Slot Port Question

I have a quy that wants 3 Memphis M312D4 and on there site they of the measurements for 2 vented subs at 11.5x2.5x13 and need to know how do I calc the 3 sub?  Also, I have get the equation to squar ports but, I don't understand it all the way when I convert from round to slot I get the sq.in of the port but I don't know with that number lets say 25sq.in how do I know how wide and on high to make the opening.  Do I just make it say 3in hight by 22in long and keep it what ever the length is.  Also, when converting to slot what do I do with multi. ports boxes.  Like my MA 1200 I have they say to have 2 4in ports 14in long.  I have 2 and I want one slot port how do I figure all that in.  I like the nice quit port noise from slot ports and I want to construct those instead of round ports.  I know this is most likely all confusing but, I need help.

Thanks

hi. all u gotta use is pie.       if u have a round port of say 4 inches diameter. u must multiply this by pie(3.14 approx). this will give you 12.56.now wat u have here is the surface area of the port.

now consider 12.56 as a single length of the port. now if ur port is 1 inch wide(slotted), and the port is 12.56 in height- 12.56multiplied by 1 = 12.56.so finally if ur port is gonna be 2 inches wide all u gotta do is divide 12.56 by 2. and this gives ur dimensions. 6.28 in height and 2 inches width.u can do this forever u can divide it by 3,4,5 but remember u always have to use both numbers as the dimensions.

also when makin the port u must consider the displacement of the mdf if it is a certain thickness it will take up space, and therefore reduce volume of the box. so making the box bigger is a must if u r using very thick mdf.

There are a bunch of calculators floating around the internet, so bookmark the ones I'm going to show you here and you'll have some tools to work with.  I went to the Memphis site and looked up recommendations for that sub.

http://www.memphiscaraudio.com/LiveImages/112/43/DocumentURL1.pdf

They recommend 1.6 cu ft per sub and tuned at 38 Hz, so that is what I used in the following figures.  With three subs, there will be 4.8 cubic feet (basic calculator).  If you were looking for a round port go to the https://www.the12volt.com/caraudio/boxcalcs.asp port volume calculator.  But you still don't know how big the port opening must be to eliminate port noise...and you want a slotted port.

This port volume calculator http://www.carstereo.com/help2/Articles.cfm?id=31 will first tell you the minimum area of port opening you will need for these subs.  Going by the specs on the Memphis site, using 9mm as Xmax ( http://www.linearteam.dk/default.aspx?pageid=unitconverter) and 38 Hz as tuning frequency along with a quantity of 3 drivers, it tells me that to have NO port noise you must have a minimum of 56 square inches of port opening.

Now go to the calculator below that one.  Here you will find port length given the numbers you input.  Use your regular calculator and find a slot opening that will fit the enclosure and be 56 square inches.  I used 11.5 X 4.8 to find a port length of 12.9 inches (by using a regular calcuator and dividing 56" by one known length of the port, which was 11.5").

This calculator wants no noise from the port so it always gives a large port opening.  If your enclosure can't take such a wide opening, use 3.5 to 4" and see what length it tells you to make it.  Also, substitute tuning frequency if you've already decided what you want it to be...and substitute overall box volume if that's been decided with all of the calculators.  Use these a couple of times and they'll be yours.

There is a long list of calculators including that HERE.

Man you guys are great that made a whole lot more sense then the other stuff i have read.  Thanks a bunch.  I have one more question since we have just started the custom box thing how do you figure in port displacement.  I thought on the tech notes of most subs one of those numbers is the speaker displace ment and I know Memphis calc's that in when giving you the dimensions.  Thanks guys you are truley great.

It sometimes makes it a little tougher when the manufacturer figures in the displacements when giving you volume.  Best thing to do is minus out the displacements and get the actual air volume for one sub and then go from there.  A port displacement, as jeffchilcott and Bigsingh both described, is the entire volume of the port including both its air and its divider walls.  Treat it as a solid object the same way you would treat a solid block of wood as a solid object.  It is taking up space within the enclosure so the volume of air around it has to amount to the cubic feet you're looking for.  I for one get lost using calculators unless I have a sketch of the box on paper in front of me.  There is where I can make my notes and get through it much easier without forgetting what the hell I'm calculating now.

It's rare that you can calculate once through everything and be done with it, especially when dealing with vented enclosures.  And that's because of the port displacement.  Any little change anywhere with the box will necessitate refiguring the port, and then the port volume will change....which then makes you have to refigure the box dimensions...and on and on....lol