Hi folks,
Are there an advantages of using double ports instead of single, assuming they are both tuned at the same frequency and air flow? Especially in a MTM configuration?
If double port would be better, how should they be placed? Side by side vertically, horizontally facing the rear of the tweeter, or at the far vertical ends, facing the mid woofers?
Are there an advantages of using double ports instead of single, assuming they are both tuned at the same frequency and air flow? Especially in a MTM configuration?
If double port would be better, how should they be placed? Side by side vertically, horizontally facing the rear of the tweeter, or at the far vertical ends, facing the mid woofers?
Hi,
It's all about surface area of the ports (the cylindrical area), the higher, the more friction (losses)
For the same volume of air (which is going to resonate) double ports will have double area.
Do not make the mistake to consider Aport = A1 + A2. It is not correct. They are 2 ports to be calculated as working in parallel.
The most efficient way is to have a single port, circular (rectangular has more losses) with the largest diameter you can afford (by means of length of the port).
Regards
It's all about surface area of the ports (the cylindrical area), the higher, the more friction (losses)
For the same volume of air (which is going to resonate) double ports will have double area.
Do not make the mistake to consider Aport = A1 + A2. It is not correct. They are 2 ports to be calculated as working in parallel.
The most efficient way is to have a single port, circular (rectangular has more losses) with the largest diameter you can afford (by means of length of the port).
Regards
There's much mention of this lately. Is there some data to show what difference the friction makes? What type of difference does it make?
Drag
yes, it's called drag
the power necessary to overcome the aerodynamic drag is:
Pd = Fd * v = 1/2 * rho * v^3 * A * Cd
where Fd = drag force
v = velocity
rho = mass density
A = cross sectional area
Cd = drag coefficient, it depends on shape and Reynolds number
PS. sorry, I don't follow all posts on a daily basis, sorry if I repeated some known facts.
yes, it's called drag
the power necessary to overcome the aerodynamic drag is:
Pd = Fd * v = 1/2 * rho * v^3 * A * Cd
where Fd = drag force
v = velocity
rho = mass density
A = cross sectional area
Cd = drag coefficient, it depends on shape and Reynolds number
PS. sorry, I don't follow all posts on a daily basis, sorry if I repeated some known facts.
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Hi, there should me gazillion papers about gasses and fluids flowing in a pipe 🙂 I'm not expert, haven't read any of papers so drawing this next comment back from my head. My hunch is that friction (area of the wall, smoothness of the surface as well as the roundings each end) affects the Reynolds number, which is kind of indication how turbulent or smooth the flow in a pipe is. How this relates to speakers with Helmholtz resonator pipe? I think it might affect the tuning, which is no problem since it always seems to be bit of from calculated and needs adjusting after measurements. Also it might affect chuffing sounds. How? Make the chuffing sound different? Chuffing at lower power? No idea, but both can be considered as worse performance so one might choose using round port, which has the lowest surface area. Reading the forums rounding the ends of a port has most notable effect and also one can change the particle speed in the port by changing the poirt volume. Just another compromise, use round if you have plumbing stock. Use square if you have extra MDF.
Advantage of two ports is ability to tune the response. You might have a lower tuning listening at home by blocking one of the ports and higher tuning with more port area for loud applications. Or be able to affect the highpass Q a bit tuning the in room response. You could make a prototype with several options to choose which is best for your application! have fun!
Advantage of two ports is ability to tune the response. You might have a lower tuning listening at home by blocking one of the ports and higher tuning with more port area for loud applications. Or be able to affect the highpass Q a bit tuning the in room response. You could make a prototype with several options to choose which is best for your application! have fun!
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Thank you. Without digging into this, resistance might suggest this is a damping issue rather than a tuning, or turbulence/velocity issue unless comparing shapes. I guess it's safe to presume that the single round port shouldn't disappoint, but by how much if any?
Reynolds number Wikipedia entry Reynolds number - Wikipedia has following paragraph:
For calculation involving flow in non-circular ducts, the hydraulic diameter can be substituted for the diameter of a circular duct, with reasonable accuracy, if the aspect ratio AR of the duct cross-section remains in the range
1/4 < AR < 4.
My conclusion is use round or rectangular port with aspect ratio higher than 1/4, which ever is more practical 🙂
ps. remembered another advantage of two ports. If you are driving them speakers with lots of power two vents could help exchange more heat, especially if one was above and one below the drivers?
For calculation involving flow in non-circular ducts, the hydraulic diameter can be substituted for the diameter of a circular duct, with reasonable accuracy, if the aspect ratio AR of the duct cross-section remains in the range
1/4 < AR < 4.
