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Ported Subwoofer Box Design Formulas & Calculator

Butterworth Fourth-Order Vented Speaker Enclosure Design Formulas & Calculator

Explanation of Terms
 Qts, Vas, and Fs, are some of the electro-mechanical parameters for your woofer. They are also commonly referred to as the "Thiele-Small" parameters. You can get these from the paperwork that came with your speakers, from the dealer where you purchased your speakers, or from the manufacturer of the speaker (they are sometimes hard to contact, however).
Vb - the required internal volume of the enclosure.
F3 - the frequency that the response is down by 3 dB at.
Fb - the frequency that the vent needs to be tuned to.
Enclosure Design Formulas
Vented Enclosure Image
Typical Vented Enclosure Design  
Vb = Qts 2.87 x 15 x Vas
Fb = Qts -0.9 x 0.42 x Fs
F3 = Qts -1.4 x 0.26 x Fs

Calculate Your Enclosure Here
 To use this form, enter the Qts, Vas, and Fs parameters for your speaker in the boxes below. Then click the "Calculate Enclosure" button, and the results will appear in the empty text boxes.
Number of Drivers : Isobaric
Yes  No
Enter Qts
Enter Vas  (ft3 )
Enter Fs Hertz
Vb = (ft3 )
F3 Hertz
Fb  Hertz
  If you want to experiment with different quantities of drivers, or the isobaric option, you  don't need to enter all of the values again, just change the value you want to adjust and click the "Calculate Enclosure" button again. When you are finished, write down your enclosure specifications, (or copy & paste them), and go here to design your port.


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