Air Flow & Bend design in a 6"x6" Square Port - AVS Forum | Home Theater Discussions And Reviews
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Old 04-17-2017, 02:43 PM - Thread Starter
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Air Flow & Bend design in a 6"x6" Square Port

Is Option 1 better than Option 2? If so, how much?



I'm in the process of wrapping up the ports, etc inside the subwoofer cabinet I'm working on in this thread:

Nearfield Bass Cabinet

I've got two square 6"x6" ports in the main volume of the cabinet for one UM18-22 @ around 11.3 cubic feet. The box should be tuned around 16-17Hz so both ports are 50" long and they both have two 90 degree bends.
As you can see below, the ports exit the cab at the bottom corners of the cabinet, they each run along one end of the cabinet, then turn up and run along the back of the cabinet, then turn in and run along the top of the cabinet a short ways before opening up into the cab interior.





My question is about airflow at the 90 degree bends. Most slot ports I see have panels fit at right angles on the outside of the corner and on the inside corner. Some, maybe most, have a roundover on the inside edge of the turn. But most are only 1" to 4" tall and don't bother with any smoothing or filling of the outside corner. I would guess that in most cases it isn't necessary and wouldn't really add anything to the function of the port and specifically the bend in the port. I know air is fluid and has a boundary layer and in most places where there are gaps, holes, troughs or recesses, the air in the recess will sit pretty still and the moving air will flow right over it. I have seen pics of turns like this where the air in the outside edge of the corner has some turbulence, but may not really flow and just acts as a boundary for the air moving around the bend.



Some builds do have a small panel at port turns at 45 degrees to help the air turn in the 90 degree bends.

On my port though, there seems to be a pretty significant recess in the outer corner of the 90deg bend where the air can sit and maybe turbulence could be a problem. Due to the shape and size of my port, I was wondering what would be best.
So my question is, between these two options:
1. With curved piece filling in the outside corner/recess of the turn
2. Without anything in the outside of the corner, just two straight pieces attached at a right angle.
(there is a small 3/4" roundover on the inside the corner)



Is Option 1 better than Option 2 for airflow?
How much better is Option 1 than Option 2? (If it is)
Are there any specific or proven advantages of the one or the other?

I don't want to put in a 45 degree panel; due to the design and build so far, that isn't an option. I'll either go with Option 1, with a curved piece of plastic or wood to help the air turn and flow at the bends. Or I'll go with Option 2, leaving it as is, trusting the fluid character of air to fill the recess and create boundaries for smooth air flow. I've seen many builds which don't address this as an issue, so I'm tempted to leave it as is and not worry about it. But if I can put something in place to improve the design in a noticeable or significant way, I'm happy to do that.

(just wanted to tag a few people who had great comments in previous port and flow threads: @diy speaker guy , @Mark Seaton , @augerpro , @LTD02 , @mtg90 , @notnyt , @head unit
like this one The Perfect Port? that thread and the Harman paper are part of the reason I put in the flanges on the inside ends of my ports)
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Old 04-17-2017, 02:47 PM - Thread Starter
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I did some research on this, but couldn't find much. There is some talk about it in the world of ducting and HVAC systems. Here are some of the analyses and solutions for those situations, including vanes at the 90 degree bend to help the air make a more laminar change of direction to try to reduce turbulence. What do you guys think of that?





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Old 04-17-2017, 02:57 PM
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I'd go with option 1 as long as you're not narrowing the port around the bend. Also remember airflow in a port very quickly changes direction. I don't think #2 is going to be much of an issue unless you're pushing your port velocities too high. You can easily just put a small 45 in the corner as well you don't need it to be that big. Just make sure that the corner to the 45 is no shorter than 6" at any point.
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Old 04-17-2017, 03:29 PM
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Option 2. It's easier, and the low frequencies that excite the port don't require any smoothing through the bend. It also will give a somewhat lower Fb, due to the extra mass of air in the corner. The HVAC example doesn't translate to what a port does. It does to a folded horn that works into the midrange, but that's a horse of a totally different color.

