We all know that laminar exit flow is key to reducing noise. Question is with respect to the meat of the port, the interior, should it be treated for turbulent flow (kill the boundary layer) for more effectiveness? Assume the port is located only a few feet from the listeners ear, so port noise must be non-existant. And does does turbulent flow or laminar flow maximize output?
CONTROLLED TURBULENCE
You want the maximum air volume, so you want to create small circular vortecies at the port-air interface that reduce friction for the center air mass. Lower friction also means lower port noise...friction=vibration=noise=heat=inefficiency.
If golf balls did not have a dimpled surface, even Tiger Woods would have difficulty reaching 200 metres with his best drive. Dimples improve the way the air flows over the surface of any object. In the case of reflex ports, they offer a significant improvement over simply flaring the port ends in reducing air flow turbulence at each end of the port; so you get less chuffing noise and less compression at high sound levels.
You want the maximum air volume, so you want to create small circular vortecies at the port-air interface that reduce friction for the center air mass. Lower friction also means lower port noise...friction=vibration=noise=heat=inefficiency.
If golf balls did not have a dimpled surface, even Tiger Woods would have difficulty reaching 200 metres with his best drive. Dimples improve the way the air flows over the surface of any object. In the case of reflex ports, they offer a significant improvement over simply flaring the port ends in reducing air flow turbulence at each end of the port; so you get less chuffing noise and less compression at high sound levels.
Apropos, just made a new port out of 2" ABS.
I noticed that the inner wall of the pipe is not smooth, as is my 1.5" and 3" ABS. It's as you describe, like the surface of a golf ball, though perhaps not quite so dramatic. I got a good picture of it, which I'll have up on my web site in a day or two. I really doubt it helps much, plus the exit of the port flares from 2" to 3", but who knows? I can't hear port noise at any output level except using test tones. This is only using a 6.5" woofer with 4mm of Xmax, though, tuned to the low 40s. 🙂
Aaron Gilbert
I noticed that the inner wall of the pipe is not smooth, as is my 1.5" and 3" ABS. It's as you describe, like the surface of a golf ball, though perhaps not quite so dramatic. I got a good picture of it, which I'll have up on my web site in a day or two. I really doubt it helps much, plus the exit of the port flares from 2" to 3", but who knows? I can't hear port noise at any output level except using test tones. This is only using a 6.5" woofer with 4mm of Xmax, though, tuned to the low 40s. 🙂
Aaron Gilbert
Beats me!
masterp2,
Sorry, I really have no idea! It would be cool if that were true, seeing that's how my speakers worked out, but I sure didn't plan it that way. 🙂 I just bought the parts at Home Depot and that's how they came. I don't know how easy it would be to add a dimpled texture to the inside of a smooth port. I know I have seen people completelyl fill a port with drinking straws, but I forget the theory behind the improvements claimed for it.
I can only speculate that there must be some reason the inside of my ABS pipe is textured the way it is, and one would certainly expect that it's not there to INCREASE friction.
Aaron Gilbert
masterp2,
Sorry, I really have no idea! It would be cool if that were true, seeing that's how my speakers worked out, but I sure didn't plan it that way. 🙂 I just bought the parts at Home Depot and that's how they came. I don't know how easy it would be to add a dimpled texture to the inside of a smooth port. I know I have seen people completelyl fill a port with drinking straws, but I forget the theory behind the improvements claimed for it.
I can only speculate that there must be some reason the inside of my ABS pipe is textured the way it is, and one would certainly expect that it's not there to INCREASE friction.
Aaron Gilbert
Yes, the pipe came with a non smooth interior.
I have purchased the same kind of pipe in 1.5" diameter and 3" diameter, and it's as smooth on the inside as on the outside. Not so this 2". The picture below should explain what I mean, and you can also see the flare, which is nothing but a 2" to 3" coupler. Total port cost is about $2.50, though it doesn't come with a fancy flange like those Precision flared ports. Oh, this was a 10' length of pipe, cost about $4.00. That will last a lot of projects. And yes, that is the back of the woofer you are looking at in the port, which is about 1" away from the end of the port. I know it's probably heretical, but so far I don't hear anything bad, and the speakers are deep enough already without moving the port out more.
