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.
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.
Can you post a reliable link with about this?
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.
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
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.
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.
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.
gcguy2169:
Telescopic ports are sold from several dealers, search and you will find!
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.
gcguy2169:
Telescopic ports are sold from several dealers, search and you will find!
Hello,
Post #3 by Arty is dead on. Yes it about physics. You measure and calculate T/S parameters. You calculate the dimensions of the box. You calculate the dimensions of the port. You revaluate your assumptions and recalculate.
Port noise is velocity noise.
For a given quantity of moving air divided by the cross sectional area of the port gives velocity or doing it the other way about, pick your maximum velocity that gives cross sectional area.
Flared ports are not required.
BTW if a flared port is used is it flared on both ends? There is turbulence at the end inside the box as well!
Like any other recipe bake until done.
DT
All just for fun!
Post #3 by Arty is dead on. Yes it about physics. You measure and calculate T/S parameters. You calculate the dimensions of the box. You calculate the dimensions of the port. You revaluate your assumptions and recalculate.
Port noise is velocity noise.
For a given quantity of moving air divided by the cross sectional area of the port gives velocity or doing it the other way about, pick your maximum velocity that gives cross sectional area.
Flared ports are not required.
BTW if a flared port is used is it flared on both ends? There is turbulence at the end inside the box as well!
Like any other recipe bake until done.
DT
All just for fun!
Same results can be achieved with non flared ports.
Flared ports come in singe end flared and both ends flared versions.
Drawback is that flared ones are a bit more sensitiv to where they are placed at.
Even so, port velocity is calculated -at least by me- at the given smallest cross section. I think it is logical.
Flared ports on the otherhand can sometimes behave quite intresting, sometimes You need them shorter than expected, sometimes longer.
Straight ports are a bit more predictable.
Yes, final lenght is tuned by mesurements, but it is a pain to run into the limits of what You can fit into the box
not to mention the air contained in the port is not part of nett. box volume.
Flared ports are allso a tricky one on this too.
Flared ports can pick up wider band of in-box resonances than straight ones, and so on. (well not in any case, but thats just a nother unknown, un-simulated variable to complicate things even more)
Actualy both straight, single flared, double flared, "slot", bendt, mmm.. "sticking out of the box", and who know how many other variations are out there have advantage, and disadvantage. Hence i consider eatchone as a compromise. Since it is.
Since most are going after the simulation predicted ports, i would assume the more predictable solution has a slight advantage. At least You are not getting that mutch off of what the simulation predicts.
DualTriode:
ports flared on one end only will have turbulence and velocity noise on the straight end, it is usualy the one inside the box. It just has more room to decay.
I think best is to go what Your personal taste tells You.
There is no superior one. Just different advantage traded for a different drawback.
Hello,
Just to straighten out a couple of things.
Noise due to air velocity in a duct (in our case a tuned port) varies ~ to the 6th power of the velocity. Put another way double the velocity the noise increases by near 16 db, note that is in the straight section of the duct not the inlet or outlet. Also note the duct (tuned port) length is a small multiple of the diameter. The duct is not long enough to provide laminar flow.
The short version of this is the flow will always be turbulent and minimizing the velocity is of primary importance for controlling noise. Flared inlets and outlets will provide little benefit.
DT
All just for fun!
Just to straighten out a couple of things.
Noise due to air velocity in a duct (in our case a tuned port) varies ~ to the 6th power of the velocity. Put another way double the velocity the noise increases by near 16 db, note that is in the straight section of the duct not the inlet or outlet. Also note the duct (tuned port) length is a small multiple of the diameter. The duct is not long enough to provide laminar flow.
The short version of this is the flow will always be turbulent and minimizing the velocity is of primary importance for controlling noise. Flared inlets and outlets will provide little benefit.
DT
All just for fun!
Keep in mind that the T/S parameters you measure will give you a point on the T/S curves and what you get are dependent on the weather and your drive.
That is one of these reasons for Arty's need to cut & try (repeat till done).
dave
I don't have much experience (in any box design), but the theory behind the duct tells that 'flared' is an improvement over 'flat' . It's the theory of fluid motion ,and air is a light fluid . The same flared ends can be seen inside an engine ,where oil has to be 'sucked' better ,and it's called the Venturi principle
(same but different
for the airbrush ) .i know about the venturi effect, but do consider that the noise is created at the scross section where its minimal. So even if You do flare both end of the ports, the part between them is plain simply a small cross ection, and do to venturi principle the air will be faster there. that makes the noise.
So actualy the goal- as pointed out a few times- is to minimise the velocity, since it is in direct relation to the noise lvl.
That calls for a large crossectional area.
That is, if the port is not a stright pipe, the size of the smallest crossectional area will be the critical bottleneck.
Allso, do noitce, while fulids can not be compressed, air can be, an it acts like a spring.
So the engine oil is not a good example.
So actualy the goal- as pointed out a few times- is to minimise the velocity, since it is in direct relation to the noise lvl.
That calls for a large crossectional area.
That is, if the port is not a stright pipe, the size of the smallest crossectional area will be the critical bottleneck.
Allso, do noitce, while fulids can not be compressed, air can be, an it acts like a spring.
So the engine oil is not a good example.
Re the flared ports, you can throw all the theory at it you want, but in a real world experiment published in Speaker Builder/audioXpress, the gains in reduction of port noise with a flared pipe were dramatic. And to be most effective flared and terminated identically inside & out.
I approach it quite differently, using long, very high ratio, high R slot vents, purposely pushing the vents into compression. In practise it works REALLY well.
dave
I approach it quite differently, using long, very high ratio, high R slot vents, purposely pushing the vents into compression. In practise it works REALLY well.
dave
Its all about compromise
If you can keep port one diameter away from anything else... But not essential... I had to install 2 ports in the side of the box... And the only option was to fit them 2 inch seperation... Apsolutely has not compromised the sound what so ever...
Lots of myths and old wives tails in the car audio world... What is critical is vb and port dimensions and solid box build... If you get those facts right... Youll get enough bass to keep you happy until you cant drive anymore...And once you have a great box... You can put almost any sub brand into it providing its the same size sub, although 10's will go into a box made for 12's and visa versa, with a ring adapter .. But not 15's, they generally require a larger vb
Changing a sub brand or line will change the shape of the frequency response
If you can keep port one diameter away from anything else... But not essential... I had to install 2 ports in the side of the box... And the only option was to fit them 2 inch seperation... Apsolutely has not compromised the sound what so ever...
Lots of myths and old wives tails in the car audio world... What is critical is vb and port dimensions and solid box build... If you get those facts right... Youll get enough bass to keep you happy until you cant drive anymore...And once you have a great box... You can put almost any sub brand into it providing its the same size sub, although 10's will go into a box made for 12's and visa versa, with a ring adapter .. But not 15's, they generally require a larger vb
Changing a sub brand or line will change the shape of the frequency response
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