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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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Ya maybe the saying should be: there is no replacement for [sensitivity] 

 

:)

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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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