Calculating box tuning...

[dubkarma]

I'm pretty sure there's a way to calculate the port tuning of a given speaker cabinet. I just don't know how to do it. Any tips?

I've got a pair of well-made speaker cabinets (1.5" thick panels of birch plywood/mdf, 360 degree bracing) with internal dimensions of 27 x 19 x 17 inches. This does not include the thickness of the acoustic foam that's on five sides of the cabinet or the volume occupied by the bracing.

There are two ports in each cabinet, each 4 inches in diameter and about 4.5-5.0 inches long (hard to be precise because they're flared ports).

When loaded with TAD TL-1601b drivers and crossed over at 650 Hz to a TAD TD-4002 with TAD TH-4001 repro horn they have a thick, congested, boomy sound to them. By contrast, another pair of cabinets with internal dimensions of 27 x 24 x 17 inches and dual ports 4 inches in diameter and 5 inches long produce lively, articulate, well-defined low frequencies with the same drivers and horn.

So, since I'm wondering if the 4 inches larger width of the second cabinet changes the box tuning a great deal, the best way to answer that question is to apply the right formula to calculate the difference.

[Horn Fanatic]

[QUOTE=dubkarma;373547]

"I'm pretty sure there's a way to calculate the port tuning of a given speaker cabinet. I just don't know how to do it. Any tips?"

There is a simple method, which involves calculating the acoustical mass reactance of the vent, and the reactance of the acoustical compliance of the box to be at unity. However, I have had very good luck with Bass Box Pro. As for the vents, I am an advocate of large diameter on the order of 6 inches. Of course, using large vents sometimes requires a bit of length depending on the woofer. You can use two 4 inch vents, or a single 6 inch depending on the baffle real estate you have available. Personally, I vent out the back, or in the the case of mid-century designs on Danish style legs I reproduce, I vent out the bottom. Using larger vents will provide a more solid, tight, and articulate bottom end.

The folks who manufacture flared vents provide tuning data for those types of vents. That may be a start, however, you may want to consult with folks most familiar with TAD woofers. I'm sure there are plenty of them on this forum.

If I may suggest. Stay away from using foam as damping material, because it doesn't provide the proper acoustic resistance. It will actually decrease the volume of the box rather than act as an acoustic tangle. Damping material is after all, an acoustical component with it's own impedance in the acoustical circuit. I have never found a reputable book on speaker enclosure design that advocates using couch type foam as damping material. Either use fiberglass, or what I use more often than not, is that damping material that is made from shredded denim material in one inch thickness. I use it sparingly, as too much will suck the life out of a full range woofer. In one particular design of mine I use one inch thick duct board, but that material needs to be installed before I seal up the enclosure because it is rigid.

http://www.bondedlogic.com/

Good luck, H.F.

[Lee in Montreal]

http://www.psp-inc.com/tools2.html

Enter the internal volume of your cabinet.

Enter port dimensions and # of ports

Enter tuning frequency (what you think it could be)

It will calculate port length. Then change the estimated tuning frequency until the results is the same as your port length.

Then you will know to which frequency you cabinet is tuned to.

BTW 27" x 19" x 17" is 5 cubic feet

27" x 24" 17" is 6.3 cubic feet

You can also use a simulator, enter the T/S parameters, you cab volume, Fb etc. And compare the curves.

[speakerdave]

Just use the larger boxes, or am I missing something. They are really nice woofers.