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! 🙂
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! 🙂
You're only 'improving' the vent's spurious HF BW based on ~ the corner plate area, i.e. acoustically tiny relative to the vent's BW, so distortion I've often needed to quell by critically damping it, ergo recommend not using them as the excess net volume [Vb] acts as an acoustic low pass filter, which will damp any spurious vent HF distortion and calc its axial path length based on 71.4.
Ditto if a box regardless of aspect ratio, so need large offset plates to randomize its eigenmodes same as one does with rooms. i.e. can easily be optimized using a room mode calculator, though least 'offensive' overall for bracing = red.
Ditto if a box regardless of aspect ratio, so need large offset plates to randomize its eigenmodes same as one does with rooms. i.e. can easily be optimized using a room mode calculator, though least 'offensive' overall for bracing = red.
OK, but why two corners? If this diagram is a plan, then you only need one bend. Avoid half your uncertainty and construction labor.
Ideally your "rounding" would be ROUND. Then the centerline is clear and the bend-length is easily calculated. Cheating with flat stock, I would eyeball the flat to the average of the quarter-circle (so more than your red, less than your grey).
Most such acoustics are not EXACT. Won't fail if 39" or 41". If you are sure high precision is needed, and you can't prove all assumptions (and as-built box dimensions and as-bought speaker parameters), put a movable part in there and adjust on test.
My depraved mind says all woofer precision is lost in real rooms with +/-10dB bass peaks/dips, but maybe your room is better.
Hey, PRR... very helpful. First, I might share your depraved mind: With so many variables in play, won't some minor variations on things like this just 'come out in the wash'? haha
Ideally, yeah, those corners would be round (thanks for the artistic rendering, by the way!). Other than breaking out some fiberglass, I'm not sure how to do that feasibly. I guess I could kerf the wood. Huh... just thought of that. Thanks!
As for eliminating the second bend as you suggested, I definitely like the idea, but I've wondered whether having the throat of the port closer to the woofer would be problematic. I have NO data to support this... just a completely baseless intuition! haha
Oh, and by the way... this enclosure will be facing backwards in a hatchback. 🙂
Ideally, yeah, those corners would be round (thanks for the artistic rendering, by the way!). Other than breaking out some fiberglass, I'm not sure how to do that feasibly. I guess I could kerf the wood. Huh... just thought of that. Thanks!
As for eliminating the second bend as you suggested, I definitely like the idea, but I've wondered whether having the throat of the port closer to the woofer would be problematic. I have NO data to support this... just a completely baseless intuition! haha
Oh, and by the way... this enclosure will be facing backwards in a hatchback. 🙂
Acoustically parabolic assuming the top/bottom plates are parallel.
True enough, though if these dimensions are near a room eigenmode then it can either add a peak or dip in the vent's response to audibly affect the speaker's performance.
Pretty much IME, so somewhat before even knowing of Dr. Geddes' existence had come to the conclusion that multiple subs in 3D was the poor man's 'sub horn as listening room' based on the earlier posted 1954 cartoon, so three at minimum.
A many decades long ROT is three diameters away from any boundary, though technically it seems like its end correction distance should in theory be sufficient, but either way, shaped as it is I imagine it needs to be simmed as a multi chambered BP4 to get any sort of ~accurate sim.
Pack with plastic foam, carve-away what don't look right.
I've built short-horns that way. (But flat-panel pyramidal horn worked as good with less lung irritation.)
If you think the foam will absorb bass, or rain, get roof-flashing "tin" and a tube of Liquid Nails to put a curved skin on it.
(I used fiberglas... what a disaster.)
Oooh... good idea! I was thinking about layering and reinforcing some chipboard to make the curve, but I think I like the idea of roof-flashing better. Hadn't even thought of that!