free plans
http://www.geocities.com/free_schematic/loudspeaker.html
(I’m still looking for the JBL port size spreadsheet, but this will do for starters)
That’s a great calculator for ports – it’s not specific to car speakers. The spreadsheet I was looking for is nothing more than results from the math on the port size calc. Use ‘Calculate Your Vent Length’ for generic speaker cabinets. This is the most useful.
The other calculator, uses Xmax, is more attuned to specific speakers. It’s good if you have the speaker specs. Xmax is ‘sort of’ the usable distance the speaker cone can move back and forth. It’s related to the speakers 'Q’
Older JBL home speaker cabinets were ported and tuned to 36hz, but the speakers were made to go down that far. Many/most cheaper speakers can’t perform down that and will just flop at lower frequencies. Cabinets should be tuned to the lowest frequency the speaker is made for, but should have a high pass to block anything lower from getting to the speaker.
Radio Shack has some cheap 10" woofers that would tune GREAT to about 98 hz. That’s not really that low for smaller bookshelf speakers. It’s pretty typical actually. But, they sounded good in JBL enclosures tuned to 40hz (the old JBL woofers crumbled from age – still got them). Yet, putting speakers incapable of producing those lower frequencies in cabinets tuned that low is not a good idea at all – those speakers still can’t perform below 90 or so.
Properly tuned enclosures give a flatter response above and down to the tuning frequency, then there is a bit of a bump (a resonance), and a strong rolloff below the tuning frequency.
Look at cabinets in the 2 to 8 cu ft ballpark with a single port diameter of 4 to 8 inches, tuned 30 to 50 hz. (get to know the speaker specs is you can)
Port sizes near 0 are no port. Tweak the numbers until the result port length is around .5 inches if it come up negative.
This is fun stuff to experiment with. I wrote an app for the computer that would spit out all that data a long time ago, but it was on the Commodore 64. I never ported it to the PC. It used the same math formulas that are used on the port tuning calc.