port elbow

[WarrenP]

Hey folks, I'm finally getting around to building my subs and have a question for you.

I need a six inch port 24" long. I'd rather not put the port on the top of the speaker, so I'll need an elbow. How do you determine the space/length of the elbow? If I used (just for conversation) two 12" sections, once they are connected with the elbow that is longer than 24".

Suggestions on how to account for the elbow of the port?

Thanks.

[collo]

The length added by the elbow is best measured though the centreline.

Lay the elbow on it's side and use a tape measure to find the length. Subtract the amount by which the straight sections intrude into the elbow and you should be left with your answer - shown as the dotted line in this drawing...

[Chrisbee]

Or, find a round object (plate? saucer?) whose outside edge exactly matches the middle of the curve of the bend when laid on top of the bend.

Measure the diameter of the round object and call it D.

The length of the curve in the bend to add to your straight sections should be Pi x D/4 (Pi = 3.142)

[Blaser]

How does this affect tune and noise?

[collo]

The tuning is worked out purely on length (with any adjustment for flares)

Any turn in a port has the potential to cause turbulence. This will depend on airspeed and minimum radius of the turn. See the "port airspeed" thread for more info on airspeed.

An AES paper suggests that the minimum turn radius be at least 20% of the port diameter. In practice, you are limited to what's available from the plumbing section of your hardware store.

Sometimes you will find elbows where the inside of the turn is a sharp 90 degrees. In that case it is worth going to an actual plumbers supplier to see if they have a smoother elbow.

If you're forced to use the sharp type, do at least sand off the sharp edge.

[Chrisbee]

In the 4" & 6" PVC sizes there are some very slow standard bends available.

These tend to be sold through the professional builders outlets rather than the DIY stores aimed more at the amateur market.

I suppose it varies a lot from country to country.

A local drainage contractor would be a good place to ask if you can't find what you need elsewhere. They often have masses of bends and pipe offcuts in a container saved up for a rainy day.

Don't use anything that has already seen sewage though. Yeuch! Check for internal discoloration before parting with any money. A bit of external bleaching from the sun is no problem though.

There are also flexible drainage pipes available (with and without a smooth lining) in sizes up to well beyond that needed for the largest imaginable LLT.

[WarrenP]

Thanks for your replies everyone.