type of port for vented box
What type of port do you prefer when you build a speaker box? A plastic tube of a specific diameter or a longer or shorter tunnel made of mdf/pal.
I've seen that the boxes having a plastic tube, have lower frequences, is it a general rule?
nah, its not a rule.
I prefer those orange drain pipes like:
there. simply since have easy acces to them.
Actualy the port diameter and lenght for a given volume gives the tuning freqvency, not the material of the port.
You can make tunnel if You like, it is just harder to predict the tuning freqvency.
The usual methood is to first of all figure out tuning freqvency suitable for the driver/task, then figure out the minimum vent area to avoid "box farting", then given these informations You can calculate the lenght of the port.
If the vent area is too small, then the cone will force air out via the tube with high speed, yielding a horrid sound. So whenever possible use the largest diameter tube You can fit. Naturaly You do have to play with the numbers a bit to make sure You can fit the port in the box.
To make a port work more or less as simulation predicts tube ports are my choice.
Allso one has to place the port 1 dimater distance from any other speaker in the box, and suposedly around 1 diameter clearance is adviced in every direction around its ends. So if Your tube is say.. 3" in diameter and You fit it in the box, the opeing of the tue inside the box should be allso at least 3" away from any walls, bracing, whatever You got.
Play with Winisd if You like, that is quite handy. There is a port calculator inside it, so You can see how does the length/area and box volume affects the whole system.
Do take notice, the air inside the port does not count towards the net volume of the box.
Ports allso do rezonate. It is never a bad idea to wrap it into wool from the outside, that does dampens the rezonances a bit. Some add bracing it ports from the outside, i do prefer that too. Helps to make sure the tube does not rezonate (as mutch as without it).
Bass reflex is actualy the world of compromise.
You do make a nice route for the sonic reflections inside the box to escape.
You do get extended bass, but You allso get group delay.
You allso get a nother source of harmonics.
Still, it is a very prefered build type.
But anyways, the short answer is that the material what the port is made out of, and its actualy shape giving the required cross sectional area is not that important.
There are correction formulas to overcome different shapes.
The most simple form is a circle, so tubes are prefered as for shape of area.
Plastic is easy to cut to size, and one can even make a tunable port from 2 plastic pipes that can slide into eatch other. Therefore changing the tuning freqvency for a given volume.
That is quite handy, once You fine tuned Your "stuff" You can get a new pipe and cut it down to size, or just fix the position with glue.
Flanged end ports are told to be better, that is what i can not confirm.
So the DIy approach is to use thick wall PVC pipe for me.
What about vent's that are bended, for example a "plastic" tube with a curved corner?
Look up bass reflex speaker design:
bass reflex speaker design - Google Search
The port size (inside diameter and length) for optimum bass is determined by the cabinet volume and the driver's Thiele-Small parameters. The port and cabinet volume make a Helmholtz resonator at a frequency below the driver's resonant frequency, and allows a bass response that extends to a lower frequency than otherwise. I hope that's not an oversimplification.
The "bended" or curved ports I've seen are that way because the port needs to be longer than the linear space inside the cabinet allows, and the bend in the port only has a minor effect on port tuning and such, but I think it's better to not have a bend in it if practical.
Have a look at Collo's site as there is heaps of info on ports, chuffing, pipe resonance etc.
Collo's DIY Subwoofer Enclosures
Good link. Be shure to use/make flared (aerodynamic) port-openings, both to increase port-output and to avoid port-ressonances. Unflared ports flow as little as 50% of their caculated capasity due to turbulence.
If you can find a way to get a telescopic "tube-in-tube" you`ll ne able to fine-tune it to you`re room.
Helmholtz resonance - Wikipedia, the free encyclopedia
actually this one is quite handy too.
oh, and if You take a look at the formulas You will see that vent material is *not* important.
Actualy it is, but it will not change the tuning freqvency. Ports made out of thick material that does not resonate easyli is better than thin material that can resonate with ease.
Flared ports are not realy my recommendation, while they offer advantage they are just a compromise as everything else.
To put it simple, straight non flared ports are more predictable. Flared ones have the less turbulence advantage.
Supposedly some ports on the market have a correction value calculated for them, but i kindof never realy did use those. the "tube in tube" tuning option is usualy the way to get proper results.
One thing is sure, You can do as mutch math as You want, reality will be different.
Square ports are the one to avoid, or if You must do it for some reason, be verry sure to use a WAY larger vent area then simulation predicts.
Best practice is to use the largest area You can get away with in any case.
Specialy true for square ones, less important but still recommended for circular ones, and never hurts with flared ended ports.
Does anyone have a primce example of how to contruct these adjustable ports? Is it basically 2 different size PVC pipe with very tight tolerances once placed together? To me, that would be wrong. If I have a 2'' PVC tube for my port and want to add an extender, and its 1.85'' would that mean that the port calculation would be incorrect? Air is intering a 2'' tube then is reduced to 1.85 thus creating more restriction? I'm sure slightly and not even noticeably but how does this work?
the methood of construction you describe is correct.
Allso, you are correct, the math will be a bit flawed by the 2 different port size.
Notice, even if You use straight pipe, constant diameter, and do the math, it will still be not accurate.
Thats why pipe-in-pipe port is suggested, as no mather how well You do Your homework, the outcome will be different. It depends on where You place the port for example. And many other factors.
You`re wrong about flared ports Arty, this is plain simple physics, no mysteries involved.
Non-flared ports makes a geat deal of turbulence and turbulence not only restricts flow but it is the main reason for unwanted port-sounds too.
About unpredictable; the final lenght of a port should not be just based on calculations, it needs to be meashured (tonesweep+ohm-meter) and tuned by trained ears.
Telescopic ports are sold from several dealers, search and you will find!
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