Laminar or Turbulent Ports?

[masterp2]

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?

[LineSource]

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.

[aaroncgi]

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

[masterp2]

So would it be best to smooth the flair, while dimpling the narrower port interior?

[aaroncgi]

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]

I just bought the parts at Home Depot and that's how they came

What parts? dimpled?

[aaroncgi]

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

[masterp2]

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?

[markp]

The question is, does turbulent airflow decrease the appearant diameter of the port?

[masterp2]

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.

The question is, does turbulent airflow decrease the appearant diameter of the port?

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.