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Old 04-17-2017, 03:47 PM
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The ends of the port are far more significant than the bend, btw.
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Old 04-17-2017, 04:01 PM
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For airflow and HVAC applications, option 1 is much better. We usually try to avoid option 2, unless there are turning vanes.

Whether or not it makes a difference in an audio application, I'll leave that for the more experienced audio guys.
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Old 04-17-2017, 09:34 PM - Thread Starter
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Thanks for the feedback guys. I'm probably over-thinking it a bit (doesn't always hurt). I do have the inside of the corners rounded over and I think that's the most important thing. I think the outer air will just sit there and not cause a significant problem.

I may still put the curved bits in there for peace of mind, or I may just keep it simple without.

I will have 3/4" round overs on the ends too with flanges in the inside.

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Old 04-18-2017, 05:35 AM
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Quote:
Originally Posted by Brazle View Post
I do have the inside of the corners rounded over and I think that's the most important thing.
If the radius is a significant percentage of a wavelength it will have an effect. Figure out the wavelength at the tuning frequency and you'll see why there's no point in doing that.

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Old 04-18-2017, 07:50 AM
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Quote:
Originally Posted by Bill Fitzmaurice View Post
If the radius is a significant percentage of a wavelength it will have an effect. Figure out the wavelength at the tuning frequency and you'll see why there's no point in doing that.
I believe you will mostly find Bill's claims here right on. I did some experimentation with large ported boxes (Not LLT, these were all tuned around 20hz) and found that the bend in the port didn't make a huge difference. The ends made a big difference, but I also found that just making the ports really large was the best option. I had put significant effort into two boxes which had fairly small slot ports (2.5" by 18" profile) with 1" flared ends and found they had horrible port chuffing. I tried experimenting with lining the ports with 30ppi reticulated foam, lowered tuning and tuning Q but made port chuffing worse. In the end none of my fancy ideas panned out and the only thing that did work were ports larger than strictly needed and/or significant flaring of the ends (With the flaired design my only flared slot port with a noticable reduction in port chuffing made use of a 3" radius router bit and tons of MDF). I had tons of measurements for this, but this was many laptops ago, so I'm not sure where they are anymore. I did post them on DIYaudio so if I can find them again, I'll repost.

I've long wondered if there was any value in smoothing the corners airflow with turning vanes but never had the desire to go through the effort of building them.

I'm curious your thoughts Bill F., if I was building a bandpass design, would you be more inclined to think that some efforts to smooth airflow is of value? The biggest negative of all my bandpass builds, including the ones built for me by Dr. Geddes has been port noise. None have been as quiet as I would have liked at their higher output levels. This is more where I've wondered about spending some time on the port airflow, in the front chamber/higher tune port.
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Old 04-18-2017, 11:34 AM
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Originally Posted by Mpoes12 View Post
I believe you will mostly find Bill's claims here right on. I did some experimentation with large ported boxes (Not LLT, these were all tuned around 20hz) and found that the bend in the port didn't make a huge difference. The ends made a big difference, but I also found that just making the ports really large was the best option. I had put significant effort into two boxes which had fairly small slot ports (2.5" by 18" profile) with 1" flared ends and found they had horrible port chuffing. I tried experimenting with lining the ports with 30ppi reticulated foam, lowered tuning and tuning Q but made port chuffing worse. In the end none of my fancy ideas panned out and the only thing that did work were ports larger than strictly needed and/or significant flaring of the ends (With the flaired design my only flared slot port with a noticable reduction in port chuffing made use of a 3" radius router bit and tons of MDF). I had tons of measurements for this, but this was many laptops ago, so I'm not sure where they are anymore. I did post them on DIYaudio so if I can find them again, I'll repost.

I've long wondered if there was any value in smoothing the corners airflow with turning vanes but never had the desire to go through the effort of building them.

