Port chracteristics |
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70,s hero
Young Croc Joined: 14 December 2014 Location: bristol Status: Offline Points: 637 |
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Posted: 25 October 2015 at 10:28am |
Anyone here who can shed some light on this subject? Looking at ports and without referring to all the tech stuff, its is more often than not a requirement to fine tune them even after calculating the length, size etc.
So in terms of cone control are there any rule of thumb generally accepted rules when altering ports, I am talking about frequencies in the sub range with a 18 inch driver. I realise that port position and orientation effect how it interacts, so if say you wanted to control cone excursion at say 30 HZ, how would that relate to port length and size. Some speakers just have perforated grills, I guess its a custom design like the Mackie 408,s. It may be that they just vent the cab precisely to control the cone excursion, thats how it seems to be, perhaps that could be extended to sub design.
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Aman Gebru
Registered User Joined: 01 February 2014 Location: Delhi Status: Offline Points: 350 |
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Normally the box size will dictate how large you can make
the ports, as if the box is too small or the tuning freq too low, then the
ports will be deeper than the box or not have very much clearance at from the rear
of the port to cabinet wall.
But if you have a big enough box or high enough tuning freq then you could in theory have a very large port. But large ports have problems. There will be more loses due to wall friction and resonances that will have harmonics that can be heard higher up. Too small a port and you face high wind velocity's and port non linearity, where the port stops resonating consistently. The trick is to make the port area just bigger than is required so that wind velocity noise is not a problem at resonance. Say you decide 36 meters per second is your the figure you wish to keep wind velocity above at resonance. Make the port the area the size it needs to be to achieve that but no bigger. You mentioned 30Hz. A port will only be able to provide loading at this frequency if the port is tuned to, or preferably lower than 30Hz. That's going to require quite a long port for a single 18" driver, especially if the box is not massive. So I don't think you will ever run into the situation where the port or ports will be suffer losses or resonances. The other thing up for debate is that larger ports have more throw due to their larger surface area. An 18" driver with very small port can lose a lot of low bass at distance. |
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DMorison
Old Croc Joined: 14 March 2007 Location: Aberdeen Status: Offline Points: 1647 |
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Any port will provide its best cone control at its tuning frequency, so if you want best cone control at 30Hz, you tune to 30Hz. That's the simple part. The trial & error/adjustment part comes when you try to actually build a port with a specific tuning frequency. Any speaker modelling program will happily spit out port areas and lengths to achieve this, but they are all based on assumptions about how the port is built into the box. The most common assumption is that one end of the port is flat to the baffle and the other is in completely open air inside the box. If your particular cabinet layout can accommodate this, then the nominal values should get you pretty close. A lot of designs don't allow this to be done at all easily however - as Aman says above, once a port is big enough to avoid noise due to airspeed in the port it often has to be near a side wall (or top/bottom) or it's back end ends up very near the back of the cabinet. As a port is moved nearer to the box wall (ie alongside or above/below the port) then there starts to be an interaction where the adjacent cabinet wall sort of encourages more of the air immediately behind the end of the port to act as if it's connected to the air in the port - ie the port behaves as if it's longer than what's actually built, giving a lower tuning frequency. The trial & error part comes in where you are working out just how much shorter the physical port component needs to be to get the correct effective length, as this interaction is not included in most modelling software. If the back end of the port is too close to the back wall of the box. then the port can get choked up and not work correctly. Bending a port so that it either follows around a corner or keeps its end in the open air in the middle of the cabinet can yield better results in such a case. Re Grilles, as long as the grille has enough open area not to start choking up the port, it should have no effect at all on box tuning. The Mackie S408 you mention looks from its Freq Response curve that it might be tuned quite high, which would mean it only needs a very shallow port. They might just have got a required port depth that is the same as the thickness of the timber from which the box is built, plus perhaps a little of the virtual extension from the side walls of the box being really close to the inside edges of the port as described above. That technique would not translate well to a 30Hz tuned sub, because the required port length always goes up as tuning frequency goes down. HTH, David.
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70,s hero
Young Croc Joined: 14 December 2014 Location: bristol Status: Offline Points: 637 |
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Thanks guys, I have a tuning frequency of 33HZ ,226 liter box and four triangular corner ports at 270 depth. I have reliable circular port dimensions that I substituted for the same square area of triangular ones but as you say, it looks like this could do with some tuning, perhaps shortening of the ports and adding another to make up for the anomalies associated with triangular ports.
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Teunos
Old Croc Joined: 23 November 2008 Location: The Netherlands Status: Offline Points: 1799 |
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triangle is the worst shape for a port. The area in the corners of the triangle will form a boundary layer so big that the effective port area will be reduced massively. Round is optimal since the ratio of surface area relative to the circumference is the most optimal. Triangle is just mehhh.
