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Old 14th April 2008, 11:59 PM   #1
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Default Help with CerwinVega please

I need a subwoofer for my next project, and funds are limited as usual.
I have a pair of CerwinVega 124 (03 ) car subs here and I have been playing with box programs.
Here are basic parametres
Vega124(03)
Fs 23Hz
Qms 4.31
Qes 0.358
Qts 0.33
Vas 2.85 Ft3
Xmax 0.748inches (19mm )
This is a huge driver 3 inch voice coil and magnet must weigh 3kilos or more, currently in 18 litre sealed boxes, and while the sound is acceptable I think it could give greater depth in the correct box.
I was interested in the possibility of an EBS alignment but I am a little confused; if I need a largish vent to lower chuffing and vent noise the size of a 6inch vent is so much longer than any one dimension of the box itself, at what point does an EBS alignment turn into a transmission line??
I am having trouble with using the MJK site, but I was under the impression that this driver had a " Q " that was a little too low for TL cabinets.
As always space is a consideration WAF and all that, she said no to the double KEF boxes.
I think she was right on that this is a small house.
Comments?? Ideas??
Any and all help gratefully accepted.
Regards Ted
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Old 15th April 2008, 12:50 AM   #2
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Playing with Jeff Bagby's box program i have come up with some possible solutions.
#1/ 43 lire box tuned to 43 Hz ( 150mm vent 1626mm long) F3 = 31hz
#2/ 95 litre box tuned to 22Hz (9150mm vent 1092nn long ) F3 = 20.3Hz

#3/ 55 litre box tuned to 23 hz (150mm 1778m long ) F3 = 30 hz, but with this box a first order passive high pass at 20hz give a flat responce and an F3 of 21.6Hz.
Capacitor bank capable of handling 100 watts at that frequency probably rules this option out, also Qms is probably a little low for this option anyway.
All of these boxes are bigger than those I calculate manually using SBB4 tables in Weems' book.
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Old 15th April 2008, 01:19 AM   #3
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I would go with this 43 lire box tuned to 43 Hz ( 150mm vent 1626mm long) F3 = 31hz

Smalest and the bass region is nice

Also as low as you tune u need more power to feel it because u cant hear it
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Old 15th April 2008, 02:17 AM   #4
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Thanks Alekander, although power is not a problem ( 500watts nominal 1000 peak) and getting an amp to do that is do-able, how do I arrange a vent with this length?? or do I do a rectangular shelf vent and calculate the length given the area of the vent, given that the vent will be 3metres long and I think I then have a TL
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Old 15th April 2008, 05:56 AM   #5
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Yeah, these specs 'scream' an end loaded, high taper ratio TQWT due to its low Vas, Qts, so at a glance, ~7.12 ft^3 with a 10:1 ratio yielding a ~360"^2, 72.4" long pipe tapered to a 36"^2 terminus seems reasonable to take advantage of all that Xmax down to ~16 Hz. Fold it up and stuff it to suit.

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Old 15th April 2008, 06:07 AM   #6
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GM thanx for those thoughts; could you please explain in a little more detail?? I am "all at sea" with those figures.
I need to fill in the bottom octave and a half from 60 Hz down but I could go from 85.
If I understand correctly; Volume of tower = 200 litre
Big mouth shallow taper and the end 1/10th the size of the starting point.
6 ft seems short; 1850mm, the wife might let me, if I could put this in the corners.
I will be building two of these
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Old 15th April 2008, 07:52 AM   #7
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GM I think I was having an "oldtimersmoment" 10:1 is a sharp taper, in other words a short fat box?? That would be very do-able with reasonable WAF. I fold the line and have the port towards the front on the same plane as the driver??
How do I measure the length of the line? centre to centre and at the midline how much room do I need to leave between the end of the box and the dividing panel?? I was thinking equal to the other two dimensions (a+b/2)

My scanner files are too big to post so no sketch yet.

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Old 16th April 2008, 07:48 AM   #8
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OK, I have quickly worked out a fold, vent to the ceiling and front firing woofer.
How critical are dimensions with TQWP's?? it fits into a 900*1200 sheet of chipboard, my supplier charges for each cut.
Does 20 or 25 mm either way make an appreciable difference, it will increase volume by 10%
These will fit in the corners and I haven't told SWMBO I'm building them yet
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Old 16th April 2008, 03:12 PM   #9
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You're welcome!

Not sure what else I left out. I mean a 10:1 TQWT of 'only' 72.4" line length has its 3rd harmonic dip well above a typical sub's BW even with a VLF Fp, so normally not an issue and certainly not with your low XO point.

Right, folded in half so the driver and terminus are next to the floor on the same baffle works for me. Think inverted BIB with the terminus ('Sm') blocked off: http://www.zillaspeak.com/bib-howtobuild.asp

To maintain an a=b=c spacing requires a width/depth ratio of 1:1.4142 AFAIK, so for a 360"^2 closed end:

depth = SQRT((360*SQRT(2)) = 22.56"
width = 360/22.56 = 15.96"
a-b-c = 22.56/2 = 11.28"

All that's left is to figure out where to cut the top boundary for the triangular 36"^2 terminus.

GM
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Old 16th April 2008, 03:20 PM   #10
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Hmm, if firing at the ceiling, seems like it would be better to make it a straight, triangular TQWT to increase ceiling loading and reduce its physical in-room presence.

WRT net Vb variance, less is always less, i.e. higher F3, while more at worst only means more stuffing required to damp any audible Fp peaking, so I always recommend erring on too large.........

GM
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