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2nd July 2013, 04:30 PM  #11  
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Quote:
Even if it’s made as a 54” long continuous perimeter vent it will still be tuned 40+ Hz, so the vents must be made much longer like is typical for an Onken alignment to get down into the 20s. Referring back to my post, the implication is you don’t even need a low mach T/S vent much less an Av = Sd one, especially one divided up into a bunch of small ones that only make them longer since the plan here is to stuff them to ‘taste’, not rely on their friction and sheer combined air mass resonant ‘slugs’ as the Onken’s does. Note too that ‘buried’ in my response is the fact that my ‘EBS’ alignment [and any I’ve recommended to others] is in some form of MLTL, which shortens the vent required, so for a given vent length it can be larger if desired. FWIW, beginning at this post, here’s one man’s take on how to achieve your goal: Altec Lansing WRT tuning choices, the 1603’s published Qts + connection wire resistance is high enough for a Fs tuning, but a passive XO and/or a high output impedance amp will shift it down to below Fs, so how much added series resistance will be in the signal chain and how accurate are the published specs? If this isn’t known, then tuning to Fs and using either some form of adjustable vent or varying stuffing density in both the vent and cab may be the ‘safe’ choice. GM
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5th July 2013, 03:22 PM  #12 
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Join Date: Jan 2010
Location: LA

OK,GM, I lost my mind. Let's start again. How about an internal Vb of 18,542.78125 cu. ins. and a Vb of 27.86 Hz. Internal dimensions would be 35.796875"x28"x18.5". There would 2 vents across the front, each 1.5"x11.75" and 2 vents on each side, each 1.5"x7". Lv would be between 4.66" and 5.1515" including Auspurger correction. Total Sd of vents is 77.25 sq. ins.
My other figures were based on a box for an Altec 4168, sorry about that. too much computer time and not enough sleep for a 76year old. Does this new set of figures seem more appropriate for the TAD 1603? 
6th July 2013, 02:18 AM  #13 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Hmm, ~10.73 ft^3 with one pair of vents larger than the other two pair means that they will have different tunings for a given length, so will spread out/average the tuning [Fb] a bit.
For a 1st approximation then, I can only average them to [6] ~1.5” x 8.5833” vents = a ~21+” vent length, which includes a certain amount of end correction and heavy damping to get to 27.86 Hz, so with no other info I have to conclude that your vents will be too short. Again, not familiar with GA’s vent design routine AFAIK, but knowing his TL work is ~spot on, hence the likelihood of his vent design being just as accurate, I’m wondering if you’re using it correctly and/or not posting all the pertinent details. Again, for semi to ~aperiodic alignments, much less vent area is required compared to a low vent mach T/S reflex and the only way short of a long TL cab I know of to get the vent length reasonably short. Yeah, know the feeling well, I’m ‘only’ ~67, but have suffered from chronic sleep deprivation for as long as I can remember, though only in recent years has it been obviously affecting my ability to do things. GM
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6th July 2013, 06:53 PM  #14 
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Join Date: Jan 2010
Location: LA

Thanks, GM.
If I have understood GA's vent correction it is as follows for a rectangular vent:the 4th root of the ratio of vent dimensions (larger number divided by smaller number times times 1.13 times the smaller dimension. The answer is then adjusted for the number of vents (4th root of the number of vents times the previous correction) So if the vent is 1.5"x8.5833 the correction would be 2.6216"times the 4th root of 6 vents or a total of 4.103".] If the Sd of a single vent is 12.875 sq. ins. then using the formula Lv=(1.463*10^7*r^2/Fb^2*Vb) 1.463*r, I get a corrected vent length of 5.30741". What am I doing wrong? Best regards, Bill 
8th July 2013, 04:04 AM  #15 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

You're welcome!
Hmm, math is for the most part is a foreign language to me, so not sure what the formula looks like, but if I’m understanding you, you’re shortening the vents for multiples when the reverse is true due to increasing frictional losses with increasing number of vents. Also, the basic Lv formula has an end correction [1.463*r], so sounds like you’re ‘double dipping’, compounding the error. GM
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10th July 2013, 05:18 PM  #16 
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Join Date: Jan 2010
Location: LA

GM
Actually the formula lengthens the vents. By my math, though it may be suspect, your design vent of 12.875 sq. ins. would have an Augspurger correction that adds 4.103 inches to the standard length formula. Could you explain how you arrived at the 21+ inch length? I'm wondering if you are using a different formula then the one I used per #14 or am I executing it incorrectly? 
11th July 2013, 04:12 AM  #17 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Oh well, shows how little I understood your explanation, so please either post the math or PM it to me.
As I noted, mine is just a 1st approximation using WinISD Pro to get the multiple vent’s basic length, end correction for a rectangular vent. The actual length will be a bit shorter due to local boundary conditions, though there’s no program available to get it really close AFAIK. When I convert it to a single large round vent tuned to 27.86 Hz using proven math, it’s 16.25” long and adding GA’s end correction for [6] ups it to 20.35”, so for now, WinISD’s calculation could be the more accurate of the two for all I know, though I would be very surprised. GM
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17th July 2013, 08:37 PM  #18 
diyAudio Member
Join Date: Jan 2010
Location: LA

Hi GM
Is the single vent mentioned in your last entry derived from the 12.875 sq. in vent or is it larger? What is the radius? I would like to try it using the math I have and try to figure out why we are so far apart. Best regards, Bill 
18th July 2013, 05:30 AM  #19 
diyAudio Member
Join Date: Jun 2003
Location: Chamblee, Ga.

Greets!
"For a 1st approximation then, I can only average them to [6] ~1.5” x 8.5833” vents......." = 6*12.87495"^2 = 77.2497”^2 = ~9.91753” dia. is what I used. GM
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18th July 2013, 07:01 PM  #20 
diyAudio Member
Join Date: Jan 2010
Location: LA

Hi GM
I see the difference now. I have been using the diameter of one vent with an Sd of 12.875 sq. ins.= 4.04882 ins. That gives me a design Lv=1.204444 ins., plus an Augspurger correction of 2.62156 ins. times 1.5650845 (correction for 6 vents).= a total Lv = 5.30741 ins. Augspurger specifically calls for calculating a single rectangular vent Lv correction and then multplying by a correction for multiple vents. That led me to believe that it is the volume of the individual vent that is the basis for the 27.86 Hertz resonant tuning, whereas you seem to be saying that the multiple vents should be lumped and the calculation for Lv made for a summed Sd of all vents. Am I correct in that asumption? If yes, perhaps you would not mind explaining further so that I understand the theory. 
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