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Old 28th December 2001, 10:15 PM   #1
pkgum is offline pkgum  Australia
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Default port design

Is it possible in a ported box to have a driver forward firing and a port that exits in the top side of the enclosure?
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Old 29th December 2001, 03:32 AM   #2
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As far as I know you can. Its not optimal for air flow, however, and can become worse depending on how your bracing is positioned.
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Old 29th December 2001, 03:40 AM   #3
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Also phase response will be less than optimal.
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Old 29th December 2001, 09:47 AM   #4
pkgum is offline pkgum  Australia
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alrite...then i won't put the port at the top

thx
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Old 29th December 2001, 08:23 PM   #5
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Gum:

I don't think it makes a whole lot of difference, especially if you plan on tuning at 18-20 Hz or so.

Most people either put the port on the front, so there will be little space difference where the port output "crosses over" to the cone output, or on the back, low, so the port output is closest both to the floor and the wall-asuming you put it against the wall.

Theoretically, the space difference between the port and cone can cause interference problems.

If you tune the box to 20 Hz or below, there will be little port output above 40 Hz. The wavelength of 40 Hz is 27.5 feet.

Generally, a separation of a quarter wavelenth or less-in this case about 7 feet-causes few, if any, problems. The interference factor declines until by the time you hit one tenth of wavelength or less-in this case 2.75 feet-there are no problems at all, theoretical or otherwise.

I like to put the port low and near the wall, but that is just me. If your design or aesthetics dictate a top firing port-go to it.

Here is a link to a review of a speaker that has the passive radiator-very similar in function to a port-on top of the speaker. Doesn't seem to cause any problems.
http://www.hometheaterhifi.com/volume_2_3/v2n3e.html
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Old 30th December 2001, 09:51 AM   #6
pkgum is offline pkgum  Australia
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hey..

After tuning a 200L box to 18Hz with a 6 inch port...

why do all 3 prgrams predict different port lengths?

unibox says 74.5cm
(no damping or leaks/basic port with no special ends)

speakersimulator 2.0 says 73.4cm

LspCAD lite says 72.7cm

Driver t/s parameters aren't necessary, so don't ask for them
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Old 30th December 2001, 04:25 PM   #7
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Air has different densities at various points above sea level. Air has different densities if it is raining out or sunny. If one program is written by a guy on a nice day in Denver, and the other written by a guy on a rainy day in New York, there's your difference right there.

Run each of those lengths through one program-it doesn't matter which one. You will find that the difference between the lengths make much less than one half of 1 Hertz!!

Nothing to worry about.
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Old 30th December 2001, 08:14 PM   #8
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Considering that the tolerances on many drivers (you'll notice that they're rarely specified, even for the expensive ones...) are as much as +-20%, it's not worth getting too worried about fractional differences in the cabinetry.
Unless you intend to measure the drivers and mate the cabinet to that <i>specific</i> driver, of course, but then you've got to buy in quantity so as to have enough drivers to match from channel to channel. Then you're left with a bunch of spare drivers and no project to put them in.

Grey
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Old 31st December 2001, 05:57 AM   #9
pkgum is offline pkgum  Australia
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is it alright for a ported box to have a very high F3 frequency?(60-70Hz)
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Old 31st December 2001, 03:00 PM   #10
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Sure, thats not all too uncommon among smaller bookshelf speakers using 5 1/4 inch or smaller woofers.
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