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Old 27th July 2013, 05:22 AM   #1
Zene is offline Zene  United States
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Default Odd porting

My single sub PPSL box will be about 400L (14 cu ft). Tuning around 40hz.
I only have one flared 6" port and even two isn't enough area. Is there a way to have one slot port and one round which I would use for tuning as it's a removable type? Not sure how to tune this combination, either.

Thanks, Zene
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Old 27th July 2013, 06:26 AM   #2
OscarS is offline OscarS  United States
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You can, but I would highly suggest not attempting it unless you are willing and able to experimentally verify and alter the slot port to achieve your desired tuning. Also, how are you achieving PPSL with only one driver?
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Old 27th July 2013, 06:33 AM   #3
Zene is offline Zene  United States
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OscarS, I was afraid of that. My last stint of designing an adjustable slot port was messy.
Do(es) that mean buying two more 6" ports? Not a great expense, I just wanted to hurry the building process.
Zene
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Old 28th July 2013, 03:27 AM   #4
OscarS is offline OscarS  United States
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Sell the 6" port and just use a large slot port, I'd say.

So what about this single-driver PPSL box???
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Old 28th July 2013, 03:51 AM   #5
Zene is offline Zene  United States
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OscarS ... or make a porta potty from the big port. Good idea for for slotted port. I will do just that. I would need 4 6" dia ports and that just ain't right.

Mis-wrote; this box is (will be) a single sub woofer PPSL, not single speaker PPSL (as that isn't possible). I did have one of my woofers in an open baffle style Slot, but it did like all open baffles, killed the bottom. It was Nelson Pass style but with just the one speaker and was looking for stereo bass, not sub bass at the time. Good for upper bass, but I have that covered, already.

Thanks, Zene
P.S. I was tempted to try an Onken PPSL, but when the beer wore off I regained sanity.
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Old 28th July 2013, 09:04 PM   #6
GM is offline GM  United States
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[4] 6" dia. = [1] 12" dia. in area, so use a nominally 12" dia. cardboard tube. If you need a slot vent, then flatten it to an oval and use it as a template for cutting the baffle and adjust it the same as you would any fixed tube vent.

GM
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Old 29th July 2013, 12:53 AM   #7
Zene is offline Zene  United States
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GM, good idea (will remember), but have recanted and would like to try a PPSL 15" Onken box. I can always plug some ports and resize the box if necessary for conventional porting.
I know you've had experience with them, so was good timing answering.
The problem is that the "n" input for Debien's Onken Caclulator needs to be around 10 to keep the ports within his 30cm length limit. He says not to use excessive value, but I don't know what excessive would be.
Can you help?

Thanks, Zene
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Old 30th July 2013, 03:52 PM   #8
GM is offline GM  United States
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You’re welcome!

Good question, one I haven’t yet tried to find out.

The best way to design a multiple vent cab IME though is to use a simming program. Conversely, use one to sim whatever the Onken calculator spits out and if high tuning accuracy isn't required, then WinISD Pro alpha is good enough, just be sure to input the highest power likely to be required at Fb, so that you can tune it based on a < 5% vent mach [< 17 m/s], though some claim it needs to be much lower, but most modern day drivers would then require virtually all low tuned reflexes not configured as some type of [ML]TL to use PRs, which has its own audible vent resonance problems if not tuned below ~20 Hz.

That said, how long the vent system's length can be is a function of the cab's intended usable BW, so for a 'sub' it can be much longer than for one designed to be used over a wide BW, just keep the vent’s harmonic decay > ~ –12 to –24 dB point depending on the XO point.

IOW, IME, for 80 Hz/4th order, it needs to be > ~ an octave away [~160 Hz/-24 dB], but this is so subjective that if one prefers the clinically ‘sharpness’ typical of sealed it will be much too low and those that find a long vent Onken’s harmonically distorted lower mids euphonic, then should be fine and maybe can be as low as ~ a half octave [~120 Hz/-12 dB], though the latter will be much easier to ‘locate’, so proper positioning is critical. MJK’s or similar software required to sim.

Some folks have claimed that as long as the vent pipe’s 3rd harmonic [~two octaves above Fb] is an octave above the XO point, it’s good enough and can be ‘critically’ damped if desired, but at this point one is better overall using an inversely tapered TQWT or tapped variant IME since vent cut lengths can exceed 4 ft when tuned low. WinISD Pro alpha’s ‘1st port resonance’ calculation is generally close enough.

GM
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Old 31st July 2013, 02:21 AM   #9
Zene is offline Zene  United States
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GM, thanks. Will use your advice and do some comparing between UniBox and the Onken spreadsheets. So far it looks as if the "N" only makes the slope order steeper. The "9" I used made it into an approx 5th order box. I haven't tried to increase the Le to see if I can duplicate the order. That's not much beyond a standard 4th order ported and it did not look like the overshoot was any worse than normal.
Bass extension was not as good, but may not care. Not looking for HT boom. It was -3dB at 33hz and -10dB at 28hz. That's quite steep.

"N" directly effects F3 and box volume what-ever that's worth.

The L' vent length does not appear to be anything magical as it's just a shortening from a constant factor times the sq root of vent area.

Zene
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