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 7th December 2004, 06:22 PM #1 diyAudio Member     Join Date: Nov 2004 Location: Oklahoma Port question? Hello all, My question concerns a 3' dia. port by 12" long. Flared at both ends. I am rebuilding the cab for a existing sub. Everything considered I wound up being 2ci^3 larger. Easily fixed. My problem is with the port. Because of re-shaping the cab, I have had to put a 45dg. elbow in the port. I know this changes things, but should the port legnth be longer, shorter or stay about the same? Any help will be greatly appreciated. Thanks
 7th December 2004, 07:51 PM #2 diyAudio Member     Join Date: Feb 2004 Location: Stockholm It is not trivial to calculate the effective length of a bent tube, but I have seen people use the centre of the tube as an approximation. My suggestion is that you start with this length, then measure and adjust accordingly. __________________ Simulate loudspeakers: Basta! Simulate the baffle step: The Edge
 8th December 2004, 10:13 PM #3 diyAudio Member     Join Date: Jun 2004 Location: Illinois Bro what's your box size and tuning and what size pipe are you using, how much air volume does the bend contain? __________________ The golden rule of DIY: Build nice, or build twice!
 8th December 2004, 11:05 PM #4 diyAudio Member     Join Date: Nov 2004 Location: Oklahoma The cab is 1.8f^3. The elbow is 3" as is the port. The part of the bend that is the shortest is 3/8" and the longest is 2.75" Both ends are flared, and I figured, like Svante suggested above, that if I measured from the middle of the port to the middle of the other end I would be pretty dead on, I hope.
 9th December 2004, 03:38 PM #5 diyAudio Member     Join Date: Jun 2004 Location: Illinois I have an idea for you, tape over one end of the bend using some electrical tape, make sure its sealed completely, no leaks. Then from the other end fill it with water and use a measuring cup to determine the amount of water, You can convert most fluid measurements to mL aka cubic centimeter and there you know the exact volume inside the bend. The port's tuning are based on three things, Box volume, Port volume, Port cross section. If you have two boxes of the same volume which both have ports of the same cross section and air volume then they will be tuned identically regardless if one has no bends in the port and the other has one or more. This is because the air inside the box acts as a spring. That spring is attached to a mass, which is the air in the port. The effective mass of the air in the port is determined by its cross section and length or volume. Determine the volume inside the bend. You didnt give me a tuning frequency, but since you said 1.8 cubic feet with a 12 inch long double flared port then it would be right around 27hz. This gives you 1384.7cm^3 of total port volume. Take whatever the bend's volume is, subtract from 1384.7 and you will have the volume you need to be in the rest of the port. divide by 45.4 and that will be the length. Here's a formula: X=Bend's volume in cm^3 (1384.7 - X)/45.4 = length needed for remaining port in cm __________________ The golden rule of DIY: Build nice, or build twice!
diyAudio Member

Join Date: Feb 2004
Location: Stockholm
Quote:
 Originally posted by BassAwdyO I have an idea for you, tape over one end of the bend using some electrical tape, make sure its sealed completely, no leaks.
This method might lead you to a close guess, but it is still not exact.
__________________
Simulate loudspeakers: Basta!
Simulate the baffle step: The Edge

 10th December 2004, 12:21 AM #7 diyAudio Member     Join Date: Jun 2004 Location: Illinois it is so close that it would never matter bro __________________ The golden rule of DIY: Build nice, or build twice!

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