45-Degree Corners in Slot Ports: Adjust Port Length? Calculations?

qObsession · Apr 4, 2022

Hi, everybody...

I'm somewhat surprised that I haven't found any good information on this, so I thought I would pose it to you smart folks!

Before posing the question... a quick preface: I'm not a total noob (maybe noob-adjacent), and although I'm not a physicist who can tell you about the more technical aspects of port dynamics (e.g., laminar flow, etc.), I am familiar with their general forms and functions and all that goes into their calculations (trying to stave off questions about why this or that port is better, how to convert one port shape to another, how to calculate port length, etc.).

That said, here's the question (maybe with a few parts, if you'll indulge me):

Let's say you have a 40" port like the one outlined in black below (and the port SHOULD be 40" long)...

(a) When you add 45-degree corners to facilitate air flow (in red on diagram), how do they affect the length of the port? Or do they? Some have suggested that they don't (and that might be true!), but to my pea-brain, if the air volume in that port affects the compliance of the driver, wouldn't changing that volume (even a little bit) have SOME effect on the overall system? I imagine that you would add a wee-bit of length to the port to account for this, but I'm not at all comfortable with that 'conclusion.'

For extra credit...

(b) When you add those 45-degree pieces to the port, how far should they 'intrude' into the port (see blue and '?' on diagram)? All the way to the mid-line (noted in gray)? Somewhere in between? Is there a rationale or calculational justification for one way or another?

And if you really want to make my day...

(c) Is there any value in 'rounding' the corners in the enclosure itself with 45-degree angles like the ones in the port? Sometimes I see it... sometimes I don't. That just something I've always wondered about!

And if you're in a "Holy cow, I'm feeling generous today!" kind of mood...

(d) Looking at this box design, do you see any 'red flags' (not aesthetically, bracing, or how it might be done otherwise)--just any "Dude, you can't do that!" reactions to the overall layout?

So, if you'd like to take a crack at (a), (b), (c) and/or (d), I would greatly appreciate it. Thanks, forum!

mikey7182 · Apr 4, 2022

You wouldn’t want to put 45s in a port like you have in that image, because it changes the width (and thus, port area) of the vent. So for example, if your vent is 3” wide x 12” tall, if you put that 45 in there, you’re likely changing the width to 1.5-2”. If you put a 45, you’ll want to mirror that on the other side of the vent to maintain width/area throughout the vent. As far as figuring length, measure down the center of the vent. 😎

qObsession · Apr 4, 2022

Hey, Mikey. Yeah, I knew vent length was measured down the center... just didn't know if changing the vent volume with the 45s would affect port tuning--and, if it did, whether it was worth adjusting the length to account for it.

Thanks for weighing in!

mikey7182 · Apr 4, 2022

By changing the vent area in the corners, you’re effectively changing the total vent volume and in doing so, also changing the tuning frequency, but it’s not just the volume that determines the tune. The area needs to be consistent through the length of the vent. Having different widths in different spots is also going to increase turbulence as it’s channeling the air into narrower spaces in those parts of the vent. It would be no different than cutting a rectangular piece and placing it against one side of the vent wall, like this:

If you’re wanting to minimize turbulence, you could do a round port with flared ends, and any “bends” could use a 90* rounded elbow.

qObsession · Apr 4, 2022

Yeah... decreasing turbulence was the goal. I'm thinking about kerfing the wood on the upper left-hand corner (of the diagram), maybe extending the port toward the woofer so there will only be one bend.

Does that seem like a decent idea? Any issue with the mouth of the port being that close to the subwoofer?

Thanks!

mikey7182 · Apr 4, 2022

I was going to suggest kerfing actually 😎

I don’t see any issue with the port location. I think the rule of thumb is you want the port at least one “port width” away from the sub. It looks plenty far away in the drawing.

qObsession · Apr 4, 2022

Cool! I'm familiar with the one-port-width 'rule,' but how does that work with a slot port? Is the long side or the short side the 'width' in this case? Thanks!!

mikey7182 · Apr 4, 2022

It’s really port area you want to stay consistent, so height and width should be uniform throughout the entire length of the port. Picture a round port- you want the entire thing to be the same diameter, which is really just height and width. You wouldn’t use a 3” diameter section, then a small 1” diameter section then back to 3”. Same applies for slot ports. So let’s say you are doing a 2cf enclosure and you want 24” of port area. So you make your port 12” x 2” and just maintain that 12” x 2” for the entire length. If you do have a 90 and want to minimize turbulence, just kerf both “sides” or corners of the 90.

qObsession · Apr 4, 2022

All of that makes good sense, mikey. I guess what I was driving at (pardon the pun) is how close this slot port can come to the subwoofer. If it's a 13" x 4" port, how close can any given edge of it come to that 12" subwoofer?

Thanks!

mikey7182 · Apr 4, 2022

I see what you mean. I would say if it’s at least 4” it should be fine. Someone else may have more info than me. You basically don’t want the sub unloading right into the port.

qObsession · Apr 4, 2022

Cool! Thanks, mikey... really appreciate it. This is my first slot-port build, and I feel better now having had this back and forth. Cheers!

45-Degree Corners in Slot Ports: Adjust Port Length? Calculations?

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