Help with port calculations

[Kenny Kenny]

Hi, long time reader, first time poster. I'm up against a time constraint and could use some help. I've done much searching here but I still need help.

I have a well made empty cab. I want to load it with two (2) JBL 2226 drivers for bass guitar use. I'm familiar with these drivers and have used them in PA and bass rigs. I'm well aware that they are designed to be PA woofers. @agedhorse has posted a very good summary of their cons for bass guitar use (dark on axis response, low roll-off frequency, poor off-axis response.) I've used these drivers in biamped bass rigs and they're "ok" for my use case. I have a limited budget and I already have these drivers so I'm using them.

The JBL specs for a single 2226 are in a 4.5 cu ft ported box tuned to 40Hz.
My cab has an internal volume of 7.8 cu ft. I've measured the bracing and factored that in, so 7.8 is the true volume (except for the drivers themselves.) I'm installing a new baffle in the cab and I need to install ports. The cab is deep but the face is a bit smalI. I would prefer two (2) 4" diameter round ports of any length although I could rear port and use larger diameters if need be.

I haven't dived in to Win ISD for years and I understand that it sometimes isn't the most accurate. I also see that there's other programs now available.
I'm not looking for subwoofer performance. And I'm not looking for "optimum" driver performance. I just want to tune this cab as best as possible using these two drivers for the lower frequencies of a bi-amped rig to use with a 4-string bass. To clarify further, when using a single 2226 I run my bass with a HPF at 40Hz (and sometimes it's even at 60Hz.)

My question is : What size ports should I use?
If any of you experts could calculate this for me I'd be most appreciative.
Thanks

 

[Kenny Kenny]

Woops, now I remember that this isn't really my first post. I have been reading for a long time but just joined a few years ago.
(Just want to be accurate, lol.)

 

[agedhorse]

Good question...

in that box, 2 round ports 6" in diameter by the 3/4" baffle thickness is pretty close to ideal, I would chose 6.25" diameter myself but close enough.

You would get the following:
F3=51Hz
F6=45Hz
F10=40Hz
Sensitivity=102dB/1W/1M
Pmech=800 watts RMS

All in all a good set of tradeoffs with a nice balance between LF extension, sensitivity and mechanical power handling. It might be little dark, but may be correctable enough w/ a little eq.

 

[Kenny Kenny]

Thanks much for the quick reply!
6" diameter is a bit big for my available baffle space. This cab won't be near any back wall. I can rear port, no problem. The cab's rear panel is 5/8" if that makes much difference? (It's well braced.)

Rear port locations: Any preference? Would appreciate your thoughts on placement.

And while you're on the line, are there any advantages to sectioning this cab into two separate ported chambers?

Again, much thanks! I'm already cutting into this box installing tilt back wheels and a rear handle.

 

[Kenny Kenny]

P.S.
The numbers look good.

 

[agedhorse]

How about a slot port in front? There will need to be some port depth though to stiffen the baffle.

If not, these ports can be located on the back, 5/8" is fine.

 

[Kenny Kenny]

Not much room in front. I'll just go with the two big holes in the back. Easy peasy
Much appreciated!

 

[Redbrangus]

If you have room for two 4" round ports, 2-2.5 inches deep would get you in the ballpark. Two triangular ports with the same cross-sectional area in the corners might enable an even smaller baffle board.

 

[Kenny Kenny]

If you have room for two 4" round ports, 2-2.5 inches deep would get you in the ballpark. Two triangular ports with the same cross-sectional area in the corners might enable an even smaller baffle board.

Thanks for that info. I'll keep those options in mind.

 

[agedhorse]

If you have room for two 4" round ports, 2-2.5 inches deep would get you in the ballpark. Two triangular ports with the same cross-sectional area in the corners might enable an even smaller baffle board.

note that there are 2 drivers in this cabinet (unless I mis-read the original post). 2 x 4" ports is awfully small and result in a box tuned too low.

 

[Kenny Kenny]

note that there are 2 drivers in this cabinet (unless I mis-read the original post). 2 x 4" ports is awfully small and result in a box tuned too low.

Yes, 2 drivers in this cab.

 

[Redbrangus]

Oops -- I stand corrected. In that case, two 4" ports would need to be 8" long. IME, shelf ports and corner ports lend themselves more readily to minimizing the size of the baffle board.

 

[agedhorse]

Oops -- I stand corrected. In that case, two 4" ports would need to be 8" long. IME, shelf ports and corner ports lend themselves more readily to minimizing the size of the baffle board.

I think you have it backwards here. Either that, or I have incorrect TS parameters.

 

[Redbrangus]

That could be -- I wasn't considering the T/S parameters of the driver(s). I was trying to calculate a port size that would yield the OP's target Fb with the given internal volume.

Edit: Well, dang -- I made the same mistake twice. The enclosure would need 4 round 4" ports, 8 inches long. Maybe I should just stay out of this. Apologies.

 

[Kenny Kenny]

That could be -- I wasn't considering the T/S parameters of the driver(s). I was trying to calculate a port size that would yield the OP's target Fb with the given internal volume.

Edit: Well, dang -- I made the same mistake twice. The enclosure would need 4 round 4" ports, 8 inches long. Maybe I should just stay out of this. Apologies.

Thanks for clearing that up..
I'll stick with the calculations that are based on the driver's T-S parameters.
I have no problem with rear ports.

 

[StephenMcLuckie]

Penn Elcom make some nice 6-inch ports you might like to check out. They're inexpensive and will give you a more pro finish than a simple hole. They'll also allow you to experiment with port length a bit, although I wouldn't stray far from @agedhorse's recommendation - say, 1.5 inch max.