Square Duct Questions For Vented Box

[kelticwizard]

Building a flat box with the Vifa P13WH008. The box volume is 1300 cubic inches, 39L, and the dimensions are 42" high by 8.125" wide by 4" deep. This is really a Mass Loaded Transmission Line.

The selected box tuning is 42 Hz.

The builder wants a really large duct, away from the floor, to prevent vent noise and floor interaction. Toward that end, we decided on a 4" by 2" duct, 13.75" long.

Since the enclosure is only 4" deep, the duct will be a channel bisecting the interior of the box for a certain length. At the top end of the channel is an aperture 4" long by 2" wide. Behind the aperture I anticipate a channel running down the interior for 10 inches or so. The interior opening will be about 6 inches abouve the bottom.

I include a couple of illustrations below.

Two questions.

A) Is there any problem building a duct like this? That is, most ducts use smooth elbows to guide the air smoothly-this one has a sharp right angle from the upward motion of the air to the outside. Shold I kep the sharp right angle in it, like in Fig A, or put in an angle piece to smooth the air flow, like in Fig B.

How long shold the duct be to count as a 13.75" long duct? I am figuring from the center, so it would be 2 inches into the center of the duct from the front of the speaker board, (actually 2.75" if you coant the 0.75" buidling material, but I'm ignoring that right now, plus 11.75" inches from the center of the duct to the open end at the bottom.

In other words, the duct should in fact be 13.75" from the open end to the inside of the top piece. Would that be right?

The duct will be built along an interior wall, (two interior walls, as a matter of fact) wold that afrfect the length requried for tuning to 42 Hz?

Anyway, here are two ilustrations. Sorry for the sloppy drawing. the first is the front view of the speaker as visualized.

[kelticwizard]

And the second drawing. Any advice is appreciated.

[pinkmouse]

Peter, any chance of a side elevation, just to confirm what I think you're doing?

[kelticwizard]

Absolutely.

I had one drawn up, thought I had posted it, but it got messed up and I ended up posting the same view as previous. I had to do another one.

This is the side view, what I originally meant to post.

[pinkmouse]

First thing - I hate square ports!

Secondly, and much more importantly, have you simulated the constriction in the box that will be caused by adding all this wood? I can't see that it would work as a MLTL.

[thalis]

Why do you hate them? They are nice! And they work well.

So,.the duct is counted from its center, as you suggested.
I believe it will also work as you have illustrated it (it is not the best duct but you don't have other choices either), but put this angle piece for sure. The duct should be considerably bigger in its surface to work well without this piece.

[kelticwizard]

Originally posted by pinkmouse
First thing - I hate square ports!

I've never used one. But Thile recommended one in his paper way back when, so I thought I would see how it where it would lead.

Originally posted by pinkmouse
Secondly, and much more importantly, have you simulated the constriction in the box that will be caused by adding all this wood? I can't see that it would work as a MLTL.

I am not sure what you mean. I did compensate for the volume taken up by the port by adding two extra inches to the height. The volume of the duct, including 1/2" building material for the sides, is 165 cu inches. I added 2" onto the height, which gives back around 65 inches, for a net loss of 100cubic inches, which is negligible.

Here is the response chart fo the Vifa in the box which is 40" high instead of 42 inches high-it shold be very close to this.

[kelticwizard]

Originally posted by thalis
Why do you hate them? They are nice! And they work well.

So,.the duct is counted from its center, as you suggested.
I believe it will also work as you have illustrated it (it is not the best duct but you don't have other choices either), but put this angle piece for sure. The duct should be considerably bigger in its surface to work well without this piece.

Thalis:

Thanks for your answer. I always was somewhat suspicious of square or rectangular ports because it would seem to be common sense that corners would be disruptive to smooth airflow. I don't have any proof of this, just guessing.

I am not building this, another member is and I am just assisting him, making suggestions, etc. The simplest way to port this is to simply make a 2" round pipe, 4.75 in long. WinISD gives this design the okay. In fact, WinISD says any round port larger than 1.7 inches in diameter is good, and 2 inches is 40% larger. The straight pipe would vent to the side.

However, the builder seems concerned that the port still might cause port noise, and suggested a port that might also be used as a brace, as well as being larger. So I came up with this design, trying to satisfy both aims.

What I am concerned about is that if you follow the centerline of the port, in order to exit the port it must make a sharp right angle turn. Although having a very large crosssection port might help, if the right angle turn causes possible point noise, I can't see making the cross section large only to hurt the cause with a design that is noisier than a straight pipe of equal cross section.

Which I am concerned this might be, even with that angle piece.

[thalis]

I believe I already answered to this, you won’t have problems with this turn, and he will not hear anything most probably even without this 45 degree angle small wood piece – but you should put it as well. As for the rectangular ports, I have constructed dozens of them and they all worked and measured like a charm. If rectangular shape was measurable disruptive to smooth air flow there would be no building ventilation system with rectangular channels, but they would use pipes instead to save power and energy.

Regards,
Thalis

[kelticwizard]

Thalis:

Thanks. I will do that, using the angle piece.

I was just curious, though, what you meant when you said,

it is not the best duct but you don't have other choices either

.