My conclusion is use round or rectangular port with aspect ratio higher than 1/4, which ever is more practical 🙂
ps. remembered another advantage of two ports. If you are driving them speakers with lots of power two vents could help exchange more heat, especially if one was above and one below the drivers?
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Hey guys, thank you for your valuable info!
By the way, is there a rule of thumb how much distance should be between the inside port end and the front wall?
By the way, is there a rule of thumb how much distance should be between the inside port end and the front wall?
If port "inlet" is close to a box wall the tuning will change from calculated a bit. Another reason to avoid port inlet near enclosure wall placement is that possible standing waves inside the box have pressure nodes at the wall boundaries so they kind of want to leak through the port. If your ports are rear of the enclosure the "leakage" might not be an issue though. Nevertheless this "leakage" is unwanted and in worst case can be as loud as the bass output which is the only thing you actually want the port to do, augment the lowest lows not ruin the mids!
Where is the best place for the port "inlet" then? Don't know! where are your woofers located? what are the dimensions of the box? Here are my thoughs: If there is problematic leakage it is almost certainly the lowest modes of the box that fall into the pass band of the woofer and are not affected by the possible stuffing. Lowest modes relate the longest dimension of the box (or the pipe resonance of too long port). These modes have pressure nodes at top and bottom of the box, 1/2, 1/3, 1/4, 1/5 way etc. Google up to visualize. MTM configuration could prevent the lowest mode to form, maybe affects some of the others as well. Anyway I would avoid boundaries, 1/2 and 1/4 position of any dimension inside the box. If this is not practical just stick it in and don't worry so much 🙂 You could make a prototype with multiple ports in different locations to be able to pick the best. Otherwise it is what it is you happen to get 😉 Quick prototype will take one evening and teach you much more than the forums. You could test some other stuff as well like bracing, round overs, overall assembly considerations for neat results... Prototyping gives peace of mind and a much better speaker overall. Have fun!
Where is the best place for the port "inlet" then? Don't know! where are your woofers located? what are the dimensions of the box? Here are my thoughs: If there is problematic leakage it is almost certainly the lowest modes of the box that fall into the pass band of the woofer and are not affected by the possible stuffing. Lowest modes relate the longest dimension of the box (or the pipe resonance of too long port). These modes have pressure nodes at top and bottom of the box, 1/2, 1/3, 1/4, 1/5 way etc. Google up to visualize. MTM configuration could prevent the lowest mode to form, maybe affects some of the others as well. Anyway I would avoid boundaries, 1/2 and 1/4 position of any dimension inside the box. If this is not practical just stick it in and don't worry so much 🙂 You could make a prototype with multiple ports in different locations to be able to pick the best. Otherwise it is what it is you happen to get 😉 Quick prototype will take one evening and teach you much more than the forums. You could test some other stuff as well like bracing, round overs, overall assembly considerations for neat results... Prototyping gives peace of mind and a much better speaker overall. Have fun!
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Thanks! My primary idea was a 51L enclosure with golden ratio internal dimensions - 60*37*22.9cm.
So I understand that putting the port in the rear facing the tweeter would violate the 1\2 rule? If so, would it make sense to put two ports that do not match these fractions? What if I make a slotted port in the center, but has a two openings using a division between the center?
What about two ports near the top and bottom of the speaker? Would this be considered a boundary?
So I understand that putting the port in the rear facing the tweeter would violate the 1\2 rule? If so, would it make sense to put two ports that do not match these fractions? What if I make a slotted port in the center, but has a two openings using a division between the center?
What about two ports near the top and bottom of the speaker? Would this be considered a boundary?
Well, the conditions inside the enclosure are pretty complex so best thing we can do without computer simulations is to simplify. Consider the exact 1/2 and 1/4 way and any boundary a bit worse than some other location. But really, all these could work and not be an issue.
A boundary here is just a surface which air particles inside the box encounter while trying to move around given some excitation by your woofers. When a moving particle collides with a boundary, velocity comes to stop and eventually reverses, this appears as pressure against the boundary 😉 If there are opposing boundaries a standing wave and it's harmonics an develop between. Harmonics have other nodes than the boundaries as well, at 1/2, 1/3, 1/4 etc. along the length.
Longest dimension of 60cm will give you standing waves at 286Hz, 572Hz, 858Hz, 1144Hz... Wavelength of a standing wave = 2L / n, where L is the length in cm and n is harmonic 1, 2, 3... These all fall to a woofer pass band, if it is a two way with a dome tweeter. 286Hz is the 1st harmonic and has pressure nodes at top and bottom boundaries of the enclosure. 572Hz has pressure node also at the 1/2 way, avoid this with your port inlet. 858Hz has pressure node at 1/3 along the way but this might already be dampened enough with stuffing. Your other dimensions are smaller and the possibly problematic only if your port is at the boundary.