😎
Aaron Gilbert
I have purchased the same kind of pipe in 1.5" diameter and 3" diameter, and it's as smooth on the inside as on the outside. Not so this 2". The picture below should explain what I mean, and you can also see the flare, which is nothing but a 2" to 3" coupler. Total port cost is about $2.50, though it doesn't come with a fancy flange like those Precision flared ports. Oh, this was a 10' length of pipe, cost about $4.00. That will last a lot of projects. And yes, that is the back of the woofer you are looking at in the port, which is about 1" away from the end of the port. I know it's probably heretical, but so far I don't hear anything bad, and the speakers are deep enough already without moving the port out more.
😎
Aaron Gilbert
Attachments
That is creative, A+. Suggestion: Where the ID meets the coupler (flair), sand down the ID to make a seamless mating without the ridge. This would eliminate a source for turbulent noise.
So this stuff comes in 3"? Other sizes? Sched 40? I could use this in a 6". Is it called drain pipe, or what?
So this stuff comes in 3"? Other sizes? Sched 40? I could use this in a 6". Is it called drain pipe, or what?
OK, let's dive into the meat of airflow. Fluid flow for dummies, beginning with dimples. I am reaching way back into some Chem E fluid dynamics courses, so here goes as best as I can remember. Manufacturers of cars, aircraft, and even golfballs introduce methods to actually create turbulence on the surface. Many times this is to reduce noise. (why would you introduce turbulence to reduce noise? hmmm)
Fact: low speed laminar airflow is the quietest airflow available. It is characterized by a velocity gradient where the velocity is highest in the center of the port and zero at the port boundary. (if you are not educated in this, please just believe it) It is also the most "viscous" of flows at higher flows.
However, rarely can a port (or golfball) find itself always surrounded by low velocity, laminar airflow. A port may (will) experience both types, lam and turb velocities, depending on output demand (velocity).
Friction losses are at their highest during laminar flow at the highest speed right before turbulent flow occurs at the boundary. I believe this was called boundary layer separation. After the boundary layer seperates, turbulent flow characteristics predominate, and this includes lower (viscous) friction losses.
so if a golf ball (or port) is going to experience turbulent flow most of the time anyway, why dimple it. Answer: to create boundary layer separation sooner, at a lower velocity, reducing friction considerably, sooner, thereby providing more airtime without the friction characteristics of lam flow. = longer drive.
So what about turbulence causing port noise? Well it does. Open your mouth and blow, then close it down to straw diameter and blow. Velocity=turbulence. That is why flairs work. Gradually increasing cross-section, slows velocity, and re-introduces quiet laminar flow. And if that it done before it leaves the port, no noise, the noise has disappeared (lam flow) when it leaves the port and heads for your ears.
But what about those dimples inside the port, aren't they noisy? (Does a dimple make a noise if there is noone in the port to here it fall?) Well here is where I am foggy, but I believe this. Turbulence inside the port will not be heard, if the air is laminar by the time it exits, faciltated by flairs (and polk powerport).
That 17 ft/sec that is thrown around, I believe is representative of the transitional velocity, lam to turb, and back. stay below 17 ft/sec (a velocity equivalent of the reynolds number for air) and you don't have to worry about turbulence at all, but it MAY (I'm not sure) reduce port output, because friction losses equal reduced output. I think people are refering to the term as compression loss. So here is where I propose that there is an analogy; dimpled port is to dimpled golfball, as port output is to longer drive. But since only a smooth surface can support lam flow (quiet), the flairs must be smooth
Correct me if I'm wrong, if you know for a fact that I am.
Answer: well hopefully with the above expalnation, you now see that the question really is an invalid question. Only diameter changes diameter. Are you refering to a calculated equivalent diam from a box program? Eq Diam is a little misunderstood, it is the actual diameter at the port exit, or equivalent if the port is not circular. Turb airflow is noisy if it exits the port that way. Increasing diameter is one way to keep that from happening.
To those who have a fresher education on this feel free to jump in.
Fact: low speed laminar airflow is the quietest airflow available. It is characterized by a velocity gradient where the velocity is highest in the center of the port and zero at the port boundary. (if you are not educated in this, please just believe it) It is also the most "viscous" of flows at higher flows.
