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chawski
11-19-2011, 06:06 PM
Hey guys, looking to build my first ported box and have a few questions. Sub in mind is a 10" Kenwood kfc-something model sub. Manufacturer recommends 1.6cuft with a 5"L x 3.5"Dia port for 35hz tuning. Running off a kenwood amp, i think around 800ish watt. I mainly listen to rock, country, a little rap here and there. Not looking for any competition type building, just filling in the low end. Box is going in a civic trunk. I plan on just making a simple rectangle box, as shallow as possible while using as much width as needed bw the wheel wells (I think I have approx 30ish inches)

Few questions:
1) is it safe going for a slightly lower tuning, say 32ish or should I stick with the 35 as suggested by kenwood.
2) is there any appreciable difference bw slot port and a cylinder, other than less wood? I only have a few days over thanksgiving break so looking to keep it simple

Thanks for any input!

hispls
11-19-2011, 06:30 PM
Stick with 35hz tuning. Round port is accoustically supperior, though you'll likely not notice any difference provided no width is way too thin. DO make sure you don't crowd the port opening into anything.

chawski
11-19-2011, 06:34 PM
awesome, I was thinking sub back, port up. should work ok?


edit:
Was looking online at ports and found some for pretty cheap from precision sound products. Only question I have is the retailer I found only has 3" and 4", no 3.5 as kenwood recommends. Is this a problem at all or just as long as its the correct volume it doesnt matter? Am I better off with the 3 or the 4?

Moble Enclosurs
11-19-2011, 08:50 PM
If you are limited on the length of the port and the area, you have the ability to correct it with chamber volume, but again, this is only if you have no other options. SO, if you get the closest to the recommended one (my recommendations may be different so Ill leave it simple for now), then you can adjust the chamber volume to get the sound you want as long as it does not cause issues by making it too small or large. -just trying to keep it simple as requested...I would not actually recommend this if you do not know the effect it will cause.

CAT MAN
11-19-2011, 08:54 PM
Stick with 35hz tuning. Round port is accoustically supperior, though you'll likely not notice any difference provided no width is way too thin. DO make sure you don't crowd the port opening into anything.

i would not say this. i find slots a bit more efficient spl wise in what ive done

Moble Enclosurs
11-19-2011, 09:20 PM
i would not say this. i find slots a bit more efficient spl wise in what ive done

It's relative. Depending on the application, situation, and preference and also position of the port. I would like to possibly do a comparison for this as well for you guys. Maybe soon. That way, it can be determined more easily for each individual. But generally, slot ports are what I use if that means anything. So, again....relative at this point. it is when the slot ports have a greater ratio than 8:1 or say 9:1 that it is really less efficient. Round ports can be good for SQ systems because of vortex propagation, much like a gun barrel that guides a bullet. Slot ports are pressure related and may have more resonances, therefore, tuning is more concentrated than overall response. If you want to have the slot port work better for SQ and not SPL, make it as square as possible. Hope that helps.

So, to the OP, maybe this will help you determine what you want to do with the port before purchasing anything.

chawski
11-19-2011, 10:25 PM
port length will not be an issue, just questioning width.
So if I used a 3.5" aeroport x 5" long, thats approx 74sqin of port area. Would it be correct to say that if I used a 4" aeroport, going 4inches long, again giving me approx 74sqin of port area, it would work, or do I really need to find a 3.5" as per manufacturer?

edit - I think the best way to phrase my question is if my box has an internal volume of 1.6cubes, how long does my 3" or 4" aero port need to be to keep my 35hz tuning frequency, given that 3.5" diameter doesnt seem to be an option

Moble Enclosurs
11-19-2011, 10:44 PM
port length will not be an issue, just questioning width.
So if I used a 3.5" aeroport x 5" long, thats approx 74sqin of port area. Would it be correct to say that if I used a 4" aeroport, going 4inches long, again giving me approx 74sqin of port area, it would work, or do I really need to find a 3.5" as per manufacturer?

Actually, a 3.5" is only roughly 9.6in^2 of area. It would be 62.16cm^2. 74in^2 port is about 9.7 inches round. Now, square inches is area, and cubic is volume(when you include length). I wouldn't concentrate on length right now until you get area down. If you use a 4" instead of a 3.5", that is not a big deal at all. Length will just need to be adjusted from the 3.5" aero tuning just a bit longer is all.
A 4" aero is about 12.59in^2.

chawski
11-19-2011, 10:55 PM
sorry, not sure if I edited before you posted a response. If I run the 4" what length do I need. Im just not sure how to calculate given the flares? Is only the actual cylinder length calculated or does the area of the flares matter as well?

Moble Enclosurs
11-19-2011, 11:38 PM
Well, if the flare is exponential, then it is generally around half the length of each flare added to the length of the port, but a good number to use is about 1.5". The nominal factor is just under 3/4" but for aero, best to use 1.5 as a simple flange is not used. So, this is added 1.5" to each length for each flare. SO, a port that is 32", should be about 29" plus the flares if the flares are on both sides of the port. Or about 30.5 plus flare for single flare port. This can vary, and you may get other answers as well, or even corrections to this, but this is based on an exponential flare rate and is very generic. For more of a hyperbolic flare, it may be shorter, or you can use the diysubwoofer page for that as well.

chawski
11-20-2011, 11:01 AM
so for calculating how long of a port I need, as long as I retain the internal volume of the 3.5dia x 5long, its golden?

