port design

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
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
 

pkgum

Member
2001-10-27 4:56 pm
sydney
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
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
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.
 
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
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Gum:

Your quote: "is it alright for a ported box to have a very high F3 frequency?(60-70Hz)"

In a ported box, the output falls at a huge 18 dB per octave, (usually higher) after the Fb-the frequency the box is tuned to.

Fb is usually, though not necessarily, fairly close to what the F3 is.

Just for the heck of it, what are the Thiele Small parameters of the speaker and what size box, tuned to which frequency, are you planning?
 
kelticwizard

U know what I'm doing,.....its a 230L ported box tuned to 18Hz with hopefully a 7" tube.
Its gotta be abit bigger than thomas's because he tuned his to 18.56Hz
The driver, its a Blueprint 15

it has an F3 of 66Hz yet can do 116Db at 18Hz because theres a little hump of 1Db at 18Hz.
After 30Hz the frequency response rises 3Db/octave to reach MAX output at 120Db at around 110Hz then flattens again


CAN SOMEONE TELL ME HOW TO UPLOAD A GRAPH FROM MY HARD DRIVE TO MY MESSAGE
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Gum:

I assume you know how to save a graphic. If not, it goes like this: Right-click the link for the item you want, and then click Save Target As.

When you save the graphic, make sure it is in the form of: gif, jpg, png, txt, zip, bmp, or jpeg.

When you click on "Reply" and the screen appears to start writing, scroll down to the bottom of the screen. You will find a section named "Attach File" with a little box next to it with a "Browse...." button beside it. Click on the "Browse...." button, and locate the file that has your graphic. Then click "Submit".

I assume you know how to find a file in Windows Explorer.

That's it.

PS: I think they just changed the board to allow you to do this. Another board I go to has the same message board program and they just modified it. Before, apparently you had to get a web page, upload the graphic to your web page, then put the URL on the reply box. What a hassle!