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Port Tuning

Capt Odyssea

Capt Odyssea

In Yamalibrium
Was thinking, if one had a ported speaker box, a fitted a sliding inner tube that could be adjusted.........
air tight, but adjusted from the outside.
The sliding portion would be external to the box volume, with a fixed port intrusion into the box volume.
One arriving at a certain determined overall length,
it could all be internalized.
However, how does the internal box volume changing once the set port length is arrived at externally change
things once put inside the box?
Make sense? Is reduction of box volume coupled with a longer internal port counter productive?
What about port diameter? same approach, reduce the port volume incrementally?
Am I off my Rock?
Current port is 4"dia x 11 1/4 internal currently. Volume of box is 3 cu ft.
Aka should the port volume (cylinder) should be subtracted from the overall volume?
Idle curiosity, to be followed by fabrication based on feedback !
 
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You can sim a port dia/length for tuning as you've described and that final port volume is counted against the enclosure volume.
 
A Helmholtz resonator (which is what a bass reflex/ported loudspeaker enclosure is) doesn't care whether the "duct" part of the port's inside or outside; the mass of air in the "port" (which is proportional to the internal volume of the port) is the important variable.
i.e., it won't matter if you slide it in or out; you have to change the length, or the diameter (or the density of the air).

You could -- sort of -- do something with a telescoping port (nested set of tubes) but it would be a bit nonideal in that the diameter of the port (and thus the volume in the port) would change as a function of the 'degree of telescoping'. In other words, translating the volume of the "ideal" port to a single, fixed port would be a little tricky. Not impossible; but tricky. Math would be involved.

Although... there's always the old go-to lazy way to determine the volume of an irregular vessel -- with water (and a graduated cylinder). :)
 
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Ok, that's scary!
Yes, telescoping, with the 11" x 4" internal, probably make a hardish foam tube to fit within to increase the port length.
I could also use the hard foam to line the 11 x 4" port to reduce its diameter.
Which then begs the question, what effect will foam have as opposed to a hard surface.
Think pool noodle foam. 1/2 thick.
 
Meanwhile i've got two other speaker projects lined up, with drivers etc on hand. Emptying the garage one project at a time!
Another sub to go with the speakers I'm asking about here, A Decware Wicked one w two 10" alpine SWS dual vc drivers.
and I got a 4 channel car amp to split the rear channels on my truck into 4 channels, two woofer channels and two
coaxial channels.
Can you tell i'm bored?
I've got two 8" JL Audio woofers, the tweeters died, and two kenwood 6" coaxials.
Amp has built in crossovers etc.
 
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