I'm curious your thoughts Bill F., if I was building a bandpass design, would you be more inclined to think that some efforts to smooth airflow is of value? The biggest negative of all my bandpass builds, including the ones built for me by Dr. Geddes has been port noise. None have been as quiet as I would have liked at their higher output levels. This is more where I've wondered about spending some time on the port airflow, in the front chamber/higher tune port.

I have built a couple of bandpass designs, both single reflex and dual reflex and used passive radiators instead of ports ( well for the low tuned chamber in dual reflex) ala Geddes. Really liked the performance and reduction in size.

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Old 04-18-2017, 12:03 PM
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Originally Posted by cubdenno View Post
I have built a couple of bandpass designs, both single reflex and dual reflex and used passive radiators instead of ports ( well for the low tuned chamber in dual reflex) ala Geddes. Really liked the performance and reduction in size.

I had two concerns with PR'S that maybe you can address. First is if the response looks ok. A bandpass design falls off more rapidly below the resonance of the PR, So what does that look like in the response of the high tuned portion of the box (front chamber).

Second is if the PR'S are noiseless. I worries they might contribute mechanical noises of their own.

Sorry to hijack the thread, feel free to PM me instead.


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Old 04-18-2017, 12:03 PM
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Why not just use a straight piece at an angle to improve the situation like in a horn without going to a lot of trouble to radius it?

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Old 04-18-2017, 03:52 PM - Thread Starter
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Quote:
Originally Posted by Bill Fitzmaurice View Post
If the radius is a significant percentage of a wavelength it will have an effect. Figure out the wavelength at the tuning frequency and you'll see why there's no point in doing that.
So you're saying because the wavelengths are so much longer than the radius, rounding over the edges won't make any difference? I find it hard to believe there's no point in smoothing airflow through the port. It seems any sharp edges in the port make it more likely to cause port noise from air crossing a sharp boundary.

I can see how it can be really hard to model and predict and even observe exactly how air behaves in a port since the air is moving back and forth, in and out so fast. There is no steady state especially with constant variance in noise/sound from movies/music.

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Old 04-18-2017, 04:17 PM - Thread Starter
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Quote:
Originally Posted by Mpoes12 View Post
I believe you will mostly find Bill's claims here right on. I did some experimentation with large ported boxes (Not LLT, these were all tuned around 20hz) and found that the bend in the port didn't make a huge difference. The ends made a big difference, but I also found that just making the ports really large was the best option. I had put significant effort into two boxes which had fairly small slot ports (2.5" by 18" profile) with 1" flared ends and found they had horrible port chuffing. I tried experimenting with lining the ports with 30ppi reticulated foam, lowered tuning and tuning Q but made port chuffing worse. In the end none of my fancy ideas panned out and the only thing that did work were ports larger than strictly needed and/or significant flaring of the ends (With the flaired design my only flared slot port with a noticable reduction in port chuffing made use of a 3" radius router bit and tons of MDF). I had tons of measurements for this, but this was many laptops ago, so I'm not sure where they are anymore. I did post them on DIYaudio so if I can find them again, I'll repost.

I've long wondered if there was any value in smoothing the corners airflow with turning vanes but never had the desire to go through the effort of building them.

I'm curious your thoughts Bill F., if I was building a bandpass design, would you be more inclined to think that some efforts to smooth airflow is of value? The biggest negative of all my bandpass builds, including the ones built for me by Dr. Geddes has been port noise. None have been as quiet as I would have liked at their higher output levels. This is more where I've wondered about spending some time on the port airflow, in the front chamber/higher tune port.
Thanks for the info, @Mpoes12 . I tried to make my ports as big possible while keeping them small enough to have some decent port velocity for tactile response. Total cross area of two 6x6 ports is 72 sq in. (vs 45 sq in for 2.5x18" port like the one you made). Port velocity should be under 15m/s 99% of the time. The square shape should flow well with decent area to boundary ratio. I'm hoping it'll be OK for port noise. If not ... well I'll come up with a solution then.

Also, what username did you use over on DIYaudio.com ? I could try looking for some of those posts. I'd be really interested in that info with your measurements.

