A Quick Port calculation comparison ?

[Scott L]

I only have one method of port calculation, so I was wondering, given the specs, if someone out there could run the numbers to see if the answer is the same ?

thanks in advance for any help.

Net Volume Box = 3.0 cubic foot
Desired Port freq. ( Fb ) = 50 Hz
Using 2x 4" i.d. ports

my answer: length of vent = 5.0 inches

[GM]

Except for some early T/S math which calcs a ~6.09" long vent (no flares), all others I have say a 5" length will be 47-48 Hz, so ~4.1" seems the most correct length, but all these assume that the cab's air mass has a ~uniform particle density, so if it's a high aspect ratio cab, then even the shorter one could be too long.

GM

[richie00boy]

I get 4.88 inches so I would say you will be OK at 5 as at least you have some spare to trim if necessary. Tuning seems dangerously high for such a big box, what are you building?

[Scott L]

Originally Posted by richie00boy

I get 4.88 inches so I would say you will be OK at 5 as at least you have some spare to trim if necessary. Tuning seems dangerously high for such a big box, what are you building?

Answer:
Since this is my most favourite forum, I thought I'd give as much info as possible. This (pair) of enclosures is the upper bass/low mid section of a full-scale assault-type of sysytem. The designated range is from 60-250Hz.
Each enclosure will house 2) 12" pro-type woofers model Eminence HO4012.
There is a woofer on the front and exactly opposite it, on the back, in a reaction cancelling arrangement. Originally, the plan called for each driver in it's own encloure of 1.5 ft^3 w/ 2x4" vents, but this required vents much too long to fit inside a reasonable enclosure. A lightbulb in my head finally lit up, and I decided a common volume of 3 cubic feet, and just one pair of 4" vents for the entire volume. The measured woofer specs are Fs 46; Qts .35
Vas 2.4 I think i'll opt to place the vents on the rear, as I really don't want to look at them. The crossover slopes are 24/db/ oct high-pass and 48 db/ oct low-pass. Since the high pass is 60, and the vents are tuned to 50, I am not so concerned with vent compression, if even I need be ?
All comments and criticisms welcome.

[richie00boy]

Seems OK but you will have more port noise, as you have halved the port area. Whether this is audible or not is another question, I suspect you will be OK as you are filtering higher than the port frequency. Give it a go and see