However, rarely can a port (or golfball) find itself always surrounded by low velocity, laminar airflow. A port may (will) experience both types, lam and turb velocities, depending on output demand (velocity).
Friction losses are at their highest during laminar flow at the highest speed right before turbulent flow occurs at the boundary. I believe this was called boundary layer separation. After the boundary layer seperates, turbulent flow characteristics predominate, and this includes lower (viscous) friction losses.
so if a golf ball (or port) is going to experience turbulent flow most of the time anyway, why dimple it. Answer: to create boundary layer separation sooner, at a lower velocity, reducing friction considerably, sooner, thereby providing more airtime without the friction characteristics of lam flow. = longer drive.
So what about turbulence causing port noise? Well it does. Open your mouth and blow, then close it down to straw diameter and blow. Velocity=turbulence. That is why flairs work. Gradually increasing cross-section, slows velocity, and re-introduces quiet laminar flow. And if that it done before it leaves the port, no noise, the noise has disappeared (lam flow) when it leaves the port and heads for your ears.
But what about those dimples inside the port, aren't they noisy? (Does a dimple make a noise if there is noone in the port to here it fall?) Well here is where I am foggy, but I believe this. Turbulence inside the port will not be heard, if the air is laminar by the time it exits, faciltated by flairs (and polk powerport).
That 17 ft/sec that is thrown around, I believe is representative of the transitional velocity, lam to turb, and back. stay below 17 ft/sec (a velocity equivalent of the reynolds number for air) and you don't have to worry about turbulence at all, but it MAY (I'm not sure) reduce port output, because friction losses equal reduced output. I think people are refering to the term as compression loss. So here is where I propose that there is an analogy; dimpled port is to dimpled golfball, as port output is to longer drive. But since only a smooth surface can support lam flow (quiet), the flairs must be smooth
Correct me if I'm wrong, if you know for a fact that I am.
Answer: well hopefully with the above expalnation, you now see that the question really is an invalid question. Only diameter changes diameter. Are you refering to a calculated equivalent diam from a box program? Eq Diam is a little misunderstood, it is the actual diameter at the port exit, or equivalent if the port is not circular. Turb airflow is noisy if it exits the port that way. Increasing diameter is one way to keep that from happening.
To those who have a fresher education on this feel free to jump in.
smooth only at outer edge
Michael << But since only a smooth surface can support lam flow (quiet), the flairs must be smooth
Correct me if I'm wrong, if you know for a fact that I am.>>
I modelled golf ball dimples in college..no jokes please....
The outer edge of the interior and exterior ports should be smooth for laminar flow, then small rounded-edge holes in slowly decreasing diameter, slowly increasing depth and slowly increasing number should be put into the port and tube to start up a controlled turbulent flow at the air-port interface. If the dimples are correctly sized and placed, small circular turbulents will be generated that act like ball bearings and increase the volume of laminar flowing air through the center of the port.
If you are a perfectionist, then don't put any bends in the tube. Have identical interior and exterior ports, and mount the interior port on an internal strut baffle. Consider putting the port on the bottom of a raised cabinet as this often allows the longest straight tube, takes advantage of gravity on the air mass, keeps rear cone midrange sound from having a direct path to the ear, and allows 270 or 360 degree propagation of the long wavelengths where it can get the largest and most uniform floor gain.
Michael << But since only a smooth surface can support lam flow (quiet), the flairs must be smooth
Correct me if I'm wrong, if you know for a fact that I am.>>
I modelled golf ball dimples in college..no jokes please....
The outer edge of the interior and exterior ports should be smooth for laminar flow, then small rounded-edge holes in slowly decreasing diameter, slowly increasing depth and slowly increasing number should be put into the port and tube to start up a controlled turbulent flow at the air-port interface. If the dimples are correctly sized and placed, small circular turbulents will be generated that act like ball bearings and increase the volume of laminar flowing air through the center of the port.
