the end, you absolutely have to build and test.

I generally agree with most of what you say here, but I would not agree that the issue has been figured out for decades.  It'd be great if all we had to worry about was making the flow area big enough, but we often have to balance many competing concerns in the overall design, most especially cabinet size.  And for any given design, there may still be 1 or 2 dB of performance that can be eked out with a tweak here or there.

Of course a lot depends on what you're trying to do.

Right now I'm experimenting with a dirt cheap 12" mid-woofer driver with a kind of whimpy motor compensated for by a very low Mms and soft suspension.  It's got quite a bit of Xmax for what it is though, so it can make some bass in an appropriate cabinet.  That cabinet needs to be relatively big to do it though, such that it's trivial to find a vent with a large enough flow area.  This can make a decent, very affordable living room speaker, but the output density is poor.

In contrast to this consider a Skhorn loaded with two 21" IPALs.  With enough power, it's easy to overload the vents on that thing, and there's no easy way to expand the vents without making the cabinet bigger --- probably a lot bigger.  The Skram, relying on a single 21", has proportionately more flow area than the Skhorn, but even it can be overloaded.  Pretty much any kind of vent-based design in which one's trying to optimize output density is going to require difficult compromise involving flow area.  Often this involves doing stuff like adding 90 degree bends to make the pipe or slot fit into the cabinet, and it's helpful to know how those features affect compression and whether the trade-off is worth it vs. shrinking the flow area to get the length to fit.

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