Help with port dimensions

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Hey there //

I have a 12" Apline type R sub, I've been reading various DIY ported box formulas, including those on By entering my Subs parameters I get Vb=1.912 ft^3, Fb=27.1 Hz

I get a minimum port diameter of 5"? does this seem too large? If I use this value of port diameter then the length has to be 34"??? how do you fit that into a box of only 1.91 ft^3?

I know if I go with a 4" port then the required port length begins to drop, but wont i get too much air noise? should I use a higher Fb then the one that was calculated? say 30 HZ??

Any help to clear this up for would be greatly appreciated, thanks.

thanks for the reply...

my sub is a SWR-1221 and I'm powering it with a Coustic 400 WRMS amp, if that changes anything.

One more question....can a port stick out the side of the box if there isnt room for it inside the box?


Hey thanks again,

I've decided to use a Vb=1.68 ft^3 box (internal volume not including sub & port displacement, I assume thats what Vb is) with a 4" port 22" long Tuned to 28.7 Hz.

Still cant figure out that dam minimum port diameter calculation :blow_up: , but oh well 4" sounds resonable.

My question is now that I have all my dimensions to work with, what is the minimum distance that the port can be placed to a wall inside the box and the Sub?

Is there a minimum distance that the sub must be from the walls as well or is this just a practical measure?

ok thanks for all the help and bearing with me since I'm a nubie at this,

Using WinISD

Volume=1.68 ft^3 (Is this Vb in WinISD?)

Tuning Freq=27.4

4" dia port @ 24.5" long for two free ends (even with an 90deg elbow I think it'll be a tight squeeze) or 24" for one flanged end.

The port air velocity peaks at 110 ft/s ( 33.5 m/s) @ 22.95 Hz with a Max input signal of 400W. My amp puts out 400 Watts RMS so at max power out the air velocity is getting close to 35 m/s.

The SWR-1221 is rated at 300 Watts RMS so air velocity at 300W peaks at 95.3 f/s or 29 m/s @ 22.5 Hz.

You say as long as I stay below 35m/s I shouldnt get alot of air noise, will I notice any at 33m/s? I guess as long as I dont crank the **** out of my system then I wont be near 35m/s and air noise shouldnt be a problem right?

If I'm using a Vb=1.68 ft^3, Vsub=0.128 ft^3 and Vport=0.196 ft^3 then my total (Gross) box volume must be 2.004 ft^3

So this means (H-1.5)*(W-1.5)*(L-1.5)=2.004 ft^3 right? (using 3/4" MDF)

on winISD they dont have any alpine subwoofers? i have two alpine 12 " SWR-1241D

what is similar two them their or what do i do?

i figured out how to creat one but i only know about half the specs

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