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| By: cplkittle on Sat, 4 Oct 03:42:44 -0500
| | The ported subwoofer design has been around for years with names such as the Hemholtz Resonator and Kentucky Resonator. The design serves the same purpose as the sealed enclosure: to isolate the forward wave from the rearward wave. The difference is in the port: The port takes advantage of, and manipulates the rearward wave, giving either more low-end extension towards the bottom octave or more SPL in the lower midbass region. One can design a ported subwoofer system to extend the low-end of the subwoofer or to enhance the SPL capabilities of the woofer.
The principal behind a ported speaker box is similar to blowing across the top of a coke bottle and hearing it whistle in a low tone.
The loudest tone is produced when the air masses that are suspended within the neck and body of the bottle are set to resonance.
In a ported speaker box, at a certain frequency, the woofer can put the air masses in the tube and the box into a common resonance. When you place a port in a closed box, you "tune" the box to a specific resonant frequency. The air mass in the port vibrates in phase with the cones movement, damping the cones motion. The cone of the driver becomes virtually motionless and the sound comes primarily from the port.
To tune a ported box to a desired frequency, use the following equation:
Fb - is the desired tuning frequency of your enclosure in Hertz.
Lv - is the length of your port in inches.
R - is the inside radius of your vent tube.
Vb - is the internal volume of your enclosure in cubic inches. To convert cubic feet to cubic inches, multiply by 1728.
If you want to use multiple ports, divide your enclosure volume by the number of ports you want to use, then use the result of this calculation as your Vb in the formula below to find out how long each port should be.
LV= [(1.463 x 10^7 x R^2) / (Fb^2 x Vb)] - 1.463xR
If you want to calculate square vents, the formula below will give you the value of R to use in the formula above.
R=square root of ( A / Pi )
In the formula above, A is the area of your square vent (height x width), and (Pi) is approximately 3.141592.
Do not be intimidated by the complexity of the appearance of this equation. It is very easy to use, we all have done this before in junior high. Remember that the radius is half of the diameter of the port opening. Simply plug in the necessary information and you have a very precise port length for the frequency you choose to tune your box. The distance you place the vent from the driver is not an issue. For frequencies ranging from 20-80hz the maximum is around 5’, obviously not an issue in car audio applications. All ports should be rounded at the opening to prevent noise from air flow.
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