If you have a big port, the inlet might span a long length and contain multiple of these pressure nodes. You could turn your port 90 degrees to have less exposure to this length 😀
It is very much fun to think about this and some imagination gets you a long way. This is not the complete picture, but somewhat educated guess is possible by thinking like this. To tackle any surprises only prototype will tell ya 😉
A boundary here is just a surface which air particles inside the box encounter while trying to move around given some excitation by your woofers. When a moving particle collides with a boundary, velocity comes to stop and eventually reverses, this appears as pressure against the boundary 😉 If there are opposing boundaries a standing wave and it's harmonics an develop between. Harmonics have other nodes than the boundaries as well, at 1/2, 1/3, 1/4 etc. along the length.
Longest dimension of 60cm will give you standing waves at 286Hz, 572Hz, 858Hz, 1144Hz... Wavelength of a standing wave = 2L / n, where L is the length in cm and n is harmonic 1, 2, 3... These all fall to a woofer pass band, if it is a two way with a dome tweeter. 286Hz is the 1st harmonic and has pressure nodes at top and bottom boundaries of the enclosure. 572Hz has pressure node also at the 1/2 way, avoid this with your port inlet. 858Hz has pressure node at 1/3 along the way but this might already be dampened enough with stuffing. Your other dimensions are smaller and the possibly problematic only if your port is at the boundary.
If you have a big port, the inlet might span a long length and contain multiple of these pressure nodes. You could turn your port 90 degrees to have less exposure to this length 😀
It is very much fun to think about this and some imagination gets you a long way. This is not the complete picture, but somewhat educated guess is possible by thinking like this. To tackle any surprises only prototype will tell ya 😉
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In this context, I remember a story where a prototyper used to move a rear larger panel with the port preinstalled around the back of the box. Exactly kind of what you describe, by trial and error. When he would finally get the correct placement of the port, he would mark it down with a pencil.
I'm not sure about the parameters he is supposed to measure though. Harmonic content through the port? Frequency response?
I'm not sure about the parameters he is supposed to measure though. Harmonic content through the port? Frequency response?
With subwoofers the "leakage" and thus port position is not that critical since there isn't much standing waves inside the box due to long wavelengths. One can use slot port along a box wall without problemos. Try a slot port bottom/top of the box with your multiway speaker and compare with port whose inlet is not against enclosure wall 🙂
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hah, thinkin' while zipping coffee, simplest way to find the influence of the inlet position would be to make the port as a periscope. Use plumbing parts, 90deg bend inside, protrude a straight part of the tube outside the box so that one can control the inlet "depth" and position (rotation) by hand from outside. position the port so that rotating the tube is possible full 360 degrees and see in realtime if there is sweet spot or is it all just mumbo jumbo 🙂
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This is a nice idea, to make the port bended. One can really fine tune the inlet.
I made a box modes simulation. The are two main modes along the depth of the speaker at 1/3 (281Hz) and 1/5 (562Hz) that run parallel to the port length. The 1/5 mode occurs at the middle and the port shouldn't be placed there.
Between 1/3 and 1/5 modes we are left with 1/15 free spaces to work with. At 38cm length, that translates to 2.5cm of space.
So one could make two slotted ports in the 6/15 and 10/15 regions. And could fill the perpendicular sides (making them narrower) to avoid boundary modes from the width of the speaker. What do you think about this idea?
I made a box modes simulation. The are two main modes along the depth of the speaker at 1/3 (281Hz) and 1/5 (562Hz) that run parallel to the port length. The 1/5 mode occurs at the middle and the port shouldn't be placed there.
Between 1/3 and 1/5 modes we are left with 1/15 free spaces to work with. At 38cm length, that translates to 2.5cm of space.
So one could make two slotted ports in the 6/15 and 10/15 regions. And could fill the perpendicular sides (making them narrower) to avoid boundary modes from the width of the speaker. What do you think about this idea?
Yeah, just put the port in there, for example somewhere around middle of the 38cm length dimension and see if there is problematic leakage. If it is good enough call it a job well done 🙂 After all it is just another compromise to choose from, between dimensions of the box, ease of build and bunch of other constrains that might be more important the inlet position. Making a prototype will help choose the relevant ones and give you insight how to solve possible problems or get to the compromise you've chosen.
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