Vent Placement
How Much Time is Needed to Make a Subwoofer Box Design? Imagine how much time you will need to calculate the length of a port by its area and the net volume of a box, then calculate the occupied volume by the port, add it to the net internal volume of a box, and only after that, calculate the dimensions of a box, taking into account the material thickness. Vented speaker enclosures require port tubes, aka port vents or flared vents.
Best acoustic coupling is accomplished when the port(s) are installed on the same side as the woofer(s). If this is not possible, installing ports on the side or back of the enclosure are good alternatives. Do not block the opening of any port(s) and always position multiple ports on the same side of the enclosure. For optimum performance, keep the port one vent-diameter away from the back and sides of the enclosure. Obstructing the output of the vent may change the tuning frequency and increase port turbulence (noise).If small enclosure design prohibits you from placing ports in their optimum location, do the best you can. Remember, the important guideline is not to block the openings of the port(s) and to position multiple ports on the same side. This applies to all common types of vents (circular, rectangular, square, Aeroport, etc..)
Long Vents
If the box design requires a bend or 'elbow' in the port, always measure down the 'middle' of the vent to determine the correct cut length.
'Rounding-Off' Slot Vents
Some enclosure designs are based upon 'slot' vents. The inherent design of these vents utilize some of the enclosure walls as part of the vent length. To minimize turbulance at the ends of the vent, we recommend to 'round-off' the inside edge of the interior part and all edges of the external parts to equlize air flow accross the thickness of the wood (see diagram.) If your enclosure is located in a trunk pressed against the back seat with the woofers facing the tail-lights, it is common to 'fire' the ports up through the rear deck. We do not recommend modifying the rear deck by cutting sheetmetal to accomodate these ports. Also, do not install the port(s) on the back due to air-restrictions from the rear seats.
Formula for Calculating Vent
| 1.463 x 107 x r2 | |||
| Lv | = | --------------- | -1.463r |
| fb2 x 1728 x Vb |
Vb = volume of box (cubic feet)r2
= radius of port (inches)fb = desired tuning frequency of enclosure (Hertz)
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Lv
Seaport Design Guidelines
= length of vent (inches)