Polk DIY power port archives...

[drakonis]

Hi all!,

A while ago there used to be a couple of files
on Polk's site about how to design a power port
(flared duct) Does any one have these files or can
point me to a place where I can get them? (Or explain me
in plain english how to calculate such ports?)

[GRollins]

Drakonis,
I'm not familiar with anything about flared ports from Polk, but was wondering...how flared is flared? My point is that a simple rounding of the edges with a router isn't going to effect the tuning of the port that much. On the other hand, if the flare rate is more drastic, and extends further down the tube, I'd think it would be appropriate to treat it as a horn. Horn math is fairly accessible.
N.B.: The one thing that horn afficionados never get around to admitting is that horn loudspeaker distortion gets really, really grotesque if you go below the low frequency cutoff of the horn itself. I'm quoting from memory here, but I think the low frequency cutoff for a horn is something like
width of horn at mouth= 1/4 wavelength of cutoff frequency
This may or may not help you, as ports are usually small enough in width or diameter that it may not be appropriate to treat them as horns.
What is your goal? To reduce turbulence? Increase effiency? Extend low frequency response? (Yes, yes, yes...I know...all of the above.)

Grey

[drakonis]

Grey,

Thanks for having the time to read and answer my post. I agree with you about horns...

My goal about these flared ports is to reduce the port length, so this port can be managed to fit into a small box.

After my post I still did some research and I was able to locate, again, the patents referring to this power port, you might want to take a look at patents:

US05475764 and US05517573

Just point your browser to 'www.delphion.com' patent server.

The basic idea is to use the walls of the baffle as part as the duct itself...

But... I remember that Polk himself published a couple of years ago a set of 'ready to use' spread sheets to calculate the power ports and a novel bandpass speaker. Unfortunately, I lost such files...

-MKT-

[GRollins]

drakonis,
I didn't read the text of the patents, but I glanced at the diagrams. Two things occur to me:
1) If you're willing to have something sticking out of your cabinet like that, simply extend the port. There's no law that says that a port must end at the surface of the speaker cabinet. All you're doing is creating a Helmholtz resonator with the same (or nearly the same) frequency as the Fs of the woofer. Wine bottles resonate quite nicely, in spite of the fact that their 'port' is external to the resonant cavity. (Although people are prone to looking at you a bit oddly for playing tunes on wine bottles...they're more forgiving if it's a beer bottle, for some reason.)
2)Another possibility is to run the tube from top to bottom in the cabinet. (We're assuming that the cabinet is taller than it is deep or wide.) Granted, it's more common to see ports in the front or back--once in a while on the side. Dare to be different. Be the first on your block to vent your port out the top! Should you decide to vent out the bottom, remember that the floor will load the port. Adjust the length of speaker feet as needed.

Grey