Can I get a double check on my cabinet?

[saabracer23]

First off I apologize for the long post and all the questions, but I need help. I'm building a "small" subwoofer cabinet to haul around with me to add a little low end to my behringer truth powered monitors. I already have a pair of the tang band w8-1363sa (like sb) drivers so I figured I would use those. I know they are somewhat of a troublesome driver because of small cabinet requirements equal extremely long ports. I assume that if I use two it will help a bit with that. They will be wired in series to a dayton 1000 watt plate amp so they'll get about 250 watts each.

I am very new to bassbox pro but find it very useful! I modeled a pair of these and it recommended a cabinet volume of 1.4 cubic feet and I stepped it up a bit to 1.75. I read on another thread that sreten suggested only a max of 12L per driver ported?!? That is only .85 cubic feet for the pair! I figured with a vas of .668 it would be closer to the 1.4 cubic feet. Did I do something wrong? Will there be a problem going 1.75 cubic feet?

So with bassbox pro I got the following:

Vb= 1.75 cu. ft
V(total)= 2.279 cu.ft
Fb= 30 cycles
F3= 31.47 cycle (perfect for music)
Fill= normal (should I use polyfill in a cabinet this large?)

I'll have one slot vent measuring 10.5 inches wide, 1 inch tall, and 15.96 inches in length (we'll call it 16). With a port opening of 10.5 square inches will i be getting port noise? On the graph from bassbox pro it shows I peak at about 29 hz with 26 m/s.

That is basically it for now
Thanks all

Dan

P.S. a few details on the cabinet.

internal dimensions of 22.5 inches tall, 10.5 inches wide, and 16.67 inches deep.

[PeteMcK]

Unibox gives me 30L = 1.06 cu ft for 2 drivers;

your 1.75 (49.5L) will give a bass hump around 30Hz

[j.michael droke]

Hi there: Ran your design in Leap program for two TB's and find the 1.75cf box to have F3=26.7hz, Fb=27.6hz, with a vent of 12.1-inches long by 3-inch diameter. Looks larger than necessary, ran 1.3cf box, F3=30.43hz, Fb=30.30, with a vent of 13.8-inches long by 3-inches diameter. Hope you get to construcion soon, post results listening evaluation always welcome. Minimum box lining is for 2-inches on 3 walls and behind the wall behind the drivers, for damping of standing waves, covering all walls 3-inches approximates 50% stuffing and will have benificial results. Keep stuffing away from the vent. You may wish to look into double flaried vent tubes (see Parts Express, catalog page 158) ...regards, Michael

[saabracer23]

Hmm why do I always seem to get different results showing than what I model? Ive used winisd with results even more off. Am I entering the specs wrong or something? I thought bassbox pro was a reliable program! I assume it's user error.

I have a 3" flared port from parts express but I figured I would get pretty bad port noise with it. At what air velocity would I expect port noise?

There is a guy on the parts express tech forum who has a pretty popular design using one of these drivers in a 3.5 cu. ft enclosure with 4" port tuned to 17hz, so I guess there is nothing "too big". LOL I would just assume using a 15" driver at that point.

If your model is reliable j.michael, then I like the looks of that 1.3 cu. ft box, with port I can keep the cabinet under 2 cubic feet total. I'll try modeling those numbers you gave me and see what bassbox says, it'll probably be off lol.

Dan

[saabracer23]

Well I ran the numbers and they were equal to j.michael's numbers, only after I changed the driver's xmax from 12 inches to 12 mm. Well I like the cabinet at 1.3 cu. ft but to make this work without having an elbow in the port I need to have the drivers an port on different baffles. Would it be better to have the port on the bottom and driver on the front? I like the idea of this because I can see what the drivers are doing, also would having the port on the bottom help hide port noise?

I grapghed these with 300 watts since they are rated at 180 each, so I figured 150 each was a good number. I'll have the extra headroom if I need it. So at 300 watts I get 12 mm xmax at 20.53 hz and I'm above 34m/s air velocity at 25-33hz, I believe it peaks at around 40. Should I be ok or should I step it up to a four inch port? I would like to use the 3" since I have it, but I want to do it right the first time otherwise it'll end up costing me more.

Thanks
Dan

[j.michael droke]

Hi there:Found a link to information on ports, by J.L.Murphy found on www.trueaudio.com titled "Questions on Vents". (topic on the technical section)... regards, Michael

[saabracer23]

Thanks for the link, I'll see if I can work it out. I know now that port position isn't a big deal but I want to see if I can eliminate port compression and turbulant noise.

Dan

[GM]

Originally posted by saabracer23
Should I be ok or should I step it up to a four inch port?

According to the Margolis-Small box program for some HP calculators, for 30 Hz Fb/440 cm^2 Sd/12 mm Xmax = 9.5 cm (3.74") min. dia./56.31 cm (22.17") long to achieve a 4% mach, so port compression is low enough to be suitable for prosound apps, but it's not until Av = Sd that there's no measurable compression and of course to attain this level of performance requires a much larger TL.

GM

[saabracer23]

Thanks for the input GM, my port is only 17 inches long, so I think I am going to go with a slot port 10.5" x 1.5". This models much better (I think) and will be just as easy to do. One thing I have noticed is that In this cabinet I am maxing out at 6mm xmax at 30 cycles with 360 watts rms (woofers rms rating together). Am I wrong in the assumption that the more air you move the louder it will get? I want to use these to their full potential.

Dan

[GM]

Greets!

Your welcome! Unfortunately, part of it was only worth what you 'paid' for it since I just noticed an input error , so this vent is only good to ~8% mach and for 4% it should be ~12.64 cm dia./104 cm long.

Yes, the more air you move the louder it gets, but the problem is that as the vent becomes 'choked up' if not large enough it puts out less output and raises Fb. Factor in that this puts more load on the driver, heating it up, the driver's output flattens out due to thermal power compression (raising its Qts).

For instance, years ago on an HT forum a member wanted more output from his sub system and some folks said increase the amp power from 500 W IIRC to 2 kW while I speculated based on some experience with the drivers that it probably wouldn't gain him any increase at best, but the allure of having that much power was too great, so he splurged (big buck$ back then).......... Some time passes and he finally admits that he measured less output cranked up with 2 kW than he did with the much smaller amp/lower gain setting.

Bottom line I don't know how much power drivers today can handle at low distortion, but it use to be in the 1/8-1/4 rated power range, though Dan Wiggins claimed Avatar/Adire Audio's original Shiva could take 1/2 power which is true of the pair I have, so I imagine good quality drivers can take a bit more than a decade ago due to increases in manufacturing and materials technology.

Little TB drivers though? Not so sure, so others with hands on experience need to chime in. For sure, your cab needs some big PRs for them to be 'all they can be'.

GM