I need some advice in how to calculate a bent port. Basically on my car audio enclosure I posted about previously, I calculated the ports out to be dual 4"ers at 20" long. I'm using the thinner 4" PVC piping for sewer use, as well as 2 90 degree bends and 2 45 degree bends. So far I was very impressed with the fit. Everything fits pretty precisely within the enclosure almost like a puzzle, but it still fits.
My target tuning freq was 33 Hz, however I gave it some tolerance due to my rather inaccurate way to measuring the internal volume of the enclosure. I filled the bottom 1/2 with water and then measured that water then vaccuumed it out with a shop vac then converted the measured water into cubic feet, then did the standard math for the top section. Then I took the driver displacement and tried to calculate the port displacement.
So basically my question is how do I accurately calculate the port length for what I have pictured? I ballparked it by putting a tape measure inside and got it as large and small as possible.
(yes those are popsicle sticks on the right side, the damn rotozip's circle cutter must have moved and the port hole was too large😡 ).
My target tuning freq was 33 Hz, however I gave it some tolerance due to my rather inaccurate way to measuring the internal volume of the enclosure. I filled the bottom 1/2 with water and then measured that water then vaccuumed it out with a shop vac then converted the measured water into cubic feet, then did the standard math for the top section. Then I took the driver displacement and tried to calculate the port displacement.
So basically my question is how do I accurately calculate the port length for what I have pictured? I ballparked it by putting a tape measure inside and got it as large and small as possible.
(yes those are popsicle sticks on the right side, the damn rotozip's circle cutter must have moved and the port hole was too large😡 ).
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
If you don't know the exact internal volume then it would be a close guess as to how long the ports should be. If you want to be accurate then download test tones off the net (20-50hz, 5 seconds each) and burn to a cd. Install the driver and play the tones. The tone that the driver moves the least is the tuning frequency. If you're lucky it will be right on. If not then add or subtract from the ports depending on if the tuning is to high or to low.
The port length is measured down the center end to end. Having the ends of the port that close to the wall will increase it's effective length. From the picture it looks like the interior end is somewhat restricted in that the full 4" cross sectional area isn't available. If so, it will have a huge effect on tuning because you no longer have a 4" port.
How did you come up the ports? They look really big for a small box. What's your interior volume net of the driver and the ports?
How did you come up the ports? They look really big for a small box. What's your interior volume net of the driver and the ports?
Thanks all for the replys.
Came up with the ports using WinISD and Perfect Box, as well as using the manuf. recommendations. The woofer is one of those high excursion woofers so it can move ALOT of air.
I calculated the port to displace approx .3 cubic foot of air, and the driver diplaces .25 according to the manuf. After this, the enclosure is approx 2.5-2.7 cubic foot.
The interior part I think has full 4" cross section available, its hard to see from that 2D pic, but I was careful about this. 🙂
Oh well, Ill just plop the driver in and try some test tones. It is for car audio so the application isnt _that_ critical hehe.
johninCR said:
Came up with the ports using WinISD and Perfect Box, as well as using the manuf. recommendations. The woofer is one of those high excursion woofers so it can move ALOT of air.
I calculated the port to displace approx .3 cubic foot of air, and the driver diplaces .25 according to the manuf. After this, the enclosure is approx 2.5-2.7 cubic foot.
The interior part I think has full 4" cross section available, its hard to see from that 2D pic, but I was careful about this. 🙂
Oh well, Ill just plop the driver in and try some test tones. It is for car audio so the application isnt _that_ critical hehe.
With the vent placement you have chosen, you will experience at least some compression.. This makes it very hard to calculate the correct vent size, but you still have the advantage of trial and error. Tune your ports to perfection. 
With such large vents, the compression shouldn't be a problem exept in the math..
BTW: the effective length of your 90 degree turn is easily found geometric by (2*pi*r)/4, where r is the centre turn radius, composed of the inner turn radius + ((outter turn darius - inner turn radius) / 2)
With such large vents, the compression shouldn't be a problem exept in the math..
BTW: the effective length of your 90 degree turn is easily found geometric by (2*pi*r)/4, where r is the centre turn radius, composed of the inner turn radius + ((outter turn darius - inner turn radius) / 2)
Just out of curiosity, have you tried dropping the subwoofer into place since installing the ports? From the looks of things, those ports and the speaker's magnet may want to occupy the same real estate.
Rocky,
What do you mean by "compression"? That's a new term for me in this context.
Those ports seem very close together which is going to have some kind of effect. Not so much on the push out, but on the in flow they will be fighting each other. Also, that close to the wall, the bottom in this case, the effective length of the ports will be a little longer.
It really does seem like risky port placement, so don't glue the PVC until it sounds perfect. With all that hard work, you deserve perfect sound.
