Bass port alignment questions

[brianpowers27]

I am constructing a new box for an old car sub that I have lying around. Using the specs I came up with a 70l box using a 17" x4" PVC pipe for a tuning frequency of 26.5hz. I plan on stuffing the tube for music use. The 70l box will yield a qtc of .55.

• WHat effect will the positioning of the port have on the sound.
• Does port position affect phasing?
• Can part of the port extend outside of the box?
• What will happen if parts of the tube are actually touching an outside wall of the box?

[j_dunavin]

what sub are you using?!
Anyway i am unsure about the possition, but i do know that you do not want the side of the port to A) touch the inside of the box and B) you want at least the dia. of the port, keep that distance away from the end of the port from the back of the box wall. So if you have a 4" dia port, it should be at least 4" away form walls. ( That's according to the box design guy at Boston )
I do belive that you can have part of the port sticking outside the box.
You could also make a slot port. We made them all the time, it does have an effect on tuning, since it is part of the outside wall, but it also doubles as box bracing, and is pretty easy to construct. Also allows for more flexabilty as you could wrap the port along two or more sides of the box.

[brianpowers27]

Just to clarify... Do you mean that no part of the tube should touch an outside wall or just the interior opening of the port?

[j_dunavin]

Lets just say that you had a 90 deg turn in your port.... I don't think it matters if the elbow would touch the inside of the box, but the opening should stay clear of the walls.

[Dirk]

• WHat effect will the positioning of the port have on the sound.
• Does port position affect phasing?
• Can part of the port extend outside of the box?
• What will happen if parts of the tube are actually touching an outside wall of the box?

As long as the exit of the port is anywhere near the woofer, there will be no affect on the sound, phase included. Remember the length of the waves involved here. A 30hz wave is almost 38 feet long. Even if the port exited the driver 4' away, it wouldn't be far enough away to be of any consequence.

Part, or all, of the port may exist outside the box. In fact, we always assume the port is outside the box (which is why you are supposed to account for the volume of the port tube when sizing your box). It could easily be a chimney! Hrm...you could even fashion it out of a chrome exhaust tip (watch for that in Wolf's next ported subwoofer build).

On the final point, I know there are potential interactions when the port is placed near or against a box wall. I believe that the wall of the box acts as if you were using a port tube where one side was longer than the other. It will probably drop Fb slightly. How much, I don't know. I am unaware of any emperical work done to really say how much it screws with things. Then again, people often do slot ports (which run along the edge of the box) and PE sells a triangular port meant for mounting in the corner of a box, so regarless, I don't think it's really a show-stopper. Just be prepared to (or at least have a method to) shorten the port tube. Don't glue things in hard before you have a chance to test.

[the kid]

ROTH:

Anything touching a wall can cause vibrations and noises. If you must run your port this way, be very sure to secure it tightly & in more than one spot.
Not recommended if at all possible

The port can stick out of the box. Beware of breakage.

You should have at least as much distance from the inside **** edge of the port to a wall as the diameter of the port. More is always a plus.

The port should not be installed directly behind a driver. You need to move it offside of the driver or install on the front baffle.

Make sure you have a sufficient diameter port for the size woofer you will use.

Do not stuff as to be blocking the port.

[j_dunavin]

i was thinking the end of the port within 2 in. of the back wall... we had a guy build a box at work, that built a port really close to the back wall and the tuning was way high.
I wonder if some woffers are more subsetibale to this than others? I also had a Boston G2 12" that the port was 4" dia, and it was within 2" of the back wall. It also made the tuning freq. a little high.
Would be a cool project to find out what effect it really does have.

[dthomas]

I am constructing a new box for an old car sub that I have lying around. Using the specs I came up with a 70l box using a 17" x4" PVC pipe for a tuning frequency of 26.5hz. I plan on stuffing the tube for music use. The 70l box will yield a qtc of .55.

• WHat effect will the positioning of the port have on the sound.
• Does port position affect phasing?
• Can part of the port extend outside of the box?
• What will happen if parts of the tube are actually touching an outside wall of the box?

Like it has been said here port position is of little consequence front, back, side or bottom all work fine and phase at these wavelengths are not a real issue. If you want more reinforcement placing it on the rear can be useful.

You should always try to keep the tube at least its diameter away from internal walls etc... so for a 3" port keep it 3" away from internal walls for it to work optimally. I have broken this rule a few times out of necessity for instance a port is placed very close to one side wall resulted in no real consequence and was verified with an impedance sweep that showed the tuning to be right where it was expected. As long as you have a clear opening to the entrance of the port that is equal to or greater than its diameter you should be ok.