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Old 04-18-2017, 04:19 PM - Thread Starter
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Quote:
Originally Posted by Vince_B View Post
Why not just use a straight piece at an angle to improve the situation like in a horn without going to a lot of trouble to radius it?

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That might be a good idea, but the box and ports are pretty much built now. I was just wondering about adding in the radius'd piece.
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Old 04-18-2017, 07:14 PM
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Originally Posted by Brazle View Post
So you're saying because the wavelengths are so much longer than the radius, rounding over the edges won't make any difference?
That's correct.
Quote:
I find it hard to believe there's no point in smoothing airflow through the port. It seems any sharp edges in the port make it more likely to cause port noise from air crossing a sharp boundary.
The only places that occurs is at the entrance and exit of the port, and it doesn't affect the low frequencies, it affects the friction sourced higher frequencies that are short enough for it to happen. It's not unlike rounding over cab edges to smooth diffraction, where it takes at least an inch and a half radius to be worthwhile, and then it only affects high frequencies.
Quote:
Why not just use a straight piece at an angle to improve the situation like in a horn
The action of angled reflectors in folded horn bends is totally different, mainly having to do with preventing a cancellation notch where the bend to bend distance is 1/4 wavelength.
Quote:
I'm curious your thoughts Bill F., if I was building a bandpass design, would you be more inclined to think that some efforts to smooth airflow is of value? The biggest negative of all my bandpass builds, including the ones built for me by Dr. Geddes has been port noise.
If the port velocity is less than 20ms or so at maximum power then the chance of port noise is slight. The problem with bandpasses is getting sufficient port cross section area to keep the velocity down without a monstrous box to house the long ports required.

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Old 04-18-2017, 09:05 PM - Thread Starter
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Originally Posted by Bill Fitzmaurice View Post
That's correct. The only places that occurs is at the entrance and exit of the port, and it doesn't affect the low frequencies, it affects the friction sourced higher frequencies that are short enough for it to happen. It's not unlike rounding over cab edges to smooth diffraction, where it takes at least an inch and a half radius to be worthwhile, and then it only affects high frequencies.
Thanks for the input Bill and sorry if my first response was a bit terse. I just like having a bit more of an explanation, so I appreciate the additional info.

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Old Yesterday, 07:55 AM
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I don't normally disagree with Bill, but I'm not sure the literature like the popular Harman paper on ports supports the idea the wavelengths are too big for relatively small shape changes to have an impact on port performance. IIRC there was a significant difference at low to medium SPL in port output and harmonic distortion depending on port flare shape. Also the poor performance of the "rough wall" using 1mm beads glued to the wall definitely suggests wavelength doesn't have much to do with the problem. This also implies - but have no evidence to prove - that a sharp change in direction will have an adverse impact. Now as with most things port related, if the port is big enough - oversize really - these problems can be minimized to practically zilch.

On the related topic of edge treatments you definitely don't need huge roundovers or chamfer to have a positive impact. Here is a test I did to verify a baffle sim of a square edge versus 3/4" chamfer, I think using the CSS LD22 tweeter (green is chamfer, red is square):
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Old Yesterday, 08:13 AM
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I'm not sure the literature like the popular Harman paper on ports supports the idea the wavelengths are too big for relatively small shape changes to have an impact on port performance. IIRC there was a significant difference at low to medium SPL in port output and harmonic distortion depending on port flare shape.
I wasn't addressing the subject of port flares. On that subject, what flares do to reduce chuffing is quite simple. The expansion of the flare reduces the port velocity at the point where most of the chuffing is created, at the port exit. It also can increase port output by reducing turbulence where the port air mass meets the outside air.

