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small box for big VAS ?
small box for big VAS ?
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Old 6th November 2017, 08:04 PM   #1
TroelsM is offline TroelsM  Denmark
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Default small box for big VAS ?

Hi.

I got an old "punch" driver that I refoamed (with some no-name ebay-rubber).
It ended up with a rather large Vas:

(Data measured with 42gram of added mass with Limp. - after 10 hours of 50hz @ 40W)
Parameter, Value, Dim
Fs, 29.63, Hz
Re, 3.40, ohms[dc]
Qt, 0.66, -
Qes, 0.75, -
Qms, 5.30, -
Mms, 97.31, grams
Rms, 3.689942, kg/s
Cms, 0.000299, m/N
Vas, 101.08, liters
Sd, 490.87, cm^2
Bl, 9.387767, Tm
ETA, 0.36, %
Lp(2.83V/1m), 91.37, dB
, ,
Le, 612.83, uH
L2, 1753.49, uH
R2, 27.85, ohms
L3, 0, uH
R3, 0, ohms

I guess its meant to go into a IB where it does sort of OK.
300Liter in Hornresp gives a -3dB around 30Hz.
Click the image to open in full size.

Now, if that driver is put into a smaller box the lower knee goes up an the response is not-so-flat. A BR only makes the hump worse.
Click the image to open in full size.

This is my benchmark. A JBL GTX450 car-sub: (the 45L punch as overlay)
Click the image to open in full size.

(The JBL is by all accounts a "better" driver, so comparison is not fair)


So, without bending the laws of physics, -how would you implement this sort of driver in 40-50 liter cab? -active amplifier and analog signal processing is allowed :-)
Goal is "PA" pop music. low-end can ca 40-60Hz. Efficiency is priority.

Kind regards TroelsM
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Last edited by TroelsM; 6th November 2017 at 08:26 PM.
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Old 6th November 2017, 08:45 PM   #2
Brian Steele is offline Brian Steele  Grenada
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Quote:
Originally Posted by TroelsM View Post
I got an old "punch" driver that I refoamed (with some no-name ebay-rubber). It ended up with a rather large Vas:
It probably started with a large Vas too

Possible Use? vented or MLTL box, 40 Hz Fb as a starter, fix the response with EQ. Focus on power-handling in the passband (Xmax is not given, so try to guesttimate it with test box and distortion measurements).

Possible alternative is a 4th order bandpass box.
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Old 6th November 2017, 10:18 PM   #3
chris661 is offline chris661  United Kingdom
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Not a great driver, really. BL is low, Vas and Qts are high.
It wants something big and tuned low, where it'll provide reasonable efficiency and go flat down low. Probably meant for car parcel shelf use, where it'll see a large slightly leaky enclosure.

So, I slapped it in a 6th order bandpass, since you can trade bandwidth for efficiency and generally mess around to get something half-useful.

S1 = 900
S2 = 900
S3=100
S4=100
L12 = 15
L23 = 0.1
L34 = 2 (18mm wood thickness)

Vrc = 40
Lrc=25
Ap=75
Lpt=25

Port airspeed is good for a couple of hundred watts (likely outside of Xmax at that point). It'll kick at 60Hz and still grunt at 40Hz, though it's 10dB down there. Port's tuned to 40Hz, so you could EQ for more extension. I wouldn't bother - the driver won't thank you.

Mount the driver so the magnet's in the small chamber and it'll get plenty of cooling. I'm picturing a 100mm square port directly over the magnet.

You could go for a normal ported box, but this is gains you a bit of efficiency at the top end, and adds in forced-air cooling for the driver.

As with all resonant alignments, some kind of highpass would be required if you want to shove some power in there. Preferably 4th order around 35Hz, but 2nd order would do it.

Chris
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Old 7th November 2017, 01:09 AM   #4
Zero D is offline Zero D  United Kingdom
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It looks ok like this

Vb = 50L
Fb = 40Hz
1 x Vent - Int Diam = 100mm x 230 Length
HPF = 24db LR @ 40Hz
Para EQ fc= 57Hz, Q = 2, -12db
Para EQ fc= 40Hz, Q = 2, +12db

If you don't have access to a Para Eq, then you should be able to tinker with a regular EQ to get something along those lines
Attached Images
File Type: png fr.png (4.4 KB, 129 views)
File Type: png Max Power.png (4.7 KB, 128 views)
File Type: png Xmax.png (2.5 KB, 9 views)
File Type: png GD.png (7.6 KB, 9 views)
File Type: png AirVel.png (3.5 KB, 9 views)
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Old 7th November 2017, 07:56 AM   #5
TroelsM is offline TroelsM  Denmark
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Thanks for the input.

I'll sim some of the suggestions when I get the time.

One thing i forgot to mention: I need the hp to be around 200Hz, so that might restrict the solutions even more.

Kind regards TroelsM
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Old 7th November 2017, 08:37 AM   #6
chris661 is offline chris661  United Kingdom
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140Hz 4th order Butterworth knocks the ~200Hz peak down on my sim so it's roughly flat to 190Hz and then falls sharply.

Chris
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Old 7th November 2017, 08:39 AM   #7
TroelsM is offline TroelsM  Denmark
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Hi Chris. Could you share the setup (screen-shot?) of the HR ?
I tried googling, but there is too many variables for me to get right for the sim to be accurate.

Thanks. TroelsM
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Old 7th November 2017, 09:15 AM   #8
TroelsM is offline TroelsM  Denmark
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a 3-min break lead to this. Please comment if its not realistic. I really just mess around with horn-resp-sliders, but apart from the top-end this looks ball-park-right.

HR-input:
Click the image to open in full size.

Schem:
Click the image to open in full size.

Power (1W/2V/4ohm) (8mm at 50Hz. Maybe on the high side)
Click the image to open in full size.

Can I add a port to the chamber to get some output filling some of the 100-200Hz range?
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Old 7th November 2017, 09:37 AM   #9
chris661 is offline chris661  United Kingdom
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We're using different drivers. The Sd on the one I'm simulating is 491, per the numbers at the top of the page.

Double click Ap1 until it says Ap, and enter some numbers to add a port. You've got a 4th order BP there which looks pretty good. Mine has a bit more output at 60Hz and a lot more at 200Hz, but loses out around 100Hz. Compromises...

Chris
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Old 7th November 2017, 10:20 AM   #10
TroelsM is offline TroelsM  Denmark
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Thanks for the input. Yes, I see that I messed up the numbers in my sim. Sorry for the confusion.

I'll do a 4th order LP and HP for the sub-signal.

TroelsM
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Last edited by TroelsM; 7th November 2017 at 10:36 AM.
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