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rady449 
Member - Posts: 15
Member spacespace
Joined: January 30, 2011
Location: Wisconsin, United States
Posted: April 12, 2013 at 11:54 AM / IP Logged  
im planning to build an enclosure for for 2 18" re XXX.
i have been using the calculators on here for volume and port depth and i have some questions.
first: when i put in the box volume for finding your port depth should i subtract the sub-woofer and bracing displacement first, or should i use the gross volume of the box before any displacement?
second: these woofers are reccomended 9cu.ft.@34htz, should i make the box so its 9cu.ft gross or after displacement, and do you subtract the volume the port will displace or just sub and bracing?
third: im i better off being over or under 9cu.ft.?
my current planed dimensions are: using 1.125"MDF 48" wide x 27.5" height x 34" deep. on the bottom the box will be 43" wide going 5.5" up to fit between wheel wells(so a 2.5" x 5.5" section off the bottom left and right sides) there will be a centered divider make two mirrored chambers. (a left and right) if all my math is right this will make each chamber 10.09902cu.ft. before any displacement. i have calculated out that after sub, bracing, and (if this is right) port displacement the final volume will be 8.95119cu.ft. does this sound right or should i be doing this differently?
i have been trying to use WinISD but i cant figure it out, it tells me crazy things. which im sure is cause i dont understand how to use it. i really want to start my build but i dont want to waste all my wood so im asking for input from the pros before hand.
soundnsecurity 
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Joined: November 10, 2008
Location: Louisiana, United States
Posted: April 12, 2013 at 10:20 PM / IP Logged  
the specs for the sub are the final internal net volume of air after the sub woofer displacement and any bracing is subtracted. if each of your chambers is 10 cubic feet then depending on what type of bracing you use you would probably end up with about 9.5 cubic feet.
this also does not include the actual volume of the port itself. the port calculator assumes the port is not part of the box's internal structure so if the port is made to go inside the box then it can be a little bit of a dance to get the port the right length to work with the airspace you are putting it in.
generally as the box gets bigger the port gets shorter but as the port gets bigger the longer it needs to be to keep a certain tuning frequency and as a port gets bigger it will take up more volume and leave you with a different box volume that needs to get plugged into the port length calculator. i usually make a first calculation that i use as an assumed port volume, then i use that to add or subtract to my box volume and then if i have some space left over i either fill that volume with bracing or i play with the box design to add or subtract volume to fit in with my tuned port. it will take some playing around with the calculators to arrive at a final design that is close enough to what you want.
there is no hard rule about if it is better to go over or under the recommended box specs for any given sub but there can be consequences if you go way too big or way too small. way too big can cause the sub to move more than designed due to lack of back pressure, which is not a very big issue with the RE XXX, and too small can cause a sub to not move enough causing it to over heat and die, same as if you stop an electric motor from moving while the power is on it will eventually cook itself.
recommended specs are what the manufacture gives you that will give you a mid level balance between loud and quality. sometimes the recommendation leans more towards loud or quality but generally you can change the box specs to go another direction. recommended specs are usually not the loudest design possible nor are they usually not the cleanest design, they just give you a basic balance of both worlds.
soundnsecurity 
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Location: Louisiana, United States
Posted: April 12, 2013 at 10:41 PM / IP Logged  
i just built a box for two 18" RE SX about 9 months ago and trust me you have a nice task ahead of you to build this box. the specs for our subs are similar, your XXX's call for 9 cubic feet per sub and the SX calls for 8 cubic feet per sub. my box is tuned at 32 HZ, double layer 3/4 MDF for the whole box so it is 1.5" thick, took 7 sheets of MDF and a whole large box of deck screws and a half a case of liquid nails. probably $300 just in materials so i definitely know how much you only want to build this box once.
rady449 
Member - Posts: 15
Member spacespace
Joined: January 30, 2011
Location: Wisconsin, United States
Posted: April 13, 2013 at 9:06 AM / IP Logged  
ok so ur saying that to calculate the port length i need to subtract the ports assumed displacement as well as the bracing and subs from the gross volume then use that volume in the calculater.
how to you calculate the ports displacement?
the way i have done it is by acting like the port is a missing area from the box (just like its not there)
my gross volume= 10.09902
sub dis.= .3
bracing dis.= .1822
port dis.= .7147 (when port internal dim. are 22.3125" x 4" x 13.8378" tuned at 34hertz) i got that port depth by subtracting the sub and brace displacement from the gross volume and using that in the port calculator.
if i do it the way u said with the assumed port displacement subtracted as well i get a depth of 15.5748
wont doing it like this be impossible to calculate cause it will change itself every time cause the displacement will change with the depth change?
how to you calculate the ports displacement cause i think im doing it to literally being that its not an entirely closed off area.
rady449 
Member - Posts: 15
Member spacespace
Joined: January 30, 2011
Location: Wisconsin, United States
Posted: April 13, 2013 at 6:13 PM / IP Logged  
ok so ive been playing with the calculators and came up with a port using the method u suggested trying to narrow down the port displacement/length ratio.
gross volume= 10.09902
sub dis= .3
bracing dis= .1822+
port dis= 1.1066
port int. dim.= 22.3125"w x 4"h x 16.75"d tunes at very close to 34hertz.
if someone reads this and can plug this into winsd or something similar and tell me what it says that would be cool cause ive tried and failed to get any real results.
soundnsecurity 
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Gold spaceThis member has made a donation to the12volt.com. Click here for more info.spacespace
Joined: November 10, 2008
Location: Louisiana, United States
Posted: April 13, 2013 at 7:00 PM / IP Logged  
port displacement is the same as its volume. a slot port is LxWxH including the thickness of the material. a round port would 3.14 x (the radius squared) x length. this will give you a port volume that you can subtract from the box.

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