Port tuning help...again

[Tom Loizeaux]

I'm trying to get some small cabinets tuned to 44Hz for a friend.
The cabinets are 3.5 cu.ft. inside.
JBL says make the ports 14 sq. in. (I assume 3/4" deep) for using the K140, etc. but don't say what the tuning freq. will be.

Can anyone tell me the port size (round) and length to get this 3.5 cu.ft. box to tune at 44Hz?

Thanks in advance.

Tom

[Zilch]

BB6P says one 6" dia. X 6.5" long, or two 4" dia. X 6.375", with typical box damping.

If you want the tuning absolutely on 44 Hz, you'll have to build and measure the actual. The ports may have to be shortened a bit from these figures....

[duaneage]

can you do a shelf port?

if your going for a 3/4 inch port I assume you are merely opening a hole in the cab and tuning that. If that is the case you could make a rectangular hole 2.75 x 2.5 inches in the cab but be warned, it is going to whistle. I would make it irregularly shaped to try and eliminate the noise.

Call me if you need help, PM me if you forgot my number.

[Tom Loizeaux]

Thanks for the replies guys.
I'm working with existing cabinets so the baffle size is a limitation. With the 15 mounted and allowing enough room (about 2") on all four sides for the grille frame, I can fit one tube 4 to maybe 4.5" in dia.
Zilch's suggestions don't quite fit, and Duaneage's suggestion of a hole in the baffle could be improved on, I believe, by using a port tube.
Attached is a photo showing the baffle with layout lines.

Tom

[whizzer]

I can't make out the layout lines very well in the photo, but it looks to me that if the penciled-in woofer circle is centered, there ought to be room for a second 4" tube situated to the opposite side exactly like the one pictured--assuming that's a 4" tube, and, as mentioned above, two at 6.375" in length; this would probably result in a very significant reduction in port air speed and eliminate the noise. You could, if need be, let the ports open to the rear, although that might not be a good idea given the nature of the intended application, but if they're to be used in a non-PA application, the rear opening ports might be a good thing, using the wall/floor boundary effect for LF reinforcement.

[Robh3606]

There is nothing that says the ports have to be front or back. You could also use the sides as well. Basically where ever it fits and works for the intended application.

Rob

[Tom Loizeaux]

I went back and highlighted that photo to show the grille frame lines and speaker cutout, ...but in that form it wouldn't load!
So, in the original photo: the speaker cutout is visable, the 4" dia. port tube in in the only place it can go and just clears the 2" border which will be covered by the grille frame. I could make the grille frame more narrow, but no enough to allow a second 4" port.

Ideas or port length suggestions?

Tom

[Zilch]

Cut arcuate notches in the grille frame to clear the ports, and even the woofer, if necessary, to make the two 4" ports fit.

JBL does it all the time, shaping the interior edges of the grille frames.

If that's a flared end on the 4" duct I'm seeing, lose the flare and just use straight.

If 4" absolutely won't fit, what size pair will?

[Tom Loizeaux]

...
If 4" absolutely won't fit, what size pair will?

Hey, I just checked and I can fit one 4" dia. AND one 3" dia. tube into these baffles at the same time. Both are presently 4" long...
so, what length should these be when working together in this cabinet to reach the 44Hz tuning?

Thanks again guys!

Tom

[Zilch]

Two 3" ports @ 2.7" = red,
Two 4" ports @ 6.375" = black.

I don't think I'd mix port sizes, and I also don't know how to calculate them.

I'd make the 4" pair fit.

Reduce the grille frame width to 1.5", maybe....

[Baron030]

I don't think I'd mix port sizes, and I also don't know how to calculate them.

Well, let me see, using the "pie r squared" formula.
The calculated the area of a 3" circle is about 7.0685 sq. inches.
And the calculated area of a 4" circle is about 12.5664 sq. inches.
Summing the two together is equal to about 19.6350 sq. inches.
Then taking 19.6350 sq. inches / pie = 6.25.
Then taking the square root of 6.25, which is equal to 2.5.
So, a 3" port and a 4" port used together will have the same area as a single 5.0" port.
After plugging in 3.5 cu ft, 44 Hz and a single 5.0" port into WinISD, the port length came in at 4.7inches.
But, someone should check this 4.7" length value with BassBox 6 Pro.
Because, I am sure that BassBox 6 Pro is more reliable than WinISD.

Baron030

[Zilch]

Yes, but the friction constants of the 3" vs. 4" ducts would be different.

For them to sum, the 3" duct would have to be shorter, no?

That's what I don't know how to calculate.

The pair has to be balanced to share proportionately, I would think....

[Baron030]

D'Oh, I forgot about fictional losses.
A single 5" port has a circumference of 15.708", while the sum of the circumferences of the 3" and 4" ports works out to 21.99". That means the air would come in contact with about 40% more surface area in the two smaller ports then it would with a single 5" port. So, a single 5" port would not be an accurate model for the 3" and 4" port combination.
Assuming that that the same 19.635 sq." of port area, were to be split into 2 equal sized tubes of 3.5355". The differences in surface areas and fictional losses would become less significant. So, 2 - 3.5355" tubes would be a closer model to the 3" and 4" combination. But, it is still would not exact. So, I have to agree with Zilch, don't mix port sizes. It’s way too hard to get an accurate computer model.
But, if you were to use a combination of 3" and 4" ports. I would think that both ports would have to be the same length. The 3" port would handle about 36% of the air, while the 4" would pickup the remaining 64%. The only down side is that it would require a lot of testing, and trial and error tweaking to get the tuning right.

Baron030