90 degree angles in PVC? (1 Viewer)

[Andrew S]

I'm making a 4.5 cubic foot Tempest sub for my car. I have my port sizes and tuning all figured out, I just have one question. My box requires me to use 90 degree elbows in my 4" PVC, and I'm not sure how to incorporate these elbows into my specified port length. My port length is supposed to be 12.5" long, but how does that work with my elbows? Anybody have any formulas or tricks to calcuting the length of 90 degree elbows in PVC?
Thanks,
Andrew.

 

[Nathan_M]

This may seem crazy, but I've been thinking about using water to figure out the effect of bends in a port. What about getting a straight port the desired length for a given tuning and then fill it with water. Then you could take a port with the 90 degree angles in it and trim the straight portions of the port the necessary length to get the exact same amount of water to fill it.

I'm sure an expert in this will come in and say why this is wrong, but it's an idea.

 

[Andrew S]

That's exactly was I was thinking, I was just waiting for someone to jump in and verify it. I've already figured out the volume of my 4" x 12.5" port (about 2.5L), so I would just seal one end of the port and fill it with 2.5 L of water and cut it at the length it fills. If anyone sees a flaw in this, please point it out.
Thanks for the response.
Andrew

 

[Dan Hine]

The length of the port is measured by the length of the center of the port. I used a seamsters measuring tape and measured from the middle of one end, around the bend, to the middle of the other end. But don't forget that the straight pieces "push in" to the elbow. For instance, my elbow pieces had a length of 8 inches and I needed 18" ports. At first you might think that I would need two 5" pieces. But the straight parts push in 1.5" on each side so I cut 7.5" pieces.
Did I make any sense? If so, I hope it helps.
Dan

 

[Andrew S]

But what I wonder is how you got the seamsters measuring tape to be in the middle of the pipe at the bend? Would it not be held against the wall of the pipe at the bend ? Sorry if I'm slow at this.

Say this crude diagram is the pipe, where p is the wall of the PVC. Would the measuring tape not hug the curve as illustrated giving you an inacurate measurement of the length if it supposed to be in the centre (Tape being "t")?

PPPPPPPPPPPPPPPPPPP
P
P tttttttttttttt
P tPPPPPPPPPPPPP
P tP
P tP
P tP
P tP
P tP

Sorry for the awful drawing.. just trying illustrate this as best as I can

 

[Brian Bunge]

Measure the length of the pipe on the inner end of the elbow and then again on the outer end of the elbow and then split the difference.

Brian

 

[Andrew Pratt]

My first sonosub used a bent elbow and I measured it on the outside of the pipe with a peice of string then measured the string. That said if you're out an ince or so it won't matter all that much. One peice of advice is to make sure you glue the elbow in very well...I thought mine was glued in tight but over time it came loose and started ratteling.

 

[Andrew S]

Thank you very much for the responses. I've posted this many times before on a couple forums but never seemed to get much of a response, I appreciate it very much.
Also, it took some searching but I found another post asking this same question and he came up with a pretty clever and precise method. Four 90 degree elbows of pvc placed end to end would form a circle. So to find the length of the middle of the elbow, you would simply find the circumference of this imaginary circle and divide it by four, except your radius would be from the middle of the circle to the exact middle of the the pvc.
Here is the original post with the diagram:
http://www.hometheaterforum.com/htfo...=circumference
Again, thanks for all the replies. I figure I'll use one method and then try another to verify.
Andrew

 

[Will Pomeroy]

Why wouldn't you just use calculus? Everybody loooooves calculus....

 

[Dave Poehlman]

Get a piece of straight PVC pipe to the length you need, plus the width of your miter saw blade.

Cut the pipe at 45°, turn one end around 180° and glue it with some serious glue to make a 90° bend. It's easier measuring than using an elbow. Otherwise, the water idea would work "swimmingly".