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port size [message #52071] Fri, 25 April 2008 08:09 Go to next message
David Morehouse is currently offline  David Morehouse
Messages: 22
Registered: May 2009
Chancellor
my delta 12lf's are in 3.5 cu.ft. boxes,with 2-4in. ports 6.25 in. each in length. 40-45 hz. These are no where near the port size Wayne uses. Why such a large difference? Should I retune?

Helmholtz formula [message #52073 is a reply to message #52071] Fri, 25 April 2008 11:36 Go to previous message
Wayne Parham is currently offline  Wayne Parham
Messages: 17580
Registered: January 2001
Illuminati (33rd Degree)

Check out the Helmholtz formula below. You can tune to the same frequency using different port sizes. As port area is increased, port length also has to be increased to tune to the same frequency. When in doubt, measure impedance and look for the minimum between the peaks.
http://www.pispeakers.com/Helmholtz.gif
Here's a little BASIC program that will calculate the formulas for you:

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

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