DukeNut
11-20-2011, 11:22 AM
try using this precision sound port calculator link Products Menu (http://www.psp-inc.com/tools.html)

chawski
11-20-2011, 11:38 AM
yea I gave that a shot. Im getting 15.5 for the straight section and 16.5 for the flared. What unit is that though

DukeNut
11-20-2011, 12:30 PM
that is inches

chawski
11-20-2011, 12:46 PM
thats almost 30 inches of port length... seems a bit on the long side, no? Manufacturer states a 3.5dia x 5" Long...kinda confused how I went from 5 inches long to 30 inches long

After re-looking my math, the 3.5"x5" port gives me 48cuin of volume. If using a 4" dia port, to retain the 48 cuin of volume, Im calculating 3.84 inches long. Sorry for so many questions, but Im just confused as to how psp's calculator is giving me 30inches long, yet my 3.14r^2 x H gives me just under 4 if using a 4inch dia port?

Moble Enclosurs
11-20-2011, 03:30 PM
Well, your'e right about the 3.5" port cubic volume. BUT.....and this is very important for everyone to understand. You cannot retain a ports physical dimensions based on volume and retain tuning!!! Here is why:
Example: Take a 8-9" round port, or a 10x6 square port...same volume. Now, if you give is a 23.25" length, you get a certain tuning, say with a chamber volume of about 2 cubic ft for a specific driver ( I am using one that I am working on right now as an example). You get about a 44Hz tuning with a 4th order Im working on for this driver. Now, if we get volume of the port and keep the chamber volume, we get 1464.75in^3. now, if we want to use a smaller port for example, say 10" and retain volume of the port...naturally it should be shorter right? Well, we know making it shorter, we are decreasing the area to maintain tuning. This is well known. But if we keep volume, making it shorter and also making it smaller, well, physics will say, you will have a pretty large area compared to the longer port if you are to keep volume. So, lets say the ideal of 44Hz for a 10" port in this example is about 5.88" round, but with retaining volume, you get about 146.48in^2 area, which is a 13.65" round port.
So, that is not going to give you the same tuning AT ALL. In fact it will be around (for my example box/driver combo) 80hz tuning! Almost a full octave above the original tuning! not going to work that way.
best to figure for minimum port area needed, then tune it based on length required afterwards. Not by retaining port volume. Not a good idea.
Note: My math is a little off only because I averaged and rounded fast to get this typed. But the idea is solid that it increased tuning by retaining port volume if making the port shorter, and vice versa due to physics.

Moble Enclosurs
11-20-2011, 03:31 PM
That is why the calculators are giving you different numbers. :)

chawski
11-20-2011, 04:03 PM
could you point me in the right direction as for calculating how long my 4" port would need to be, assuming a rectangle box. ideal box size is 1.6 cu, 35hz

Moble Enclosurs
11-20-2011, 04:39 PM
Yes. To start, you know your volume of the chamber, and your desired tuning so far. the complex way is to get everything together, but for simplicity and lack of accuracy, i can give you an idea of what to do. First, you know to subtract displacement form the chamber volume of the driver and port, or simply the driver, and add the port to the exterior of the volume. I am going to keep it very simple and basic for now. Try using a port(round) of half the area of the driver or around that. For an 8, 3", for a 10 4", for a 12 6", for a 15 8" or two 4", etc. This is very very basic and is not a way to be accurate 9and some may chime in and say other things as well, but this is what I can do without getting too involved, unless you want me to do a design for you.
Now, you have tuning, chamber volume, and port area. You need length. So, say you are using the 4" port and you want a 35Hz tuning. Well, if you cannot change chamber volume for fine tuning, you have to get the length right on. So, the best way to do this is with this:
Take the radius of the port, say 2". Square it to get 4. Then take 10^7 and multiply it by 4. Then take 1.463 times that. Write that number down. You should get 58520000. Then take the 1.463 and multiply by the radius of the port (which is 2). Write that down, which should give you 2.926. Then take the box volume, 1.6 cubic ft. Multiply by 1728 to get cubic in. You get 2764.8. Then take the tuning of 35Hz^2, which is 1225. Multiply 1225 by 2764.8 to get the third needed number to write down which is 3386880. Then divide the first number, 58520000 by the third number 3386880 to get 17.28. Then subtract the second number of 2.926 to get 14.35" port. This is the common form to get the length and is based on a restricted chamber volume. hope that helps. :)

chawski
11-20-2011, 06:56 PM
If I got you the order emailed to you tonight, how long would it take for a basic design? btw, I was off on the speaker model. Ported box actually comes to 1.5cu, not 1.6. I think ill just do a slot port.