原帖由 午夜凶铃 于 21-9-2007 12:59 AM 发表 
真的有那么大的影响吗?不是只有玩SPL的还比较注重吗?
原帖由 smswailoon 于 21-9-2007 01:18 AM 发表
红色的diameter要长或短比较好？那黄色呢？
What does "tuning" of an enclosure or port mean?
Have you ever blown over the top of a bottle and heard the note it produces? Have you ever changed that note by adding or pouring out liquid? A vented loudspeaker enclosure works under a similar principle. When liquid is added to or poured out of a bottle, its internal air volume is changed and the resonant note is raised or lowered. Enclosure tuning is affected by the ratio of air volume in the duct/port (equivalent to a bottle's bottleneck) to the enclosure's interior volume. Tuning of a loudspeaker enclosure is the result of manipulating the difference in effective air mass between the enclosure interior and the air in the duct. The bottle-like nature of a vented enclosure is known as a "Helmholtz Resonator". The vents/ducts in a vented enclosure work only over a narrow band of frequencies near the chosen tuned frequency, producing the same effect as when blowing across a bottleneck.
The port length and diameter (or area) must be tuned for the enclosure volume and speaker that it contains. When choosing diameters, you should follow these general guidelines.
A minimum of 2" diameter for an 8" woofer,
A minimum diameter of 3" for a 10" woofer and
a minimum diameter of 4" for 12" and larger woofers.
If you can use a larger port diameter then the minimumm listings, do so. Larger diameters are less likely to cause port noise. Rounding off the inside edges also will help in regards to that. To Calculate the vent length for a given diameter (D), Box Volume in cubic feet (Vb) and Tuning Frequency (Fb), use the following formula:
Length = 14.630.00 X (D/2) X (D/2) - (1.463 X [D/2])
Fb X Fb X Vb X 1728
Heres an example calculation using a 3" diameter port (D) with a 1.5 cubic foot box (Vb) tuned to 40 Hz (Fb).
Length = 14.630.00 X (3/2) X (3/2) - (1.463 X [3/2])
40 X 40 X 1.5 X 1728
Length = 14.630.00 X 1.5 X 1.5 - (1.463 X 1.5)
40 X 40 X 1.5 X 1728
Length = 32917500 - 2.19
4147200
Length = 7.94 - 2.19
Length = 5.75"
When determining where to place the vent in the enclosure, keep in mind that placing a vent opening too close to the speaker or back box wall may have an effect on how the port works. a good rule of thumb is to try to keep the openings the diameter of the vent away from the wall and twice the port diameter away from the speaker.
Occassionally, you may find that the lenght of the vent won't fit the box design you have in mind. When this happens there several different options available - Allow the vent to stick out, use a smaller diameter, use multiple ports (which will have smaller diameters) or "bend" the port inside the box. If you're using PVC pipe, simply use an elbow to accomplish the bend. Measure the length through the center when determining vent length. Using rectangular vents make it easier to integrate a log vent into the enclosure, because the vent walls can also the walls of the box itself. 
Vent Lengths for 4 Inch Diameter (12.57 Square Inches Area) Vents
 版主，请准许我用英文。。。
[ 本帖最后由 Tenchi 于 21-9-2007 10:11 AM 编辑 ] | 
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