Use the formula below to calculate the port size. For the Alpha 10's used in two π's, you'll want to tune the box to 40Hz.
Here's a little BASIC program that will calculate the formulas for you, if you can still find an old BASIC interpreter around somewhere.
10 INPUT"Enclosure Volume";VE
20 INPUT"Diameter of Port";PD
30 INPUT"Length of Port";PL
40 VB=VE*1728:PI=3.1415926535:AP=PI*((PD/2)^2):LC=PL+((8*PD)/(3*PI))
50 FR=(13548/(2*PI))*(AP/(VB*LC))^.5
60 PRINT"Fr =";FR;"Hz."
70 GOTO 10
5000 cubic inches is 2.9ft3, which is a nice size for the Alpha 10 driver. You could go with a 4" diameter port that's 4" long or a 3" diameter port that's 2" long.
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