This section outlines some
information you will need to manually design your enclosure if you are using a scientific
calculator, or if you are interested in the formulas involved. You will need the
Thiele/Small specifications for the driver you want to configure, and at least a
rudimentary knowledge of math. Check out the Software section, which lists some free programs to download for
designing your enclosure.
Note: Unless otherwise specified, all formulas use English measurements. ^ = exponent, for example 10^3
To help you design the correct enclosure for the driver you are using, first find the Efficiency Bandwidth Product of the driver:
For more information about EBP and designing sealed and ported enclosures, check out Enclosure Dilemma: Ported vs. Sealed.
You may substitute any Qtc between 0.50 and 1.50 in place of 0.70 in both equations (both must have same value) to experiment with enclosure size. Qtc of 0.70 is generally considered an optimum alignment, with very good transient response, low cut-off frequency, and flattest response to the cut-off - See Qtc.
Find alpha: Then calculate enclosure volume: System resonant frequency: To find the theoretical cut-off frequency,
use the following chart to find the
Then:
These formulas were engineered by D.B. Keele Jr. using the vented enclosure alignments developed by A.N. Thiele. Enclosure volume: Theoretical cut-off
frequency: Tuning frequency: If an ideal box is to large for your application, choose an enclosure size in cu.ft. then Find: New tuning frequency:
Determine Vr with above formula for a sealed enclosure. The 4th order bandpass design is optimum with a total system Q of 0.70, using a Qtc and S of 0.70 for Vr (sealed portion) and Vf (ported portion).
Vf The system F3 for a properly designed bandpass will be lower than a similarly damped sealed enclosure. Depending on the System Q and tuning, F3 can be up to approximately 1/3 octave lower (or more) if system Q parameters are the same for both types of enclosures using identical drivers. Bandpass enclosures are best designed with a good software program, as tuning and enclosure changes can produce endless alignment variations viewable on your pc. Because these enclosures attenuate mid-range frequencies and higher at 12 dB/octave, they are good only for subwoofer duty.
These are general guidelines, and you may use a smaller size port if desired, especially for ported enclosures. Try and use the recommended values if possible, especially for bandpass. Recommended port values are for the minimizing of port turbulence and possible noise.
If you want to use more than one port, you can solve for the equivalent cross-sectional area of the multiple ports that will equal a single, larger port. Two (or more) circular ports of diameter A and B (C, ect.) can be substituted for a single port with the larger diameter X:
You may input as many ports as you wish into the equation. Just take the radius of X and input this into the port length equation. The final length will apply to all ports being used, i.e. if you want to use 2 ports that are 3" in diameter, you will solve for a single equivalent port which would need to be 4.24" in diameter. Once you solve for the length of the 4.24" port, just make both 3" ports the same length to tune the box to Fb. |

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