When porting a cab, do you...?

m.oreilly · #1 10-31-2010, 01:32 PM

...go by the driver's free air resonance, or it's 'usable' low frequency? also, is it better to use one large port over two or more smaller diameter (round port) ones? i'm just about to do some cutting, and needed to know before i went too far

will33 · #2 10-31-2010, 01:57 PM

Do yourself a favor and don't cut anything yet. You got WinISD?

billfitzmaurice · #3 10-31-2010, 02:00 PM

Quote:

Originally Posted by will33

Do yourself a favor and don't cut anything yet. You got WinISD?

+1. The asking of the question says you aren't ready to do any cutting.

m.oreilly · #4 10-31-2010, 02:01 PM

Quote:

Originally Posted by will33

Do yourself a favor and don't cut anything yet. You got WinISD?

nope. i went with one of these online calculators, and came up w/ a 6" duct with varying length for different tunings. this is for two deltalite 10" in a 3.0 box...

m.oreilly · #5 10-31-2010, 02:02 PM

also would love to go simple, and use a slotted port, triangular being even better (like my berg cab, and just one cut across a corner of the baffle)

m.oreilly · #6 10-31-2010, 02:21 PM

winisd gives me a 4" port @ 1.1 " length...

m.oreilly · #7 10-31-2010, 02:32 PM

...or a 6" at over 2" for a res of 60hz...like i had originally. so what should one tune for? the free res or the 'usable' lowest frequency???

m.oreilly · #8 10-31-2010, 02:38 PM

anyone?

DukeLeJeune · #9 10-31-2010, 03:26 PM

Quote:

Originally Posted by m.oreilly

...or a 6" at over 2" for a res of 60hz...like i had originally. so what should one tune for? the free res or the 'usable' lowest frequency???

Neither is what you go by.

What you do is, you model a lot of different tunings, looking at the frequency response curve and excursion-limited power handling while maintaining sufficient port area to avoid chuffing at high levels, all while keeping build-practicalitiy in mind. Programs are written for round ports; as you move to other shapes that have larger perimeters relative to their cross-sectional area (like a rectangular slot port), the port lengths predicted by the program become less reliable; the actual required length is reduced, though I'm not sure why.

Any tuning you choose will be a juggling of tradeoffs, a compromise among several factors (some of which are mentioned above). Overlook or over-emphasize one factor and the result can be poor performance.

5StringPocket · #10 10-31-2010, 03:38 PM

Duke, what program would you recommend to calculate excursion-limited power handling? WinISD is a nice utility but doesn't seem to have that feature. I don't know if WinISD Pro provides this but it locks up on my PC running Vista (aaargh, hate that OS).

DukeLeJeune · #11 10-31-2010, 04:03 PM

5StringPocket, you might take a look at Fdeck's Excel spreadsheet, which you can download from this page:

http://personalpages.tds.net/~fdeck/bass/#projects

m.oreilly · #12 10-31-2010, 04:11 PM

Quote:

Originally Posted by DukeLeJeune

Neither is what you go by.

What you do is, you model a lot of different tunings, looking at the frequency response curve and excursion-limited power handling while maintaining sufficient port area to avoid chuffing at high levels, all while keeping build-practicalitiy in mind. Programs are written for round ports; as you move to other shapes that have larger perimeters relative to their cross-sectional area (like a rectangular slot port), the port lengths predicted by the program become less reliable; the actual required length is reduced, though I'm not sure why.

Any tuning you choose will be a juggling of tradeoffs, a compromise among several factors (some of which are mentioned above). Overlook or over-emphasize one factor and the result can be poor performance.

what do i look for in the winisd chart?
here is what i get with the 6" duct for 53hz:

and 60hz:

DukeLeJeune · #13 10-31-2010, 04:44 PM

M.orielly, I don't use WinISD. You can look at those curves vs other tuning frequencies to see what gives the shape curve that will best suit your needs. But, that will not tell you the whole story.

Without knowing what the excursion-limited power handling is, I would suggest you tune somewhere in the 42 to 48 Hz ballpark, as that usually works pretty well. 53 Hz is pretty high unless it's a relatively small enclosures, and the result can be premature fartout if you don't use a highpass filter to protect the woofer from over-excursion. 60 Hz is quite likely to result in early onset of fartout without a protective high-pass filter set pretty high. So in this case, my off-the-cuff suggestion is to lower the tuning frequency down to 48 Hz or maybe even a little lower. You lose some meat in the 60 to 120 Hz region, but in exchange you gain some real-world headroom, and imho end up with a better balance of characteristics.

m.oreilly · #14 10-31-2010, 05:03 PM

thanks for that

billfitzmaurice · #15 10-31-2010, 05:37 PM

Quote:

Originally Posted by m.oreilly

what do i look for in the winisd chart?

The box is too small. Let the program calc the ideal configuration, per driver it will give 2.16 cu ft /50 Hz.

Quote:

WinISD Pro provides this but it locks up on my PC running Vista

Run it in Windows 98 compatability mode.

will33 · #16 10-31-2010, 06:28 PM

WinISD Pro Alpha? It's easy to calculate excursion at various frequencies, varying amounts of power, etc., what different tunings do to the low end, port air velocity, what applying eq will do, etc. Translating what your ear wants to hear to what that looks like on paper is the kicker sometimes.

It is good for predicting what will happen in the lows. As far as what the mids/highs will do, consult the spec sheet for the driver. It will figure it out for you but consider that the "technically" best box for the speaker may result in something that's too big to be practical or tries to reach deep lows that aren't needed to make a bass guitar sound good and could be traded off for more real power handling where it counts.

m.oreilly · #17 10-31-2010, 08:37 PM

bill, here is an eminence 'medium' box for the two drivers (mine is actually a little larger)

Growly Lytes · #18 10-31-2010, 09:45 PM

Having No Port Chuffing is important & must be under 0.16 to no whistle...
Having one big port over two smaller ones wont make much difference, this is what ive been told anyway.Main thing with them is the ports shouldnt be more than about 5-10% shorter or longer than calculated so it must be done properly to get the right tuning.
I definitly had problems with tuning at first but after it was set proper i got the sound i need.I found out the hard way that you cant just stick speakers in a cab & off you go.
My rig now fills out all the bass needed to get the job done @ any volume basically....Anyway
I went for 42hz tuning with the BP102.
Who said the 102 is no good ??

will33 · #19 10-31-2010, 10:24 PM

Do the area of a circle/volume of a cylinder thing and see how a little bigger circle gets you a lot more area. The limit is bigger pipes need to be longer to get the same tuning, happy mediums in there.

m.oreilly · #20 10-31-2010, 10:31 PM

i'm waiting on my duct (6" by 6", with a mounting lip on one end) to get here before doing the cutting. i'll play with various lengths, though it looks like i may need a bit longer one to hit in the 40s.