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Old Yesterday, 08:39 AM
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Isn't chuffing caused by eddy currents or turbulence? If so wouldn't any sharp-ish change in shape be expected to generate at least some eddy currents or turbulence?
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Old Yesterday, 09:15 AM
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Thinking about it more, I guess my main objection to wavelength being involved is that I don't believe sound of the fundamental tone is travelling through the port as a wave. If I understand correctly for the fundamental tone there is no compression/rarefaction happening in the port. The air moves as a solid mass. The compression/rarefaction generating a wave happens at the surface of this "piston" where it meets the surrounding air. Now there is compression/rarefaction (and thus a wave) being created in the port, that is what the port resonances are. Similarly this is why a passive radiator works as a port: a piston creates a wave in the surrrounding air, which then travels to our ears. Also we wouldn't say the sound moves through the diaphragm of the passive radiator (or any normal speaker) as a wave when reproducing music. The only time this happens is at diaphragm resonance - in other words breakup.

So I guess it doesn't seem to me to be an acoustic problem. It is a fluid flow problem. Anyway that's my .02!
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Old Yesterday, 01:05 PM - Thread Starter
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Quote:
Originally Posted by augerpro View Post
... This also implies - but have no evidence to prove - that a sharp change in direction will have an adverse impact.

On the related topic of edge treatments you definitely don't need huge roundovers or chamfer to have a positive impact.
Thanks for that data.

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Originally Posted by Bill Fitzmaurice View Post
I wasn't addressing the subject of port flares. On that subject, what flares do to reduce chuffing is quite simple. The expansion of the flare reduces the port velocity at the point where most of the chuffing is created, at the port exit. It also can increase port output by reducing turbulence where the port air mass meets the outside air.
The rest of auger's post addresses my concern with sharp changes causing additional chuffing or port noise. He's not just talking about port flares. And I guess that brings me to my internal turmoil, as to whether or not making changes to the bends will improve any port noise I might be worried about, or if I need to look primarily at the port entrance and exit. This thread is encouraging me to focus on the latter.

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Isn't chuffing caused by eddy currents or turbulence? If so wouldn't any sharp-ish change in shape be expected to generate at least some eddy currents or turbulence?
Good question. A lot of discussion I have seen is about how to reduce chuffing or port noise. The very specific physical nature and cause of chuffing seems debatable. Fast air meets slow air? Air flow past a hard surface with insufficient contouring (port flaring)? I think of the blowing over a bottle effect, it seems to me that much port noise sounds a lot like that.

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Originally Posted by augerpro View Post
...

So I guess it doesn't seem to me to be an acoustic problem. It is a fluid flow problem. Anyway that's my .02!
Again, I would tend to agree with this, and that was sort of the premise of my post and thinking behind this approach.

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Old Yesterday, 01:13 PM
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Don't take my word for it, I might be full of s#@t! Attached is the Harman port study.
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Old Yesterday, 02:18 PM - Thread Starter
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Originally Posted by augerpro View Post
Don't take my word for it, I might be full of s#@t! Attached is the Harman port study.
Yes, that is the Harman white paper I've used as reference for much of my thoughts and designs. I should have posted a link to it at the beginning of this thread. Mea culpa.

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Old Yesterday, 03:31 PM
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Don't take my word for it, I might be full of s#@t! Attached is the Harman port study.
That study is very accurate with respect to what happens at the port entrance and exit, but it doesn't in any way address the OP's question. I haven't done a lot of research with respect to using reflectors in a port bend, just enough to conclude what I stated in my first post. I've done more research than any other source that I'm aware of with respect to reflectors in the bends of folded horns, and that research reinforces my conclusions with respect to the bends in ports. At higher frequencies the bend configuration has a major effect, at low frequencies it has none, other than the issue of when a bend to bend distance is 1/4 wavelength.
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Bill Fitzmaurice Loudspeaker Design

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Old Today, 01:36 PM
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Option 2. It's easier, and the low frequencies that excite the port don't require any smoothing through the bend...The HVAC example doesn't translate to what a port does.

I strongly disagree.

Helmholz resonance is purely mechanical (poirt air is mass, box air is spring), not acoustic, so wavelength is irrelevant.

Air velocities are high, and many loudspeaker texts show examples of port compression, and that's with straight ports.

IMO HVAC principles are relevant.

That's not to say it will make a material difference in your particular use conditions.

Noah

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