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Old 23rd April 2004, 09:37 AM   #1
garlach is offline garlach  Luxembourg
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Default woofer/midrange interference ?

I have recently acquired two 60-litre cabinets which I want to use for PA speakers for my band. I have put an Eminence Delta 12LF and and Eminence Alpha 6 driver into each cabinet, plus a piezo for very HF. The crossover is 1st-order at 1.5kHz.

The problem is distorted sounds coming from the Alpha 6 at low frequency/high volume (bass guitar). This happens even if I disconnect the Alpha unit!! The cone moves in sympathy with the Delta (mounted close by because of the cabinet size), even when disconnected.

Is this some kind of induced current in the Alpha's coil? What can I do to shield it if so (I don't really want to rebuild the cabinets if I can avoid it ...!)

I have never heard of this problem before, but I guess this is a good place to ask....
Thanks
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Old 23rd April 2004, 10:49 AM   #2
sreten is offline sreten  United Kingdom
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You need to close off the back of the Alpha 6 with a near 6 litre enclosure.

Use a strong tapered plastic bucket or a strong tubing, thick
cardboard tubbing is good if you can find it. For the tubing
cut a single piece in two at an angle and glue on end pieces.

For both cases fill with acoustic foam.

sreten.
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Old 23rd April 2004, 11:33 AM   #3
garlach is offline garlach  Luxembourg
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Sorry, I forgot to mention - it is not mechanical interference, The cabinets are ported, not sealed and the Alpha 6 is enclosed in a piece of plastic drainpipe with a sealed end. It's not 6 litres, only a couple, but the Alpha is only working above 1.5kHz so I thought it would be OK. It's definitely electrical/magnetic (which? both?) interference. If I unmount and remove the Alpha slowly out and away from the front baffle the cone movement stops, then returns as I put it back into position.

The drivers are probably just too close together (the rims almost touch on the front baffle) but I wanted to screen to get rid of the problem rather than have to remount the Alpha further away and seal the old cutout.

Thanks a lot for the reply anyway.
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Old 23rd April 2004, 05:38 PM   #4
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I honestly cannot see this being anykind of electromagetic induction from the woofers voice coil to the mids. If it was then you could remove the mid from the box and it would still continue. It sounds to me like it is the woofers rear wave loading the 6's like sreten mentioned. You need to make sure that the boxes that you have the 6's in is Completely air tight. It sound like there might be some leaks.

Somebody did tests on inductors and induction from adjacent coils, to see if mounting two coils in close proximity does indeed alter the inductors performance. He put HUGE currents through them, well on a par with whats normally put through voice coils and the induction from one coil to another was negligable.
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Old 24th April 2004, 06:57 PM   #5
garlach is offline garlach  Luxembourg
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You're right - I've cracked it I think. It's a crossover problem,
and the "induced current" is, ah, experimental error. I think what
happened is that as I moved the Alpha in and out of its mounting hole
the wire I'd disconnected moved and remade the connection, allowing
the signal to go to the unit. The rest is just that the 1st order
Xover was not steep enough. A second order Xover seems to cure the
problem.

Thanks a lot for your help - and sorry, didn't mean to be a
time-waster!
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Old 24th April 2004, 08:03 PM   #6
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Oh it doesnt matter about time the thing is you learned something from the exercise and solved the problem.
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