Better go for a rectangular or square port with loads of surface area. Also if you can keep the number of ports to a minimal. Better have 2 large ports than 4 smaller ones.
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Best regards,
Teun. |
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ceharden
The 10,000 Points Club Joined: 05 June 2005 Location: Southampton Status: Offline Points: 11776 |
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I would argue that although triangular ports have their issues, if you can fit much larger triangular ports than you can with other shapes and adjust the tuning to compensate, they can still be very effective.
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70,s hero
Young Croc Joined: 14 December 2014 Location: bristol Status: Offline Points: 637 |
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Mucking about with the sim inputs for circular vents throw up all sorts of sizes and lengths, what would be the best approach, I have a circular port size of 100 mm giving 12.57 inches square in Wins but I have the same area in a triangle actually built, I see that I will need to sim the Triangle vent as it will need to be shortened to allow for its position.
Cant seem to find a triangle port calculator
Edited by 70,s hero - 27 October 2015 at 5:20pm |
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b grade
Young Croc Joined: 05 October 2012 Location: Portland OR USA Status: Offline Points: 1337 |
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Making the vent smaller or the box bigger should allow for a shorter vent. Also, if you have more than 1 vent, having less vents will make them each shorter. Don't make it too short though. I find short vents do not seem as predicable for functioning the way they did in sim.
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70,s hero
Young Croc Joined: 14 December 2014 Location: bristol Status: Offline Points: 637 |
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Thanks for that, I can see that making the cab bigger shortens the vent, trying to get an idea of the triangular ones as it seems they need to be either bigger than a equivalent sized circular one or shorter
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DMorison
Old Croc Joined: 14 March 2007 Location: Aberdeen Status: Offline Points: 1647 |
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Yup, this goes back to a couple of the points mentioned above. If you built a triangular vent situated in the middle of a large area of baffle away from the cabinet walls (not that you would for any practical reason, but for the sake of argument...) then it would behave fairly close to a circular vent of the same cross sectional area, with the 1 exception that Teunos pointed out above. That is: as it has more internal surface area on the port walls it would be less efficient (ie behaving as if it was a smaller cross sectional area). This actually has 2 knock-on effects: 1 is that you may need to reduce it's length to maintain the tuning frequency and the 2nd is that it will choke up more easily as airspeed rises, so you'd need to start with a bigger cross sectional area (ie model for a lower airspeed) in the first place. The other thing is as I said, when you build a port in against the cabinet sides (& even more so against 2 sides simultaneously for a corner), more than half of the port walls are much longer than the remaining panel that forms the port itself, thus yielding an even lower tuning than predicted. There isn't much consensus on exactly how much you need to adjust the panel length to compensate for this, which is why you're unlikely to find specific calculators for this style of port, hence the trial & error (aka Prototyping if you want to sound posh ) part. If you're willing to do a little number crunching outside of your modelling program I have seen one estimate for how much to alter port lengths, for their position in the cab, here, but that still doesn't account for the loss of efficiency from point one. So, back to prototyping if you really want to get your tuning frequency spot on... David.
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b grade
Young Croc Joined: 05 October 2012 Location: Portland OR USA Status: Offline Points: 1337 |
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I don't personally own the real version, just the demo version, but the Eminence Designer program can do triangles if you select "other" for the vent shape. It is actually more informative than WinISD, but if you don't pay for it, it changes back to the Sigma Pro Driver every time you try to run a new plot, and it won't let you save the sims you run either. (Workaround is screenshots) but still, its not very expensive. Might be worth it. I always get different values with every different program I use, so I always take it all with a grain of salt and in the end I just pick one and pretend to trust it and hope for the best.
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70,s hero
Young Croc Joined: 14 December 2014 Location: bristol Status: Offline Points: 637 |
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Thanks fellas, much appreciated information, I have two 18 reflex boxes that sound really nice,tight and punchy but also go really low when needs be and tow boxes active 18 cabs that I use with them,the active ones are quite a bit larger to accomodate the amps, overall the sound is very good and the idea was to tune the actives a bit lower given the cab volume but it seems that the active ones arent quite up to what I expected, so I guess the easiest way would be to insert some panels at the front of the cabs to block off the triangle ports and go with the manuafacturers design for all parameters,that will mean reducing the cab volume by some way as well and making 4 round vents at 4" diameter.The bind is that I have to vent top and bottom for cooling otherwise I would use one vent.
Still, I think all of the work will be worth it, a thought occurred that i may have enough room to install a hf section within the cabs.... oh no, what am I saying
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