If you are a perfectionist, then don't put any bends in the tube. Have identical interior and exterior ports, and mount the interior port on an internal strut baffle. Consider putting the port on the bottom of a raised cabinet as this often allows the longest straight tube, takes advantage of gravity on the air mass, keeps rear cone midrange sound from having a direct path to the ear, and allows 270 or 360 degree propagation of the long wavelengths where it can get the largest and most uniform floor gain.
Forget about the dimples, they are a gimick as far as ports are concerned. By far the most important factor for reducing noise and distortion in ports is the shape of the inlet/outlet. You want a nice curved shape, not an abrupt cutoff. The curved shape leads to a smaller velocity gradient (a smooth transition between the high air velocity in the port and the low velocity outside the port). This increases the maximum velocity through the port before you start getting vortex shreading at the entry/exit.
Check out a paper in the JAES in 2002, I believe, written by some JBL engineers. They reviewed just about every study on ports there is, including dimple studies IIRC. Excellent paper.
John
Check out a paper in the JAES in 2002, I believe, written by some JBL engineers. They reviewed just about every study on ports there is, including dimple studies IIRC. Excellent paper.
John
Do you mean "flairs"?
Won't EQ take care of this? Is there another way to accomplish this when ports exit the top of the cabinet?
A couple of things to consider. First, the Polk PowerPort technology was created years (a decade?) ago, long before todays ultra-high excursion drivers were developed. Second, Polk has never used ultra-high excursion drivers like the HE-15, to my recollection.
The PowerPort design forces the air to make one or two 90 degree turns. This in Jon and my opinion could be a recipie for chuffing with ultra-high excursion drivers like the HE-15.
A clear benefit and prime goal of the PowerPort concept is to reduce the required port volume for a given box tuning- but we haven't seen any hard data comparing port velocity issues and the onset of port compression or chuffing.
Also one should note that the PowerPort doc says that a PowerPort "should" give performance similar to a conventional port. They do not specify if that performance is similar to a straight or flared port. Nor do they provide any actual comparisons...
If we were in your shoes, the way we'd approach this issue is with the construction of prototypes that provide a direct comparison between a PP box and one using the AeroPort dual flares. We had a similar situation in deciding between the original Stryke cube design with multiple PRs, vs the AS-15 design- so we built and tested both. This kind of construction and comparison is pretty much an inevitable part (if time consuming) of the design, development, and evaluation processs. At least, the acoustical proto's don't have to be pretty. ;^)
BTW the biggest area of concern for chuffing (besides too small a port diameter) is the terminus between the port and the baffle. That's where one should focus their design energy, not fixating on the texture inside a PVC pipe.
Finally with some 80yrs combined speaker building experience, we respectfully suggest that no port exists should or could be placed in close proximity to the listener's ears ..... while we've designed and built nearfield monitors, a near field sub is an interesting challenge- any out of band elements will be much more audible, since there probably won't be much masking from program. Boundary propagation aids deep bass in a conventional room, with distance the out of band products fall off quickly. Time alignment to the mains should be interesting, also.
Regards
ThomasW
The PowerPort design forces the air to make one or two 90 degree turns. This in Jon and my opinion could be a recipie for chuffing with ultra-high excursion drivers like the HE-15.
A clear benefit and prime goal of the PowerPort concept is to reduce the required port volume for a given box tuning- but we haven't seen any hard data comparing port velocity issues and the onset of port compression or chuffing.
Also one should note that the PowerPort doc says that a PowerPort "should" give performance similar to a conventional port. They do not specify if that performance is similar to a straight or flared port. Nor do they provide any actual comparisons...
If we were in your shoes, the way we'd approach this issue is with the construction of prototypes that provide a direct comparison between a PP box and one using the AeroPort dual flares. We had a similar situation in deciding between the original Stryke cube design with multiple PRs, vs the AS-15 design- so we built and tested both. This kind of construction and comparison is pretty much an inevitable part (if time consuming) of the design, development, and evaluation processs. At least, the acoustical proto's don't have to be pretty. ;^)
BTW the biggest area of concern for chuffing (besides too small a port diameter) is the terminus between the port and the baffle. That's where one should focus their design energy, not fixating on the texture inside a PVC pipe.