If you have some left over resin and fiberglass, the cab would definitely benefit from bracing. Use part of the circle cutout for a brace in the bottom between the vents, which would prevent interference between the 2. Some braces that attach the baffle to the sides is a definite. Use foam rubber between the ports and bottom, and between the car and the bottom and sides to prevent rattles and give the whole thing more rigidity and resonance dampening.
What do you mean by "compression"? That's a new term for me in this context.
Those ports seem very close together which is going to have some kind of effect. Not so much on the push out, but on the in flow they will be fighting each other. Also, that close to the wall, the bottom in this case, the effective length of the ports will be a little longer.
It really does seem like risky port placement, so don't glue the PVC until it sounds perfect. With all that hard work, you deserve perfect sound.
If you have some left over resin and fiberglass, the cab would definitely benefit from bracing. Use part of the circle cutout for a brace in the bottom between the vents, which would prevent interference between the 2. Some braces that attach the baffle to the sides is a definite. Use foam rubber between the ports and bottom, and between the car and the bottom and sides to prevent rattles and give the whole thing more rigidity and resonance dampening.
johninCR said:
Compression is what happens when the vents inner mouth is placed to close to an obstacle, like a wall. The air is not properly distributed in the box to create an even pressure, but is "trapped" in a high (or low) pressure "pocket".. It's like if you blow through a tube, keeping your hand or finger in front of the end, you'll notice it takes more effort to blow as closer your hand is to the tube end....
johninCR said:
Not 100% sure I must say, but I don't think the vents will "fight each other" like you suggests... Assume you are right; wouldn't this phenomenon apply to splitted vents as well? like two intakes joined into one exhaust (or the other way around)? Wouldn't the same laws of physics apply here?
Rocky said:
Thanks, that helps greatly.
Jim85IROC said:
Yes the woofer fits in 😀
johninCR said:
Perhaps rocky meant power compression? In PA woofers alot of the times the compression is listed, but never in car audio or home it seems. Its basically when you put power into the woofer and some of it turns to heat, loosing dB so the dB to power ratio is not perfectly logarithmic.
I put the PVC in with silicone, so it is easily removed if need be, but at the same time temporarily sealing the hole. I may try 3" ports as well since they displace less room, wont need to be as huge and I probably wont even need to put a bend in it.
The reason I went with 4" is cause I thought the air turbulence would be less. I spent only $4 buying those elbows since I alrady had the pipe left over from a previous project.
🙂
Forgot to mention:
Yes I plan to brace the top, the bottom and the sides. I probably should have done it while I was building the box but this was the first time at FG for me I wasnt sure how it would come out, fortunately now I have some experience with it and its pretty forgiving to work with. ALso I wanted to see where the major flexing action was so I wasnt pointlessly adding bracing to an area not needed etc. The baffle and sides definately need some bracing. I was playing 10-20 hz sine waves loudly and that thing was movin hehe.
Yes I plan to brace the top, the bottom and the sides. I probably should have done it while I was building the box but this was the first time at FG for me I wasnt sure how it would come out, fortunately now I have some experience with it and its pretty forgiving to work with. ALso I wanted to see where the major flexing action was so I wasnt pointlessly adding bracing to an area not needed etc. The baffle and sides definately need some bracing. I was playing 10-20 hz sine waves loudly and that thing was movin hehe.
eRiCdWoNg said:
No.. air compression..
To give an example; what happens if you replace your vents with narrower ones? You get away with shorter vents, right.. Why is this? because it takes more effort to push air through a narrow vent than through a wide one.. Why do we use wide vents? To avoid blowing sounds from the ports, because the narrower the vent; the higher the air pressure in it - and the faster wind speed through it. What happens if we use a infinitly long vent of a narrow diameter? The driver does not have enough force to push the air through the vent (hard work!), and acts more and more like a closed box the narrower/longer the vent gets..
Same thing occurs, though in a smaller scale, when the air intakes from the ports are obstacled.. A thumb-rule is minimum 7cm free space from air intake to a blocking wall to avoid air compression.😉
To get an estimate of the acoustic length of the tube you could measure the physical length of the tube at the centre of the cross-section of the tube. Add two end corrections of 0.6-0.85 times the tube radius. Then put everything in place, and measure the resonance from the electrical impedance curve. If it is too low, make the tubes shorter, if it is too high make them longer (good luck!) or narrower.
It is perfectly OK to have two ports, as long as they are of approximately the same size. The port placement in the box is typically uncritical.
It is perfectly OK to have two ports, as long as they are of approximately the same size. The port placement in the box is typically uncritical.
You could try to find the resonant frequency of the ports by tuning a sound source at one end (eg: a raw 4" speaker) until you find the lowest resonant frequency of the tube. This way you're measuring what actually happens rather than guessing the effective length. It's the same concept as producing a tone by blowing air across the lip of a bottle. (Outside of the box that is.)
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