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Discussion Starter #1
Hi

I am just learning about ports here and at http://www.subwoofer-builder.com
Based on the port’s diameter, can anyone suggest a sensible ratio or guideline for the minimum radius for bends in the port?

The project:
I’m going to use two boxes I’ve had cut, internally 93 litres/ 3.3 cubic feet, as 3 foot/ 900 mm long coffee table EBS boxes (for Peerless XLS 12’s) tuned to 15 Hz. Not the ultimate low and loud, but I have the drivers and the wood cut and I’m happy with the modelled response . .
(I figure that if four ports are used (eg each of 5 cm diameter ie 2”) instead of one big vent, the bass output is spread more evenly throughout the room than, for a slightly more even bass response).
The port volume is about one third the box volume, and each port needs one turn.

Inspired by Collo, I’m likely to use external ports, maybe something like the exhaust manifolds (“extractors”) on a hotted up car engine, two a side; and/ or for more even bass output, two facing one direction and two the other way.

Anyhow, each port needs two turns: might there be a minimum radius for bends in a port?
Or as 15 Hz wavelengths are so large . . won’t it matter, so eg for ease of build, use plumbers PVC pipe and matching small radius pre-fab PVC bends?

Thanks
 

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Discussion Starter #3
Thanks Mike, so radius isn’t an issue
I didn’t know about the differences in wall thickness . .

Cheers
 

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Discussion Starter #5
Thanks Mike, a picture tells a 1000 words
I live in the same country as Collo, so that could be handy

Cheers
 

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"Thanks Mike, so radius isn’t an issue"

The link didn't work for me so I can't see what's being referred to, but it's definitely an issue.

Too small of a radius and/or too small of a diameter and the air just slams into the wall of the turn instead of flow smoothly around, creating turbulence, noise, and loss of output.
 

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You'll find that there are two types of 90 degree (and all other radius) bends available from plambers supply stores. Some have an edge (sharp feeling) on the inside of the bend. The others have a smooth curve for the inside and the outside. I haven't tried the ones with a corner but wouldn't use them as I'm certain it would create turbulence. If your not using the stock ones available I can see a fair amount of work heading your way but there's no reason to not make a mould plug of your ports then fibreglas around them or pour concrete around them.

Personally would avoid bends in the ports alltogether and try to keep them under 4 times their diameter.
 

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I haven't tried the ones with a corner but wouldn't use them as I'm certain it would create turbulence.
An obstacle much less than a wavelength in dimension is acoustically invisible to a passing wave, so with the fifteen foot plus wavelengths passing through ducts turbulence is not an issue. Bends are also acoustically invisible to low frequencies.
 
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