From: jsc52962@uxa.cso.uiuc.edu (Jeffrey S. Curtis)
Subject: Re: crossover point?
Organization: University of Illinois at Urbana
Date: Tue, 26 Jan 1993 03:31:43 GMT

In article <1k27mqINNef@bigboote.WPI.EDU> walkerr@teak.WPI.EDU (Robert A Walker) writes:
}	I'm setting up a system in my bronco, I've got two 12 inch subs in
}the way back and a pair of 2-way 6X9s in the wall about a foot behind the
}front seats.  Now don't laugh, but i have two RadioShack amplifiers.  I'm

Don't sweat it - I think probably 90% of the people here have had RadShack
stuff in their systems at one point or another.  I'm using Realistic 2-way
passive xovers right now, as a matter of fact..  :-)

}broke, and i got them on sale plus a 10% discount from a friend.  I also
}bought the subs used from a friend.  Anyways, one of hte amps is the
}120 watt sub amp which i have crossed over at 100 hz using the little
}switch on the amp, the other is the 80X80 powering the 6X9s, i know that
}is messed up, but i can't afford an electronic crossover.  I have 220 uF 
}caps on the 6X9s which crosses them over a little less than 200 hz.  Is this
}a reasonable crossover point, i had it lower around 90 but the speakers 
}couldn't handle it and complained immensly.  Should i cross the subs over at
}200 hz also when i get up some money to purchase a crossover, or should i
}put my money in a pair of speakers to put in the front doors?  THanx for 
}any help and please don't laugh :)

Do not - I repeat, do *not* cross the sub at 200Hz.  There are a number
of reasons.  First, your 220uF caps - assuming you've placed them in a
first-order configuration on the speakers (in series with the speaker) -
are providing you with a -6dB/octave slope with your -3dB point at about
190Hz.  This means that at 95Hz, your 6x9s are still playing bass at a
level (perception and A-weighting arguments notwithstanding) only 9dB
lower than the "flat" portion of their signal, which, depending on who
you're talking to, is somewhere in the neighborhood of one-half to
one-third as loud as the rest of the music from the 6x9s.  Your subs
take over at 100Hz and below.  You didn't mention the cutoff slope of
the amp's crossover, but I'm assuming it's probably -12dB/octave, which
means that at 200Hz, your subs are playing at 15dB quieter than the rest
of their signal (below 80 or 90Hz, where the "flat" portion of their
curve begins).  So between 100 and 190Hz - your two xover points - you
have a little bit of power from your 12s, and a fair bit from your 6x9s,
which combine to give you a respectable response in that range.  Combine
that fact with the fact that in *most* automotive applications (this may
be slightly skewed for a Bronco), the car's acoustics lend themselves to
a resonance near 150-200Hz, so you don't need extra power in that range.
So your response in the 100-200Hz range, while you may think that it's
empty on paper, I would bet, if you actually measured it, you have a much
flatter response curve in that range in your actual application than you
may think.  This is the main reason why I suggest *not* crossing your
12s at 200Hz.  The other, slightly more obvious reason, is that your 12s
might sound quite bad crossed over at 200Hz.  I think you'd be happier
listening to them where they are at 100Hz.

Hope this helped,

Jeff
-- 
Jeffrey S. Curtis sidewinder@uiuc.edu | "Resplendent and full again, they revel
 The Power of 37  free2207 at uiucvmd | in their role as self-appointed critics
 Sony - Phase Linear - Sanyo - Sennet | of the establishment..."
 Jensen - Polk - StreetWires - Proton |   -- INXS _Welcome to Wherever You Are_
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