Port Diameter Question


diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Vd is the volume displaced by the driver. It is the area of the driver times the excursion. If you are going to calculate it using metric measurements, remember: area of the driver is given in CENTIMETERS, the excursion is given in MILLIMETERS, (one tenth of a centimeter. Be sure to convert your excursion to centimeters before calculating).

We run into the ever present problem of calculating excursion from center to forward,, or from all the way back to all the way forward. I calculated fro mthe center. Your speaker's excursion is 12 mm, or about half an inch.

In this case, the Vd is 369 cubic centimeters, or 59 cubic inches. Counting the excursion from the center, not peak to peak.

Yes, I think 3 inches is okay. I used to have a formula, but drat!, I lost it. But 3 inches is usually good for a 10 inch woofer.

PS: Oops! Make that 24 cubic inches Vd measuring excursion from center. Forgot to divide by 2.5 that last time.

[Edited by kelticwizard on 11-03-2001 at 12:14 PM]
Port Size

Actually the size of the port is variable, but it will depend on the size of the enclosure in relation to the values of the driver. I can't give you a math formula, but go to your program and plug in the values. I use Bass Box, so yours may be different. After I determine the enclosure size, I put in the the desired port diameter and the computer determines the length. But keep this in mind. The amount or wave pressure for the proper tune is fixed. The diameter and length can vary. For example, if a 2" diameter pipe is used, and it calls for a 5" port length, then a 3" Diameter pipe will be much longer, such as 8.5". Also, a 1" diameter tube would be less than 5", more like 2.5". You just have to adjust the parameters to the enclosure's inside dimensions.
Suppose you have an enclosure that is only 8" from front to rear and you want to use a 3" diameter pipe. If the figured length of the pipe calls for a 7" long tube, you are in trouble, because the opening of the pipe cannot be that close to the wall or it will not function as desired. You must either turn the tube with an elbow or decrease the diameter of the pipe. My advice is for you to make the pipe diameter smaller and go for a less sexy look. Which brings up something else.
Unless you are going to use the enclosures as a true bookshelf speaker, I suggest you place the port to the rear. This way you will not have the midrange frequencies muddled by the waves exiting the tube and mixing with the front cone. After all, the port is for low frequencies, which will be nondirectional and can reflect off the wall