Finally with some 80yrs combined speaker building experience, we respectfully suggest that no port exists should or could be placed in close proximity to the listener's ears ..... while we've designed and built nearfield monitors, a near field sub is an interesting challenge- any out of band elements will be much more audible, since there probably won't be much masking from program. Boundary propagation aids deep bass in a conventional room, with distance the out of band products fall off quickly. Time alignment to the mains should be interesting, also.
Regards
ThomasW
Hello ThomasW,
I definately don't think the texture of the ports was worth any effort, for what little theoretical help it might provide, so this was a fun discussion on fluids.
I am more focused on positioning the ports (compared to listening position) and port termination. Sorry to mix threads here, but I know you are familiar with what I am doing in the other thread. Since my termination area is the equivalent of 2-7" round ports, I don't think turbulence noise (chuffing) is an issue if close tolerances are applied on the 2 powerports.
Could you explain? Do you mean the smooth transition from flair to baffle?
I may just redesign a little so I can turn the box over. In your opinion, with a sub behind the listeners sofa, would bottom firing ports reduce it's presence/directionality?
I definately don't think the texture of the ports was worth any effort, for what little theoretical help it might provide, so this was a fun discussion on fluids.
I am more focused on positioning the ports (compared to listening position) and port termination. Sorry to mix threads here, but I know you are familiar with what I am doing in the other thread. Since my termination area is the equivalent of 2-7" round ports, I don't think turbulence noise (chuffing) is an issue if close tolerances are applied on the 2 powerports.
Could you explain? Do you mean the smooth transition from flair to baffle?
I may just redesign a little so I can turn the box over. In your opinion, with a sub behind the listeners sofa, would bottom firing ports reduce it's presence/directionality?
Yes smoothest transition possible between the port and the baffle. Deep flares to this best.
Yeap probably the best option. One must avoid having the port outlet itself too close to the floor boundry (too close creates additional resistive load), mininium distance is port/flare diameter. More space is better up to a point, just use common sense...
Thanks ThomasW,
My port design has a 3.5" OD pipe inside of a 6" ID pipe (times 2). When the air exits, it is side firing, along the baffles plane, and all of it emminates from a 1.25" wide, 10" diameter, circumferential slot, so to speak, that has the equivalent output area of a 7" conventional port. 2 of these. Question: Would the proximity to the floor be an issue, since it is not "resisted" further by a horizontal boundary (like a downfiring port would encounter firing into the floor)?
If I have not been clear, I will re-state or clarify.
My port design has a 3.5" OD pipe inside of a 6" ID pipe (times 2). When the air exits, it is side firing, along the baffles plane, and all of it emminates from a 1.25" wide, 10" diameter, circumferential slot, so to speak, that has the equivalent output area of a 7" conventional port. 2 of these. Question: Would the proximity to the floor be an issue, since it is not "resisted" further by a horizontal boundary (like a downfiring port would encounter firing into the floor)?
If I have not been clear, I will re-state or clarify.
I've provided the principles for a standard port. Since I have no hands on experience with a PowerPort design, I don't know what will occur. And speculation isn't something I'm good at .... :^)
Well neither of those are breaking any new ground....
In the end it's all about deciding on which compromises you're willing to live with. There are problems with ports, and there are problems with PRs.
Having built one of the PRed Stryke cubes, and having not been satisfied with it's performance, we created the AS-15.
Now in a perfect world I'd have used a 8"-10" dia port. But the length would have made the box absolutely huge. That's the primary reason we went with the 6" AeroPort dual flares.
Anther option for your design would be to remove the PowerPorts, and install a side firing 8" dia port. Use your router bit to create flares. Or you might also consider using a 90degree elbow and mount the port outside the box, given that everything's hidden by the sofa.
In the end it's all about deciding on which compromises you're willing to live with. There are problems with ports, and there are problems with PRs.
Having built one of the PRed Stryke cubes, and having not been satisfied with it's performance, we created the AS-15.
Now in a perfect world I'd have used a 8"-10" dia port. But the length would have made the box absolutely huge. That's the primary reason we went with the 6" AeroPort dual flares.
Anther option for your design would be to remove the PowerPorts, and install a side firing 8" dia port. Use your router bit to create flares. Or you might also consider using a 90degree elbow and mount the port outside the box, given that everything's hidden by the sofa.
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