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Old 22nd March 2007, 09:46 PM   #11
Dr_EM is offline Dr_EM  United Kingdom
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So in that arrangement is one driver placed behind the other, both forward firing but with only one visable? So one will be placed on a baffle inside the cabinet? Would the TL start behind the internal driver or between the two if that is the case?

Still prefer to have both drivers on show like the PMC model mind . Actually does that also cause problems with the drivers having differing effective line lenghths

I plan to use MDF for construction, get it cut at the shop
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Old 22nd March 2007, 09:51 PM   #12
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No. What you've described is an isobaric arrangement. Push-push is bipolar. Imagine a sealed box, for sake of example, with one unit mounted on the front panel, firing forward, and one on the rear directly behind it, firing backward. That's what Dave's talking about, though in this case, it would have to be with one driver mounted on each side.

Having the drivers at slightly different positions along the line doesn't really affect things much. One excites some resonant modes more than the other does, that's all.

You can get fairly acceptable results out of a 100in line, 3:1 taper, So=3Sd, Sl=1Sd, centre-point between the drivers 10.5in down from the internal top. Graph attached. It doesn't look pretty, but this is a 1/2 space graph. The LF roll-off should actually match room-gain quite well well & end up being fairly flat to the mid 20Hz regions. XO to your main units at ~80Hz or so and you should be in business (I wouldn't try crossing over higher up -over 80Hz, sound becomes increasingly directional, so you'd need two subs to preserve stereo imaging).
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Old 22nd March 2007, 09:54 PM   #13
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Quote:
Originally posted by Dr.EM
So in that arrangement is one driver placed behind the other,
No. Configured as you want the box to be, drivers would be placed on the sides.

The arrangment (in a over the top implementation) is like this.

Click the image to open in full size.

Quote:
I plan to use MDF for construction, get it cut at the shop
You are spending all this money on drivers, and spending the time building them, why degrade the box performance to save a bit of money by using MDF?

dave
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Old 22nd March 2007, 10:02 PM   #14
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Here is a real-world example of a push-push woofer (with room gain should get into the low 20s)

Implemented in 18mm BB it is 247x544x510mm

dave
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Old 22nd March 2007, 10:07 PM   #15
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Suitably menacing Dave. What were the woofers in those?
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Old 22nd March 2007, 10:22 PM   #16
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Quote:"...degrade the box performance to save a bit of money by using MDF?"

Dave, how does MDF degrade the box performance? Acoustically? Mechanically? Aesthetically?
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Old 22nd March 2007, 10:26 PM   #17
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Each has 2 Adire Extremis 6,8. Actively XOed at 106 hz to the sealed FE167. Woofers driven with Bryston 4B, sats with a Red Light District. Sealed FE167 centre & 3 FE127 diyRef monopoles built into the walls & ceiling for surround. Centre + surrounds use another 4 channels of RLD. All Fostex are modified. FE167 phase plugs match the veneer & the bases on the RLD.

XO is not yet published but is a gyrator based tube XO.

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Old 22nd March 2007, 10:35 PM   #18
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Quote:
Originally posted by MJL21193
ow does MDF degrade the box performance? Acoustically? Mechanically? Aesthetically?
Acoustically, because it is not as stiff as plywood, has higher energy storage, and panel resonances have lower Q so -- coupled with lower panel resonance frequency -- are more likely to be excited. The biggest thing in a sub would be energy storage issues. It also won't be as mechanically strong. 12mm ply will outperform 19mm MDF. For a sub 15 mm is probably the most practical.

On top of that push-push with its active newtonian motion cancellation will transfer far less energy to the box (which is why you could make it with 12 or 15mm ply.

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Old 22nd March 2007, 11:10 PM   #19
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Mechanically yes, I don't dispute you could build a stronger box with bb plywood, but since most of us are not constructing stage equipment, I don't thing it's that much of an advantage.
As I am not from the tenth planet , I have to admit that I cannot hear the difference between the two materials. Everything resonates and stores energy, but how much? My drivers drown that out..
Absolutely true about the opposite motion of push-push mounted drivers cancelling out vibration, especially if they are identical, with very similar T/S specs.
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Old 22nd March 2007, 11:17 PM   #20
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Oh, you can hear the difference easily enough in A/B comparisons. The MDF box has considerably more colouration / smearing in the time domain due to it's abrupt break-up mode. Remember, down low, around the cabinet tuning frequency, driver output is minimal -it's the box doing all the work. And things only get worse as the box gets bigger, with larger panel areas to vibrate.
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